/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*
 * Copyright 2019 Joyent, Inc.
 */

/*      Copyright (c) 1984, 1986, 1987, 1988, 1989 AT&T */
/*        All Rights Reserved   */


#include <sys/types.h>
#include <sys/param.h>
#include <sys/sysmacros.h>
#include <sys/signal.h>
#include <sys/user.h>
#include <sys/systm.h>
#include <sys/sysinfo.h>
#include <sys/var.h>
#include <sys/errno.h>
#include <sys/cmn_err.h>
#include <sys/debug.h>
#include <sys/inline.h>
#include <sys/disp.h>
#include <sys/class.h>
#include <sys/bitmap.h>
#include <sys/kmem.h>
#include <sys/cpuvar.h>
#include <sys/vtrace.h>
#include <sys/cpupart.h>
#include <sys/lgrp.h>
#include <sys/pg.h>
#include <sys/cmt.h>
#include <sys/bitset.h>
#include <sys/schedctl.h>
#include <sys/atomic.h>
#include <sys/dtrace.h>
#include <sys/sdt.h>
#include <sys/archsystm.h>
#include <sys/smt.h>

#include <vm/as.h>

#define BOUND_CPU       0x1
#define BOUND_PARTITION 0x2
#define BOUND_INTR      0x4

/* Dispatch queue allocation structure and functions */
struct disp_queue_info {
        disp_t  *dp;
        dispq_t *olddispq;
        dispq_t *newdispq;
        ulong_t *olddqactmap;
        ulong_t *newdqactmap;
        int     oldnglobpris;
};
static void     disp_dq_alloc(struct disp_queue_info *dptr, int numpris,
    disp_t *dp);
static void     disp_dq_assign(struct disp_queue_info *dptr, int numpris);
static void     disp_dq_free(struct disp_queue_info *dptr);

/* platform-specific routine to call when processor is idle */
static void     generic_idle_cpu();
void            (*idle_cpu)() = generic_idle_cpu;

/* routines invoked when a CPU enters/exits the idle loop */
static void     idle_enter();
static void     idle_exit();

/* platform-specific routine to call when thread is enqueued */
static void     generic_enq_thread(cpu_t *, int);
void            (*disp_enq_thread)(cpu_t *, int) = generic_enq_thread;

pri_t   kpreemptpri;            /* priority where kernel preemption applies */
pri_t   upreemptpri = 0;        /* priority where normal preemption applies */
pri_t   intr_pri;               /* interrupt thread priority base level */

#define KPQPRI  -1              /* pri where cpu affinity is dropped for kpq */
pri_t   kpqpri = KPQPRI;        /* can be set in /etc/system */
disp_t  cpu0_disp;              /* boot CPU's dispatch queue */
disp_lock_t     swapped_lock;   /* lock swapped threads and swap queue */
int     nswapped;               /* total number of swapped threads */
void    disp_swapped_enq(kthread_t *tp);
static void     disp_swapped_setrun(kthread_t *tp);
static void     cpu_resched(cpu_t *cp, pri_t tpri);

/*
 * If this is set, only interrupt threads will cause kernel preemptions.
 * This is done by changing the value of kpreemptpri.  kpreemptpri
 * will either be the max sysclass pri or the min interrupt pri.
 */
int     only_intr_kpreempt;

extern void set_idle_cpu(int cpun);
extern void unset_idle_cpu(int cpun);
static void setkpdq(kthread_t *tp, int borf);
#define SETKP_BACK      0
#define SETKP_FRONT     1
/*
 * Parameter that determines how recently a thread must have run
 * on the CPU to be considered loosely-bound to that CPU to reduce
 * cold cache effects.  The interval is in hertz.
 */
#define RECHOOSE_INTERVAL 3
int     rechoose_interval = RECHOOSE_INTERVAL;

/*
 * Parameter that determines how long (in nanoseconds) a thread must
 * be sitting on a run queue before it can be stolen by another CPU
 * to reduce migrations.  The interval is in nanoseconds.
 *
 * The nosteal_nsec should be set by platform code cmp_set_nosteal_interval()
 * to an appropriate value.  nosteal_nsec is set to NOSTEAL_UNINITIALIZED
 * here indicating it is uninitiallized.
 * Setting nosteal_nsec to 0 effectively disables the nosteal 'protection'.
 *
 */
#define NOSTEAL_UNINITIALIZED   (-1)
hrtime_t nosteal_nsec = NOSTEAL_UNINITIALIZED;
extern void cmp_set_nosteal_interval(void);

id_t    defaultcid;     /* system "default" class; see dispadmin(8) */

disp_lock_t     transition_lock;        /* lock on transitioning threads */
disp_lock_t     stop_lock;              /* lock on stopped threads */

static void     cpu_dispqalloc(int numpris);

/*
 * This gets returned by disp_getwork/disp_getbest if we couldn't steal
 * a thread because it was sitting on its run queue for a very short
 * period of time.
 */
#define T_DONTSTEAL     (kthread_t *)(-1) /* returned by disp_getwork/getbest */

static kthread_t        *disp_getwork(cpu_t *to);
static kthread_t        *disp_getbest(disp_t *from);
static kthread_t        *disp_ratify(kthread_t *tp, disp_t *kpq);

void    swtch_to(kthread_t *);

/*
 * dispatcher and scheduler initialization
 */

/*
 * disp_setup - Common code to calculate and allocate dispatcher
 *              variables and structures based on the maximum priority.
 */
static void
disp_setup(pri_t maxglobpri, pri_t oldnglobpris)
{
        pri_t   newnglobpris;

        ASSERT(MUTEX_HELD(&cpu_lock));

        newnglobpris = maxglobpri + 1 + LOCK_LEVEL;

        if (newnglobpris > oldnglobpris) {
                /*
                 * Allocate new kp queues for each CPU partition.
                 */
                cpupart_kpqalloc(newnglobpris);

                /*
                 * Allocate new dispatch queues for each CPU.
                 */
                cpu_dispqalloc(newnglobpris);

                /*
                 * compute new interrupt thread base priority
                 */
                intr_pri = maxglobpri;
                if (only_intr_kpreempt) {
                        kpreemptpri = intr_pri + 1;
                        if (kpqpri == KPQPRI)
                                kpqpri = kpreemptpri;
                }
                v.v_nglobpris = newnglobpris;
        }
}

/*
 * dispinit - Called to initialize all loaded classes and the
 *            dispatcher framework.
 */
void
dispinit(void)
{
        id_t    cid;
        pri_t   maxglobpri;
        pri_t   cl_maxglobpri;

        maxglobpri = -1;

        /*
         * Initialize transition lock, which will always be set.
         */
        DISP_LOCK_INIT(&transition_lock);
        disp_lock_enter_high(&transition_lock);
        DISP_LOCK_INIT(&stop_lock);

        mutex_enter(&cpu_lock);
        CPU->cpu_disp->disp_maxrunpri = -1;
        CPU->cpu_disp->disp_max_unbound_pri = -1;

        /*
         * Initialize the default CPU partition.
         */
        cpupart_initialize_default();
        /*
         * Call the class specific initialization functions for
         * all pre-installed schedulers.
         *
         * We pass the size of a class specific parameter
         * buffer to each of the initialization functions
         * to try to catch problems with backward compatibility
         * of class modules.
         *
         * For example a new class module running on an old system
         * which didn't provide sufficiently large parameter buffers
         * would be bad news. Class initialization modules can check for
         * this and take action if they detect a problem.
         */

        for (cid = 0; cid < nclass; cid++) {
                sclass_t        *sc;

                sc = &sclass[cid];
                if (SCHED_INSTALLED(sc)) {
                        cl_maxglobpri = sc->cl_init(cid, PC_CLPARMSZ,
                            &sc->cl_funcs);
                        if (cl_maxglobpri > maxglobpri)
                                maxglobpri = cl_maxglobpri;
                }
        }

        /*
         * Historically, kpreemptpri was set to v_maxsyspri + 1 -- which is
         * to say, maxclsyspri + 1.  However, over time, the system has used
         * more and more asynchronous kernel threads, with an increasing number
         * of these doing work on direct behalf of higher-level software (e.g.,
         * network processing).  This has led to potential priority inversions:
         * threads doing low-priority lengthy kernel work can effectively
         * delay kernel-level processing of higher-priority data. To minimize
         * such inversions, we set kpreemptpri to be v_maxsyspri; anything in
         * the kernel that runs at maxclsyspri will therefore induce kernel
         * preemption, and this priority should be used if/when an asynchronous
         * thread (or, as is often the case, task queue) is performing a task
         * on behalf of higher-level software (or any task that is otherwise
         * latency-sensitve).
         */
        kpreemptpri = (pri_t)v.v_maxsyspri;
        if (kpqpri == KPQPRI)
                kpqpri = kpreemptpri;

        ASSERT(maxglobpri >= 0);
        disp_setup(maxglobpri, 0);

        mutex_exit(&cpu_lock);

        /*
         * Platform specific sticky scheduler setup.
         */
        if (nosteal_nsec == NOSTEAL_UNINITIALIZED)
                cmp_set_nosteal_interval();

        /*
         * Get the default class ID; this may be later modified via
         * dispadmin(8).  This will load the class (normally TS) and that will
         * call disp_add(), which is why we had to drop cpu_lock first.
         */
        if (getcid(defaultclass, &defaultcid) != 0) {
                cmn_err(CE_PANIC, "Couldn't load default scheduling class '%s'",
                    defaultclass);
        }
}

/*
 * disp_add - Called with class pointer to initialize the dispatcher
 *            for a newly loaded class.
 */
void
disp_add(sclass_t *clp)
{
        pri_t   maxglobpri;
        pri_t   cl_maxglobpri;

        mutex_enter(&cpu_lock);
        /*
         * Initialize the scheduler class.
         */
        maxglobpri = (pri_t)(v.v_nglobpris - LOCK_LEVEL - 1);
        cl_maxglobpri = clp->cl_init(clp - sclass, PC_CLPARMSZ, &clp->cl_funcs);
        if (cl_maxglobpri > maxglobpri)
                maxglobpri = cl_maxglobpri;

        /*
         * Save old queue information.  Since we're initializing a
         * new scheduling class which has just been loaded, then
         * the size of the dispq may have changed.  We need to handle
         * that here.
         */
        disp_setup(maxglobpri, v.v_nglobpris);

        mutex_exit(&cpu_lock);
}


/*
 * For each CPU, allocate new dispatch queues
 * with the stated number of priorities.
 */
static void
cpu_dispqalloc(int numpris)
{
        cpu_t   *cpup;
        struct disp_queue_info  *disp_mem;
        int i, num;

        ASSERT(MUTEX_HELD(&cpu_lock));

        disp_mem = kmem_zalloc(NCPU *
            sizeof (struct disp_queue_info), KM_SLEEP);

        /*
         * This routine must allocate all of the memory before stopping
         * the cpus because it must not sleep in kmem_alloc while the
         * CPUs are stopped.  Locks they hold will not be freed until they
         * are restarted.
         */
        i = 0;
        cpup = cpu_list;
        do {
                disp_dq_alloc(&disp_mem[i], numpris, cpup->cpu_disp);
                i++;
                cpup = cpup->cpu_next;
        } while (cpup != cpu_list);
        num = i;

        pause_cpus(NULL, NULL);
        for (i = 0; i < num; i++)
                disp_dq_assign(&disp_mem[i], numpris);
        start_cpus();

        /*
         * I must free all of the memory after starting the cpus because
         * I can not risk sleeping in kmem_free while the cpus are stopped.
         */
        for (i = 0; i < num; i++)
                disp_dq_free(&disp_mem[i]);

        kmem_free(disp_mem, NCPU * sizeof (struct disp_queue_info));
}

static void
disp_dq_alloc(struct disp_queue_info *dptr, int numpris, disp_t *dp)
{
        dptr->newdispq = kmem_zalloc(numpris * sizeof (dispq_t), KM_SLEEP);
        dptr->newdqactmap = kmem_zalloc(((numpris / BT_NBIPUL) + 1) *
            sizeof (long), KM_SLEEP);
        dptr->dp = dp;
}

static void
disp_dq_assign(struct disp_queue_info *dptr, int numpris)
{
        disp_t  *dp;

        dp = dptr->dp;
        dptr->olddispq = dp->disp_q;
        dptr->olddqactmap = dp->disp_qactmap;
        dptr->oldnglobpris = dp->disp_npri;

        ASSERT(dptr->oldnglobpris < numpris);

        if (dptr->olddispq != NULL) {
                /*
                 * Use kcopy because bcopy is platform-specific
                 * and could block while we might have paused the cpus.
                 */
                (void) kcopy(dptr->olddispq, dptr->newdispq,
                    dptr->oldnglobpris * sizeof (dispq_t));
                (void) kcopy(dptr->olddqactmap, dptr->newdqactmap,
                    ((dptr->oldnglobpris / BT_NBIPUL) + 1) *
                    sizeof (long));
        }
        dp->disp_q = dptr->newdispq;
        dp->disp_qactmap = dptr->newdqactmap;
        dp->disp_q_limit = &dptr->newdispq[numpris];
        dp->disp_npri = numpris;
}

static void
disp_dq_free(struct disp_queue_info *dptr)
{
        if (dptr->olddispq != NULL)
                kmem_free(dptr->olddispq,
                    dptr->oldnglobpris * sizeof (dispq_t));
        if (dptr->olddqactmap != NULL)
                kmem_free(dptr->olddqactmap,
                    ((dptr->oldnglobpris / BT_NBIPUL) + 1) * sizeof (long));
}

/*
 * For a newly created CPU, initialize the dispatch queue.
 * This is called before the CPU is known through cpu[] or on any lists.
 */
void
disp_cpu_init(cpu_t *cp)
{
        disp_t  *dp;
        dispq_t *newdispq;
        ulong_t *newdqactmap;

        ASSERT(MUTEX_HELD(&cpu_lock));  /* protect dispatcher queue sizes */

        if (cp == cpu0_disp.disp_cpu)
                dp = &cpu0_disp;
        else
                dp = kmem_alloc(sizeof (disp_t), KM_SLEEP);
        bzero(dp, sizeof (disp_t));
        cp->cpu_disp = dp;
        dp->disp_cpu = cp;
        dp->disp_maxrunpri = -1;
        dp->disp_max_unbound_pri = -1;
        DISP_LOCK_INIT(&cp->cpu_thread_lock);
        /*
         * Allocate memory for the dispatcher queue headers
         * and the active queue bitmap.
         */
        newdispq = kmem_zalloc(v.v_nglobpris * sizeof (dispq_t), KM_SLEEP);
        newdqactmap = kmem_zalloc(((v.v_nglobpris / BT_NBIPUL) + 1) *
            sizeof (long), KM_SLEEP);
        dp->disp_q = newdispq;
        dp->disp_qactmap = newdqactmap;
        dp->disp_q_limit = &newdispq[v.v_nglobpris];
        dp->disp_npri = v.v_nglobpris;
}

void
disp_cpu_fini(cpu_t *cp)
{
        ASSERT(MUTEX_HELD(&cpu_lock));

        disp_kp_free(cp->cpu_disp);
        if (cp->cpu_disp != &cpu0_disp)
                kmem_free(cp->cpu_disp, sizeof (disp_t));
}

/*
 * Allocate new, larger kpreempt dispatch queue to replace the old one.
 */
void
disp_kp_alloc(disp_t *dq, pri_t npri)
{
        struct disp_queue_info  mem_info;

        if (npri > dq->disp_npri) {
                /*
                 * Allocate memory for the new array.
                 */
                disp_dq_alloc(&mem_info, npri, dq);

                /*
                 * We need to copy the old structures to the new
                 * and free the old.
                 */
                disp_dq_assign(&mem_info, npri);
                disp_dq_free(&mem_info);
        }
}

/*
 * Free dispatch queue.
 * Used for the kpreempt queues for a removed CPU partition and
 * for the per-CPU queues of deleted CPUs.
 */
void
disp_kp_free(disp_t *dq)
{
        struct disp_queue_info  mem_info;

        mem_info.olddispq = dq->disp_q;
        mem_info.olddqactmap = dq->disp_qactmap;
        mem_info.oldnglobpris = dq->disp_npri;
        disp_dq_free(&mem_info);
}

/*
 * End dispatcher and scheduler initialization.
 */

/*
 * See if there's anything to do other than remain idle.
 * Return non-zero if there is.
 *
 * This function must be called with high spl, or with
 * kernel preemption disabled to prevent the partition's
 * active cpu list from changing while being traversed.
 *
 * This is essentially a simpler version of disp_getwork()
 * to be called by CPUs preparing to "halt".
 */
int
disp_anywork(void)
{
        cpu_t           *cp = CPU;
        cpu_t           *ocp;
        volatile int    *local_nrunnable = &cp->cpu_disp->disp_nrunnable;

        if (!(cp->cpu_flags & CPU_OFFLINE)) {
                if (CP_MAXRUNPRI(cp->cpu_part) >= 0)
                        return (1);

                for (ocp = cp->cpu_next_part; ocp != cp;
                    ocp = ocp->cpu_next_part) {
                        ASSERT(CPU_ACTIVE(ocp));

                        /*
                         * Something has appeared on the local run queue.
                         */
                        if (*local_nrunnable > 0)
                                return (1);
                        /*
                         * If we encounter another idle CPU that will
                         * soon be trolling around through disp_anywork()
                         * terminate our walk here and let this other CPU
                         * patrol the next part of the list.
                         */
                        if (ocp->cpu_dispatch_pri == -1 &&
                            (ocp->cpu_disp_flags & CPU_DISP_HALTED) == 0)
                                return (0);
                        /*
                         * Work can be taken from another CPU if:
                         *      - There is unbound work on the run queue
                         *      - That work isn't a thread undergoing a
                         *      - context switch on an otherwise empty queue.
                         *      - The CPU isn't running the idle loop.
                         */
                        if (ocp->cpu_disp->disp_max_unbound_pri != -1 &&
                            !((ocp->cpu_disp_flags & CPU_DISP_DONTSTEAL) &&
                            ocp->cpu_disp->disp_nrunnable == 1) &&
                            ocp->cpu_dispatch_pri != -1)
                                return (1);
                }
        }
        return (0);
}

/*
 * Called when CPU enters the idle loop
 */
static void
idle_enter()
{
        cpu_t           *cp = CPU;

        new_cpu_mstate(CMS_IDLE, gethrtime_unscaled());
        CPU_STATS_ADDQ(cp, sys, idlethread, 1);
        set_idle_cpu(cp->cpu_id);       /* arch-dependent hook */
}

/*
 * Called when CPU exits the idle loop
 */
static void
idle_exit()
{
        cpu_t           *cp = CPU;

        new_cpu_mstate(CMS_SYSTEM, gethrtime_unscaled());
        unset_idle_cpu(cp->cpu_id);     /* arch-dependent hook */
}

/*
 * Idle loop.
 */
void
idle()
{
        struct cpu      *cp = CPU;              /* pointer to this CPU */
        kthread_t       *t;                     /* taken thread */

        idle_enter();

        /*
         * Uniprocessor version of idle loop.
         * Do this until notified that we're on an actual multiprocessor.
         */
        while (ncpus == 1) {
                if (cp->cpu_disp->disp_nrunnable == 0) {
                        (*idle_cpu)();
                        continue;
                }
                idle_exit();
                swtch();

                idle_enter(); /* returned from swtch */
        }

        /*
         * Multiprocessor idle loop.
         */
        for (;;) {
                /*
                 * If CPU is completely quiesced by p_online(2), just wait
                 * here with minimal bus traffic until put online.
                 */
                while (cp->cpu_flags & CPU_QUIESCED)
                        (*idle_cpu)();

                if (cp->cpu_disp->disp_nrunnable != 0) {
                        idle_exit();
                        swtch();
                } else {
                        if (cp->cpu_flags & CPU_OFFLINE)
                                continue;
                        if ((t = disp_getwork(cp)) == NULL) {
                                if (cp->cpu_chosen_level != -1) {
                                        disp_t *dp = cp->cpu_disp;
                                        disp_t *kpq;

                                        disp_lock_enter(&dp->disp_lock);
                                        /*
                                         * Set kpq under lock to prevent
                                         * migration between partitions.
                                         */
                                        kpq = &cp->cpu_part->cp_kp_queue;
                                        if (kpq->disp_maxrunpri == -1)
                                                cp->cpu_chosen_level = -1;
                                        disp_lock_exit(&dp->disp_lock);
                                }
                                (*idle_cpu)();
                                continue;
                        }
                        /*
                         * If there was a thread but we couldn't steal
                         * it, then keep trying.
                         */
                        if (t == T_DONTSTEAL)
                                continue;
                        idle_exit();
                        swtch_to(t);
                }
                idle_enter(); /* returned from swtch/swtch_to */
        }
}


/*
 * Preempt the currently running thread in favor of the highest
 * priority thread.  The class of the current thread controls
 * where it goes on the dispatcher queues. If panicking, turn
 * preemption off.
 */
void
preempt()
{
        kthread_t       *t = curthread;
        klwp_t          *lwp = ttolwp(curthread);

        if (panicstr)
                return;

        TRACE_0(TR_FAC_DISP, TR_PREEMPT_START, "preempt_start");

        thread_lock(t);

        if (t->t_state != TS_ONPROC || t->t_disp_queue != CPU->cpu_disp) {
                /*
                 * this thread has already been chosen to be run on
                 * another CPU. Clear kprunrun on this CPU since we're
                 * already headed for swtch().
                 */
                CPU->cpu_kprunrun = 0;
                thread_unlock_nopreempt(t);
                TRACE_0(TR_FAC_DISP, TR_PREEMPT_END, "preempt_end");
        } else {
                if (lwp != NULL)
                        lwp->lwp_ru.nivcsw++;
                CPU_STATS_ADDQ(CPU, sys, inv_swtch, 1);
                THREAD_TRANSITION(t);
                CL_PREEMPT(t);
                DTRACE_SCHED(preempt);
                thread_unlock_nopreempt(t);

                TRACE_0(TR_FAC_DISP, TR_PREEMPT_END, "preempt_end");

                swtch();                /* clears CPU->cpu_runrun via disp() */
        }
}

extern kthread_t *thread_unpin();

/*
 * disp() - find the highest priority thread for this processor to run, and
 * set it in TS_ONPROC state so that resume() can be called to run it.
 */
static kthread_t *
disp()
{
        cpu_t           *cpup;
        disp_t          *dp;
        kthread_t       *tp;
        dispq_t         *dq;
        int             maxrunword;
        pri_t           pri;
        disp_t          *kpq;

        TRACE_0(TR_FAC_DISP, TR_DISP_START, "disp_start");

        cpup = CPU;
        /*
         * Find the highest priority loaded, runnable thread.
         */
        dp = cpup->cpu_disp;

reschedule:
        /*
         * If there is more important work on the global queue with a better
         * priority than the maximum on this CPU, take it now.
         */
        kpq = &cpup->cpu_part->cp_kp_queue;
        while ((pri = kpq->disp_maxrunpri) >= 0 &&
            pri >= dp->disp_maxrunpri &&
            (cpup->cpu_flags & CPU_OFFLINE) == 0 &&
            (tp = disp_getbest(kpq)) != NULL) {
                if (disp_ratify(tp, kpq) != NULL) {
                        TRACE_1(TR_FAC_DISP, TR_DISP_END,
                            "disp_end:tid %p", tp);
                        return (tp);
                }
        }

        disp_lock_enter(&dp->disp_lock);
        pri = dp->disp_maxrunpri;

        /*
         * If there is nothing to run, look at what's runnable on other queues.
         * Choose the idle thread if the CPU is quiesced.
         * Note that CPUs that have the CPU_OFFLINE flag set can still run
         * interrupt threads, which will be the only threads on the CPU's own
         * queue, but cannot run threads from other queues.
         */
        if (pri == -1) {
                if (!(cpup->cpu_flags & CPU_OFFLINE)) {
                        disp_lock_exit(&dp->disp_lock);
                        if ((tp = disp_getwork(cpup)) == NULL ||
                            tp == T_DONTSTEAL) {
                                tp = cpup->cpu_idle_thread;
                                (void) splhigh();
                                THREAD_ONPROC(tp, cpup);
                                cpup->cpu_dispthread = tp;
                                cpup->cpu_dispatch_pri = -1;
                                cpup->cpu_runrun = cpup->cpu_kprunrun = 0;
                                cpup->cpu_chosen_level = -1;
                        }
                } else {
                        disp_lock_exit_high(&dp->disp_lock);
                        tp = cpup->cpu_idle_thread;
                        THREAD_ONPROC(tp, cpup);
                        cpup->cpu_dispthread = tp;
                        cpup->cpu_dispatch_pri = -1;
                        cpup->cpu_runrun = cpup->cpu_kprunrun = 0;
                        cpup->cpu_chosen_level = -1;
                }
                TRACE_1(TR_FAC_DISP, TR_DISP_END,
                    "disp_end:tid %p", tp);
                return (tp);
        }

        dq = &dp->disp_q[pri];
        tp = dq->dq_first;

        ASSERT(tp != NULL);
        ASSERT(tp->t_schedflag & TS_LOAD);      /* thread must be swapped in */

        DTRACE_SCHED2(dequeue, kthread_t *, tp, disp_t *, dp);

        /*
         * Found it so remove it from queue.
         */
        dp->disp_nrunnable--;
        dq->dq_sruncnt--;
        if ((dq->dq_first = tp->t_link) == NULL) {
                ulong_t *dqactmap = dp->disp_qactmap;

                ASSERT(dq->dq_sruncnt == 0);
                dq->dq_last = NULL;

                /*
                 * The queue is empty, so the corresponding bit needs to be
                 * turned off in dqactmap.   If nrunnable != 0 just took the
                 * last runnable thread off the
                 * highest queue, so recompute disp_maxrunpri.
                 */
                maxrunword = pri >> BT_ULSHIFT;
                dqactmap[maxrunword] &= ~BT_BIW(pri);

                if (dp->disp_nrunnable == 0) {
                        dp->disp_max_unbound_pri = -1;
                        dp->disp_maxrunpri = -1;
                } else {
                        int ipri;

                        ipri = bt_gethighbit(dqactmap, maxrunword);
                        dp->disp_maxrunpri = ipri;
                        if (ipri < dp->disp_max_unbound_pri)
                                dp->disp_max_unbound_pri = ipri;
                }
        } else {
                tp->t_link = NULL;
        }

        /*
         * Set TS_DONT_SWAP flag to prevent another processor from swapping
         * out this thread before we have a chance to run it.
         * While running, it is protected against swapping by t_lock.
         */
        tp->t_schedflag |= TS_DONT_SWAP;
        cpup->cpu_dispthread = tp;              /* protected by spl only */
        cpup->cpu_dispatch_pri = pri;
        ASSERT(pri == DISP_PRIO(tp));
        thread_onproc(tp, cpup);                /* set t_state to TS_ONPROC */
        disp_lock_exit_high(&dp->disp_lock);    /* drop run queue lock */

        ASSERT(tp != NULL);
        TRACE_1(TR_FAC_DISP, TR_DISP_END,
            "disp_end:tid %p", tp);

        if (disp_ratify(tp, kpq) == NULL)
                goto reschedule;

        return (tp);
}

/*
 * swtch()
 *      Find best runnable thread and run it.
 *      Called with the current thread already switched to a new state,
 *      on a sleep queue, run queue, stopped, and not zombied.
 *      May be called at any spl level less than or equal to LOCK_LEVEL.
 *      Always drops spl to the base level (spl0()).
 */
void
swtch()
{
        kthread_t       *t = curthread;
        kthread_t       *next;
        cpu_t           *cp;

        TRACE_0(TR_FAC_DISP, TR_SWTCH_START, "swtch_start");

        if (t->t_flag & T_INTR_THREAD)
                cpu_intr_swtch_enter(t);

        if (t->t_intr != NULL) {
                /*
                 * We are an interrupt thread.  Setup and return
                 * the interrupted thread to be resumed.
                 */
                (void) splhigh();       /* block other scheduler action */
                cp = CPU;               /* now protected against migration */
                ASSERT(CPU_ON_INTR(cp) == 0);   /* not called with PIL > 10 */
                CPU_STATS_ADDQ(cp, sys, pswitch, 1);
                CPU_STATS_ADDQ(cp, sys, intrblk, 1);
                next = thread_unpin();
                TRACE_0(TR_FAC_DISP, TR_RESUME_START, "resume_start");
                resume_from_intr(next);
        } else {
#ifdef  DEBUG
                if (t->t_state == TS_ONPROC &&
                    t->t_disp_queue->disp_cpu == CPU &&
                    t->t_preempt == 0) {
                        thread_lock(t);
                        ASSERT(t->t_state != TS_ONPROC ||
                            t->t_disp_queue->disp_cpu != CPU ||
                            t->t_preempt != 0); /* cannot migrate */
                        thread_unlock_nopreempt(t);
                }
#endif  /* DEBUG */
                cp = CPU;
                next = disp();          /* returns with spl high */
                ASSERT(CPU_ON_INTR(cp) == 0);   /* not called with PIL > 10 */

                /* OK to steal anything left on run queue */
                cp->cpu_disp_flags &= ~CPU_DISP_DONTSTEAL;

                if (next != t) {
                        hrtime_t now;

                        now = gethrtime_unscaled();
                        pg_ev_thread_swtch(cp, now, t, next);

                        /*
                         * If t was previously in the TS_ONPROC state,
                         * setfrontdq and setbackdq won't have set its t_waitrq.
                         * Since we now finally know that we're switching away
                         * from this thread, set its t_waitrq if it is on a run
                         * queue.
                         */
                        if ((t->t_state == TS_RUN) && (t->t_waitrq == 0)) {
                                t->t_waitrq = now;
                        }

                        /*
                         * restore mstate of thread that we are switching to
                         */
                        restore_mstate(next);

                        CPU_STATS_ADDQ(cp, sys, pswitch, 1);
                        cp->cpu_last_swtch = t->t_disp_time = ddi_get_lbolt();
                        TRACE_0(TR_FAC_DISP, TR_RESUME_START, "resume_start");

                        if (dtrace_vtime_active)
                                dtrace_vtime_switch(next);

                        resume(next);
                        /*
                         * The TR_RESUME_END and TR_SWTCH_END trace points
                         * appear at the end of resume(), because we may not
                         * return here
                         */
                } else {
                        if (t->t_flag & T_INTR_THREAD)
                                cpu_intr_swtch_exit(t);
                        /*
                         * Threads that enqueue themselves on a run queue defer
                         * setting t_waitrq. It is then either set in swtch()
                         * when the CPU is actually yielded, or not at all if it
                         * is remaining on the CPU.
                         * There is however a window between where the thread
                         * placed itself on a run queue, and where it selects
                         * itself in disp(), where a third party (eg. clock()
                         * doing tick processing) may have re-enqueued this
                         * thread, setting t_waitrq in the process. We detect
                         * this race by noticing that despite switching to
                         * ourself, our t_waitrq has been set, and should be
                         * cleared.
                         */
                        if (t->t_waitrq != 0)
                                t->t_waitrq = 0;

                        pg_ev_thread_remain(cp, t);

                        DTRACE_SCHED(remain__cpu);
                        TRACE_0(TR_FAC_DISP, TR_SWTCH_END, "swtch_end");
                        (void) spl0();
                }
        }
}

/*
 * swtch_from_zombie()
 *      Special case of swtch(), which allows checks for TS_ZOMB to be
 *      eliminated from normal resume.
 *      Find best runnable thread and run it.
 *      Called with the current thread zombied.
 *      Zombies cannot migrate, so CPU references are safe.
 */
void
swtch_from_zombie()
{
        kthread_t       *next;
        cpu_t           *cpu = CPU;

        TRACE_0(TR_FAC_DISP, TR_SWTCH_START, "swtch_start");

        ASSERT(curthread->t_state == TS_ZOMB);

        next = disp();                  /* returns with spl high */
        ASSERT(CPU_ON_INTR(CPU) == 0);  /* not called with PIL > 10 */
        CPU_STATS_ADDQ(CPU, sys, pswitch, 1);
        ASSERT(next != curthread);
        TRACE_0(TR_FAC_DISP, TR_RESUME_START, "resume_start");

        pg_ev_thread_swtch(cpu, gethrtime_unscaled(), curthread, next);

        restore_mstate(next);

        if (dtrace_vtime_active)
                dtrace_vtime_switch(next);

        resume_from_zombie(next);
        /*
         * The TR_RESUME_END and TR_SWTCH_END trace points
         * appear at the end of resume(), because we certainly will not
         * return here
         */
}

#if defined(DEBUG) && (defined(DISP_DEBUG) || defined(lint))

/*
 * search_disp_queues()
 *      Search the given dispatch queues for thread tp.
 *      Return 1 if tp is found, otherwise return 0.
 */
static int
search_disp_queues(disp_t *dp, kthread_t *tp)
{
        dispq_t         *dq;
        dispq_t         *eq;

        disp_lock_enter_high(&dp->disp_lock);

        for (dq = dp->disp_q, eq = dp->disp_q_limit; dq < eq; ++dq) {
                kthread_t       *rp;

                ASSERT(dq->dq_last == NULL || dq->dq_last->t_link == NULL);

                for (rp = dq->dq_first; rp; rp = rp->t_link)
                        if (tp == rp) {
                                disp_lock_exit_high(&dp->disp_lock);
                                return (1);
                        }
        }
        disp_lock_exit_high(&dp->disp_lock);

        return (0);
}

/*
 * thread_on_queue()
 *      Search all per-CPU dispatch queues and all partition-wide kpreempt
 *      queues for thread tp. Return 1 if tp is found, otherwise return 0.
 */
static int
thread_on_queue(kthread_t *tp)
{
        cpu_t           *cp;
        struct cpupart  *part;

        ASSERT(getpil() >= DISP_LEVEL);

        /*
         * Search the per-CPU dispatch queues for tp.
         */
        cp = CPU;
        do {
                if (search_disp_queues(cp->cpu_disp, tp))
                        return (1);
        } while ((cp = cp->cpu_next_onln) != CPU);

        /*
         * Search the partition-wide kpreempt queues for tp.
         */
        part = CPU->cpu_part;
        do {
                if (search_disp_queues(&part->cp_kp_queue, tp))
                        return (1);
        } while ((part = part->cp_next) != CPU->cpu_part);

        return (0);
}

#else

#define thread_on_queue(tp)     0       /* ASSERT must be !thread_on_queue */

#endif  /* DEBUG */

/*
 * like swtch(), but switch to a specified thread taken from another CPU.
 *      called with spl high..
 */
void
swtch_to(kthread_t *next)
{
        cpu_t                   *cp = CPU;
        hrtime_t                now;

        TRACE_0(TR_FAC_DISP, TR_SWTCH_START, "swtch_start");

        /*
         * Update context switch statistics.
         */
        CPU_STATS_ADDQ(cp, sys, pswitch, 1);

        TRACE_0(TR_FAC_DISP, TR_RESUME_START, "resume_start");

        now = gethrtime_unscaled();
        pg_ev_thread_swtch(cp, now, curthread, next);

        /* OK to steal anything left on run queue */
        cp->cpu_disp_flags &= ~CPU_DISP_DONTSTEAL;

        /* record last execution time */
        cp->cpu_last_swtch = curthread->t_disp_time = ddi_get_lbolt();

        /*
         * If t was previously in the TS_ONPROC state, setfrontdq and setbackdq
         * won't have set its t_waitrq.  Since we now finally know that we're
         * switching away from this thread, set its t_waitrq if it is on a run
         * queue.
         */
        if ((curthread->t_state == TS_RUN) && (curthread->t_waitrq == 0)) {
                curthread->t_waitrq = now;
        }

        /* restore next thread to previously running microstate */
        restore_mstate(next);

        if (dtrace_vtime_active)
                dtrace_vtime_switch(next);

        resume(next);
        /*
         * The TR_RESUME_END and TR_SWTCH_END trace points
         * appear at the end of resume(), because we may not
         * return here
         */
}

static void
cpu_resched(cpu_t *cp, pri_t tpri)
{
        int     call_poke_cpu = 0;
        pri_t   cpupri = cp->cpu_dispatch_pri;

        if (cpupri != CPU_IDLE_PRI && cpupri < tpri) {
                TRACE_2(TR_FAC_DISP, TR_CPU_RESCHED,
                    "CPU_RESCHED:Tpri %d Cpupri %d", tpri, cpupri);
                if (tpri >= upreemptpri && cp->cpu_runrun == 0) {
                        cp->cpu_runrun = 1;
                        aston(cp->cpu_dispthread);
                        if (tpri < kpreemptpri && cp != CPU)
                                call_poke_cpu = 1;
                }
                if (tpri >= kpreemptpri && cp->cpu_kprunrun == 0) {
                        cp->cpu_kprunrun = 1;
                        if (cp != CPU)
                                call_poke_cpu = 1;
                }
        }

        /*
         * Propagate cpu_runrun, and cpu_kprunrun to global visibility.
         */
        membar_enter();

        if (call_poke_cpu)
                poke_cpu(cp->cpu_id);
}

/*
 * setbackdq() keeps runqs balanced such that the difference in length
 * between the chosen runq and the next one is no more than RUNQ_MAX_DIFF.
 * For threads with priorities below RUNQ_MATCH_PRI levels, the runq's lengths
 * must match.  When per-thread TS_RUNQMATCH flag is set, setbackdq() will
 * try to keep runqs perfectly balanced regardless of the thread priority.
 */
#define RUNQ_MATCH_PRI  16      /* pri below which queue lengths must match */
#define RUNQ_MAX_DIFF   2       /* maximum runq length difference */
#define RUNQ_LEN(cp, pri)       ((cp)->cpu_disp->disp_q[pri].dq_sruncnt)

/*
 * Macro that evaluates to true if it is likely that the thread has cache
 * warmth. This is based on the amount of time that has elapsed since the
 * thread last ran. If that amount of time is less than "rechoose_interval"
 * ticks, then we decide that the thread has enough cache warmth to warrant
 * some affinity for t->t_cpu.
 */
#define THREAD_HAS_CACHE_WARMTH(thread) \
        ((thread == curthread) ||       \
        ((ddi_get_lbolt() - thread->t_disp_time) <= rechoose_interval))
/*
 * Put the specified thread on the back of the dispatcher
 * queue corresponding to its current priority.
 *
 * Called with the thread in transition, onproc or stopped state
 * and locked (transition implies locked) and at high spl.
 * Returns with the thread in TS_RUN state and still locked.
 */
void
setbackdq(kthread_t *tp)
{
        dispq_t *dq;
        disp_t          *dp;
        cpu_t           *cp;
        pri_t           tpri;
        int             bound;
        boolean_t       self;

        ASSERT(THREAD_LOCK_HELD(tp));
        ASSERT((tp->t_schedflag & TS_ALLSTART) == 0);
        ASSERT(!thread_on_queue(tp));   /* make sure tp isn't on a runq */

        /*
         * If thread is "swapped" or on the swap queue don't
         * queue it, but wake sched.
         */
        if ((tp->t_schedflag & (TS_LOAD | TS_ON_SWAPQ)) != TS_LOAD) {
                disp_swapped_setrun(tp);
                return;
        }

        self = (tp == curthread);

        if (tp->t_bound_cpu || tp->t_weakbound_cpu)
                bound = 1;
        else
                bound = 0;

        tpri = DISP_PRIO(tp);
        if (ncpus == 1)
                cp = tp->t_cpu;
        else if (!bound) {
                if (tpri >= kpqpri) {
                        setkpdq(tp, SETKP_BACK);
                        return;
                }

                /*
                 * We'll generally let this thread continue to run where
                 * it last ran...but will consider migration if:
                 * - The thread probably doesn't have much cache warmth.
                 * - SMT exclusion would prefer us to run elsewhere
                 * - The CPU where it last ran is the target of an offline
                 *   request.
                 * - The thread last ran outside its home lgroup.
                 */
                if ((!THREAD_HAS_CACHE_WARMTH(tp)) ||
                    !smt_should_run(tp, tp->t_cpu) ||
                    (tp->t_cpu == cpu_inmotion) ||
                    !LGRP_CONTAINS_CPU(tp->t_lpl->lpl_lgrp, tp->t_cpu)) {
                        cp = disp_lowpri_cpu(tp->t_cpu, tp, tpri);
                } else {
                        cp = tp->t_cpu;
                }

                if (tp->t_cpupart == cp->cpu_part) {
                        int     qlen;

                        /*
                         * Perform any CMT load balancing
                         */
                        cp = cmt_balance(tp, cp);

                        /*
                         * Balance across the run queues
                         */
                        qlen = RUNQ_LEN(cp, tpri);
                        if (tpri >= RUNQ_MATCH_PRI &&
                            !(tp->t_schedflag & TS_RUNQMATCH))
                                qlen -= RUNQ_MAX_DIFF;
                        if (qlen > 0) {
                                cpu_t *newcp;

                                if (tp->t_lpl->lpl_lgrpid == LGRP_ROOTID) {
                                        newcp = cp->cpu_next_part;
                                } else if ((newcp = cp->cpu_next_lpl) == cp) {
                                        newcp = cp->cpu_next_part;
                                }

                                if (smt_should_run(tp, newcp) &&
                                    RUNQ_LEN(newcp, tpri) < qlen) {
                                        DTRACE_PROBE3(runq__balance,
                                            kthread_t *, tp,
                                            cpu_t *, cp, cpu_t *, newcp);
                                        cp = newcp;
                                }
                        }
                } else {
                        /*
                         * Migrate to a cpu in the new partition.
                         */
                        cp = disp_lowpri_cpu(tp->t_cpupart->cp_cpulist, tp,
                            tp->t_pri);
                }
                ASSERT((cp->cpu_flags & CPU_QUIESCED) == 0);
        } else {
                /*
                 * It is possible that t_weakbound_cpu != t_bound_cpu (for
                 * a short time until weak binding that existed when the
                 * strong binding was established has dropped) so we must
                 * favour weak binding over strong.
                 */
                cp = tp->t_weakbound_cpu ?
                    tp->t_weakbound_cpu : tp->t_bound_cpu;
        }
        /*
         * A thread that is ONPROC may be temporarily placed on the run queue
         * but then chosen to run again by disp.  If the thread we're placing on
         * the queue is in TS_ONPROC state, don't set its t_waitrq until a
         * replacement process is actually scheduled in swtch().  In this
         * situation, curthread is the only thread that could be in the ONPROC
         * state.
         */
        if ((!self) && (tp->t_waitrq == 0)) {
                hrtime_t curtime;

                curtime = gethrtime_unscaled();
                (void) cpu_update_pct(tp, curtime);
                tp->t_waitrq = curtime;
        } else {
                (void) cpu_update_pct(tp, gethrtime_unscaled());
        }

        dp = cp->cpu_disp;
        disp_lock_enter_high(&dp->disp_lock);

        DTRACE_SCHED3(enqueue, kthread_t *, tp, disp_t *, dp, int, 0);
        TRACE_3(TR_FAC_DISP, TR_BACKQ, "setbackdq:pri %d cpu %p tid %p",
            tpri, cp, tp);

        ASSERT(tpri >= 0 && tpri < dp->disp_npri);

        THREAD_RUN(tp, &dp->disp_lock);         /* set t_state to TS_RUN */
        tp->t_disp_queue = dp;
        tp->t_link = NULL;

        dq = &dp->disp_q[tpri];
        dp->disp_nrunnable++;
        if (!bound)
                dp->disp_steal = 0;
        membar_enter();

        if (dq->dq_sruncnt++ != 0) {
                ASSERT(dq->dq_first != NULL);
                dq->dq_last->t_link = tp;
                dq->dq_last = tp;
        } else {
                ASSERT(dq->dq_first == NULL);
                ASSERT(dq->dq_last == NULL);
                dq->dq_first = dq->dq_last = tp;
                BT_SET(dp->disp_qactmap, tpri);
                if (tpri > dp->disp_maxrunpri) {
                        dp->disp_maxrunpri = tpri;
                        membar_enter();
                        cpu_resched(cp, tpri);
                }
        }

        if (!bound && tpri > dp->disp_max_unbound_pri) {
                if (self && dp->disp_max_unbound_pri == -1 && cp == CPU) {
                        /*
                         * If there are no other unbound threads on the
                         * run queue, don't allow other CPUs to steal
                         * this thread while we are in the middle of a
                         * context switch. We may just switch to it
                         * again right away. CPU_DISP_DONTSTEAL is cleared
                         * in swtch and swtch_to.
                         */
                        cp->cpu_disp_flags |= CPU_DISP_DONTSTEAL;
                }
                dp->disp_max_unbound_pri = tpri;
        }
        (*disp_enq_thread)(cp, bound);
}

/*
 * Put the specified thread on the front of the dispatcher
 * queue corresponding to its current priority.
 *
 * Called with the thread in transition, onproc or stopped state
 * and locked (transition implies locked) and at high spl.
 * Returns with the thread in TS_RUN state and still locked.
 */
void
setfrontdq(kthread_t *tp)
{
        disp_t          *dp;
        dispq_t         *dq;
        cpu_t           *cp;
        pri_t           tpri;
        int             bound;

        ASSERT(THREAD_LOCK_HELD(tp));
        ASSERT((tp->t_schedflag & TS_ALLSTART) == 0);
        ASSERT(!thread_on_queue(tp));   /* make sure tp isn't on a runq */

        /*
         * If thread is "swapped" or on the swap queue don't
         * queue it, but wake sched.
         */
        if ((tp->t_schedflag & (TS_LOAD | TS_ON_SWAPQ)) != TS_LOAD) {
                disp_swapped_setrun(tp);
                return;
        }

        if (tp->t_bound_cpu || tp->t_weakbound_cpu)
                bound = 1;
        else
                bound = 0;

        tpri = DISP_PRIO(tp);
        if (ncpus == 1)
                cp = tp->t_cpu;
        else if (!bound) {
                if (tpri >= kpqpri) {
                        setkpdq(tp, SETKP_FRONT);
                        return;
                }
                cp = tp->t_cpu;
                if (tp->t_cpupart == cp->cpu_part) {
                        /*
                         * We'll generally let this thread continue to run
                         * where it last ran, but will consider migration if:
                         * - The thread last ran outside its home lgroup.
                         * - The CPU where it last ran is the target of an
                         *   offline request (a thread_nomigrate() on the in
                         *   motion CPU relies on this when forcing a preempt).
                         * - The thread isn't the highest priority thread where
                         *   it last ran, and it is considered not likely to
                         *   have significant cache warmth.
                         */
                        if (!LGRP_CONTAINS_CPU(tp->t_lpl->lpl_lgrp, cp) ||
                            cp == cpu_inmotion ||
                            (tpri < cp->cpu_disp->disp_maxrunpri &&
                            !THREAD_HAS_CACHE_WARMTH(tp))) {
                                cp = disp_lowpri_cpu(tp->t_cpu, tp, tpri);
                        }
                } else {
                        /*
                         * Migrate to a cpu in the new partition.
                         */
                        cp = disp_lowpri_cpu(tp->t_cpupart->cp_cpulist,
                            tp, tp->t_pri);
                }
                ASSERT((cp->cpu_flags & CPU_QUIESCED) == 0);
        } else {
                /*
                 * It is possible that t_weakbound_cpu != t_bound_cpu (for
                 * a short time until weak binding that existed when the
                 * strong binding was established has dropped) so we must
                 * favour weak binding over strong.
                 */
                cp = tp->t_weakbound_cpu ?
                    tp->t_weakbound_cpu : tp->t_bound_cpu;
        }

        /*
         * A thread that is ONPROC may be temporarily placed on the run queue
         * but then chosen to run again by disp.  If the thread we're placing on
         * the queue is in TS_ONPROC state, don't set its t_waitrq until a
         * replacement process is actually scheduled in swtch().  In this
         * situation, curthread is the only thread that could be in the ONPROC
         * state.
         */
        if ((tp != curthread) && (tp->t_waitrq == 0)) {
                hrtime_t curtime;

                curtime = gethrtime_unscaled();
                (void) cpu_update_pct(tp, curtime);
                tp->t_waitrq = curtime;
        } else {
                (void) cpu_update_pct(tp, gethrtime_unscaled());
        }

        dp = cp->cpu_disp;
        disp_lock_enter_high(&dp->disp_lock);

        TRACE_2(TR_FAC_DISP, TR_FRONTQ, "frontq:pri %d tid %p", tpri, tp);
        DTRACE_SCHED3(enqueue, kthread_t *, tp, disp_t *, dp, int, 1);

        ASSERT(tpri >= 0 && tpri < dp->disp_npri);

        THREAD_RUN(tp, &dp->disp_lock);         /* set TS_RUN state and lock */
        tp->t_disp_queue = dp;

        dq = &dp->disp_q[tpri];
        dp->disp_nrunnable++;
        if (!bound)
                dp->disp_steal = 0;
        membar_enter();

        if (dq->dq_sruncnt++ != 0) {
                ASSERT(dq->dq_last != NULL);
                tp->t_link = dq->dq_first;
                dq->dq_first = tp;
        } else {
                ASSERT(dq->dq_last == NULL);
                ASSERT(dq->dq_first == NULL);
                tp->t_link = NULL;
                dq->dq_first = dq->dq_last = tp;
                BT_SET(dp->disp_qactmap, tpri);
                if (tpri > dp->disp_maxrunpri) {
                        dp->disp_maxrunpri = tpri;
                        membar_enter();
                        cpu_resched(cp, tpri);
                }
        }

        if (!bound && tpri > dp->disp_max_unbound_pri) {
                if (tp == curthread && dp->disp_max_unbound_pri == -1 &&
                    cp == CPU) {
                        /*
                         * If there are no other unbound threads on the
                         * run queue, don't allow other CPUs to steal
                         * this thread while we are in the middle of a
                         * context switch. We may just switch to it
                         * again right away. CPU_DISP_DONTSTEAL is cleared
                         * in swtch and swtch_to.
                         */
                        cp->cpu_disp_flags |= CPU_DISP_DONTSTEAL;
                }
                dp->disp_max_unbound_pri = tpri;
        }
        (*disp_enq_thread)(cp, bound);
}

/*
 * Put a high-priority unbound thread on the kp queue
 */
static void
setkpdq(kthread_t *tp, int borf)
{
        dispq_t *dq;
        disp_t  *dp;
        cpu_t   *cp;
        pri_t   tpri;

        tpri = DISP_PRIO(tp);

        dp = &tp->t_cpupart->cp_kp_queue;
        disp_lock_enter_high(&dp->disp_lock);

        TRACE_2(TR_FAC_DISP, TR_FRONTQ, "frontq:pri %d tid %p", tpri, tp);

        ASSERT(tpri >= 0 && tpri < dp->disp_npri);
        DTRACE_SCHED3(enqueue, kthread_t *, tp, disp_t *, dp, int, borf);
        THREAD_RUN(tp, &dp->disp_lock);         /* set t_state to TS_RUN */
        tp->t_disp_queue = dp;
        dp->disp_nrunnable++;
        dq = &dp->disp_q[tpri];

        if (dq->dq_sruncnt++ != 0) {
                if (borf == SETKP_BACK) {
                        ASSERT(dq->dq_first != NULL);
                        tp->t_link = NULL;
                        dq->dq_last->t_link = tp;
                        dq->dq_last = tp;
                } else {
                        ASSERT(dq->dq_last != NULL);
                        tp->t_link = dq->dq_first;
                        dq->dq_first = tp;
                }
        } else {
                if (borf == SETKP_BACK) {
                        ASSERT(dq->dq_first == NULL);
                        ASSERT(dq->dq_last == NULL);
                        dq->dq_first = dq->dq_last = tp;
                } else {
                        ASSERT(dq->dq_last == NULL);
                        ASSERT(dq->dq_first == NULL);
                        tp->t_link = NULL;
                        dq->dq_first = dq->dq_last = tp;
                }
                BT_SET(dp->disp_qactmap, tpri);
                if (tpri > dp->disp_max_unbound_pri)
                        dp->disp_max_unbound_pri = tpri;
                if (tpri > dp->disp_maxrunpri) {
                        dp->disp_maxrunpri = tpri;
                        membar_enter();
                }
        }

        cp = tp->t_cpu;
        if (tp->t_cpupart != cp->cpu_part) {
                /* migrate to a cpu in the new partition */
                cp = tp->t_cpupart->cp_cpulist;
        }
        cp = disp_lowpri_cpu(cp, tp, tp->t_pri);
        disp_lock_enter_high(&cp->cpu_disp->disp_lock);
        ASSERT((cp->cpu_flags & CPU_QUIESCED) == 0);

        if (cp->cpu_chosen_level < tpri)
                cp->cpu_chosen_level = tpri;
        cpu_resched(cp, tpri);
        disp_lock_exit_high(&cp->cpu_disp->disp_lock);
        (*disp_enq_thread)(cp, 0);
}

/*
 * Remove a thread from the dispatcher queue if it is on it.
 * It is not an error if it is not found but we return whether
 * or not it was found in case the caller wants to check.
 */
int
dispdeq(kthread_t *tp)
{
        disp_t          *dp;
        dispq_t         *dq;
        kthread_t       *rp;
        kthread_t       *trp;
        kthread_t       **ptp;
        int             tpri;

        ASSERT(THREAD_LOCK_HELD(tp));

        if (tp->t_state != TS_RUN)
                return (0);

        /*
         * The thread is "swapped" or is on the swap queue and
         * hence no longer on the run queue, so return true.
         */
        if ((tp->t_schedflag & (TS_LOAD | TS_ON_SWAPQ)) != TS_LOAD)
                return (1);

        tpri = DISP_PRIO(tp);
        dp = tp->t_disp_queue;
        ASSERT(tpri < dp->disp_npri);
        dq = &dp->disp_q[tpri];
        ptp = &dq->dq_first;
        rp = *ptp;
        trp = NULL;

        ASSERT(dq->dq_last == NULL || dq->dq_last->t_link == NULL);

        /*
         * Search for thread in queue.
         * Double links would simplify this at the expense of disp/setrun.
         */
        while (rp != tp && rp != NULL) {
                trp = rp;
                ptp = &trp->t_link;
                rp = trp->t_link;
        }

        if (rp == NULL) {
                panic("dispdeq: thread not on queue");
        }

        DTRACE_SCHED2(dequeue, kthread_t *, tp, disp_t *, dp);

        /*
         * Found it so remove it from queue.
         */
        if ((*ptp = rp->t_link) == NULL)
                dq->dq_last = trp;

        dp->disp_nrunnable--;
        if (--dq->dq_sruncnt == 0) {
                dp->disp_qactmap[tpri >> BT_ULSHIFT] &= ~BT_BIW(tpri);
                if (dp->disp_nrunnable == 0) {
                        dp->disp_max_unbound_pri = -1;
                        dp->disp_maxrunpri = -1;
                } else if (tpri == dp->disp_maxrunpri) {
                        int ipri;

                        ipri = bt_gethighbit(dp->disp_qactmap,
                            dp->disp_maxrunpri >> BT_ULSHIFT);
                        if (ipri < dp->disp_max_unbound_pri)
                                dp->disp_max_unbound_pri = ipri;
                        dp->disp_maxrunpri = ipri;
                }
        }
        tp->t_link = NULL;
        THREAD_TRANSITION(tp);          /* put in intermediate state */
        return (1);
}


/*
 * dq_sruninc and dq_srundec are public functions for
 * incrementing/decrementing the sruncnts when a thread on
 * a dispatcher queue is made schedulable/unschedulable by
 * resetting the TS_LOAD flag.
 *
 * The caller MUST have the thread lock and therefore the dispatcher
 * queue lock so that the operation which changes
 * the flag, the operation that checks the status of the thread to
 * determine if it's on a disp queue AND the call to this function
 * are one atomic operation with respect to interrupts.
 */

/*
 * Called by sched AFTER TS_LOAD flag is set on a swapped, runnable thread.
 */
void
dq_sruninc(kthread_t *t)
{
        ASSERT(t->t_state == TS_RUN);
        ASSERT(t->t_schedflag & TS_LOAD);

        THREAD_TRANSITION(t);
        setfrontdq(t);
}

/*
 * See comment on calling conventions above.
 * Called by sched BEFORE TS_LOAD flag is cleared on a runnable thread.
 */
void
dq_srundec(kthread_t *t)
{
        ASSERT(t->t_schedflag & TS_LOAD);

        (void) dispdeq(t);
        disp_swapped_enq(t);
}

/*
 * Change the dispatcher lock of thread to the "swapped_lock"
 * and return with thread lock still held.
 *
 * Called with thread_lock held, in transition state, and at high spl.
 */
void
disp_swapped_enq(kthread_t *tp)
{
        ASSERT(THREAD_LOCK_HELD(tp));
        ASSERT(tp->t_schedflag & TS_LOAD);

        switch (tp->t_state) {
        case TS_RUN:
                disp_lock_enter_high(&swapped_lock);
                THREAD_SWAP(tp, &swapped_lock); /* set TS_RUN state and lock */
                break;
        case TS_ONPROC:
                disp_lock_enter_high(&swapped_lock);
                THREAD_TRANSITION(tp);
                wake_sched_sec = 1;             /* tell clock to wake sched */
                THREAD_SWAP(tp, &swapped_lock); /* set TS_RUN state and lock */
                break;
        default:
                panic("disp_swapped: tp: %p bad t_state", (void *)tp);
        }
}

/*
 * This routine is called by setbackdq/setfrontdq if the thread is
 * not loaded or loaded and on the swap queue.
 *
 * Thread state TS_SLEEP implies that a swapped thread
 * has been woken up and needs to be swapped in by the swapper.
 *
 * Thread state TS_RUN, it implies that the priority of a swapped
 * thread is being increased by scheduling class (e.g. ts_update).
 */
static void
disp_swapped_setrun(kthread_t *tp)
{
        ASSERT(THREAD_LOCK_HELD(tp));
        ASSERT((tp->t_schedflag & (TS_LOAD | TS_ON_SWAPQ)) != TS_LOAD);

        switch (tp->t_state) {
        case TS_SLEEP:
                disp_lock_enter_high(&swapped_lock);
                /*
                 * Wakeup sched immediately (i.e., next tick) if the
                 * thread priority is above maxclsyspri.
                 */
                if (DISP_PRIO(tp) > maxclsyspri)
                        wake_sched = 1;
                else
                        wake_sched_sec = 1;
                THREAD_RUN(tp, &swapped_lock); /* set TS_RUN state and lock */
                break;
        case TS_RUN:                            /* called from ts_update */
                break;
        default:
                panic("disp_swapped_setrun: tp: %p bad t_state", (void *)tp);
        }
}

/*
 *      Make a thread give up its processor.  Find the processor on
 *      which this thread is executing, and have that processor
 *      preempt.
 *
 *      We allow System Duty Cycle (SDC) threads to be preempted even if
 *      they are running at kernel priorities.  To implement this, we always
 *      set cpu_kprunrun; this ensures preempt() will be called.  Since SDC
 *      calls cpu_surrender() very often, we only preempt if there is anyone
 *      competing with us.
 */
void
cpu_surrender(kthread_t *tp)
{
        cpu_t   *cpup;
        int     max_pri;
        int     max_run_pri;
        klwp_t  *lwp;

        ASSERT(THREAD_LOCK_HELD(tp));

        if (tp->t_state != TS_ONPROC)
                return;
        cpup = tp->t_disp_queue->disp_cpu;      /* CPU thread dispatched to */
        max_pri = cpup->cpu_disp->disp_maxrunpri; /* best pri of that CPU */
        max_run_pri = CP_MAXRUNPRI(cpup->cpu_part);
        if (max_pri < max_run_pri)
                max_pri = max_run_pri;

        if (tp->t_cid == sysdccid) {
                uint_t t_pri = DISP_PRIO(tp);
                if (t_pri > max_pri)
                        return;         /* we are not competing w/ anyone */
                cpup->cpu_runrun = cpup->cpu_kprunrun = 1;
        } else {
                cpup->cpu_runrun = 1;
                if (max_pri >= kpreemptpri && cpup->cpu_kprunrun == 0) {
                        cpup->cpu_kprunrun = 1;
                }
        }

        /*
         * Propagate cpu_runrun, and cpu_kprunrun to global visibility.
         */
        membar_enter();

        DTRACE_SCHED1(surrender, kthread_t *, tp);

        /*
         * Make the target thread take an excursion through trap()
         * to do preempt() (unless we're already in trap or post_syscall,
         * calling cpu_surrender via CL_TRAPRET).
         */
        if (tp != curthread || (lwp = tp->t_lwp) == NULL ||
            lwp->lwp_state != LWP_USER) {
                aston(tp);
                if (cpup != CPU)
                        poke_cpu(cpup->cpu_id);
        }
        TRACE_2(TR_FAC_DISP, TR_CPU_SURRENDER,
            "cpu_surrender:tid %p cpu %p", tp, cpup);
}

/*
 * Commit to and ratify a scheduling decision
 */
/*ARGSUSED*/
static kthread_t *
disp_ratify(kthread_t *tp, disp_t *kpq)
{
        pri_t   tpri, maxpri;
        pri_t   maxkpri;
        cpu_t   *cpup;

        ASSERT(tp != NULL);
        /*
         * Commit to, then ratify scheduling decision
         */
        cpup = CPU;
        if (cpup->cpu_runrun != 0)
                cpup->cpu_runrun = 0;
        if (cpup->cpu_kprunrun != 0)
                cpup->cpu_kprunrun = 0;
        if (cpup->cpu_chosen_level != -1)
                cpup->cpu_chosen_level = -1;
        membar_enter();
        tpri = DISP_PRIO(tp);
        maxpri = cpup->cpu_disp->disp_maxrunpri;
        maxkpri = kpq->disp_maxrunpri;
        if (maxpri < maxkpri)
                maxpri = maxkpri;
        if (tpri < maxpri) {
                /*
                 * should have done better
                 * put this one back and indicate to try again
                 */
                cpup->cpu_dispthread = curthread;       /* fixup dispthread */
                cpup->cpu_dispatch_pri = DISP_PRIO(curthread);
                thread_lock_high(tp);
                THREAD_TRANSITION(tp);
                setfrontdq(tp);
                thread_unlock_nopreempt(tp);

                tp = NULL;
        }
        return (tp);
}

/*
 * See if there is any work on the dispatcher queue for other CPUs.
 * If there is, dequeue the best thread and return.
 */
static kthread_t *
disp_getwork(cpu_t *cp)
{
        cpu_t           *ocp;           /* other CPU */
        cpu_t           *ocp_start;
        cpu_t           *tcp;           /* target local CPU */
        kthread_t       *tp;
        kthread_t       *retval = NULL;
        pri_t           maxpri;
        disp_t          *kpq;           /* kp queue for this partition */
        lpl_t           *lpl, *lpl_leaf;
        int             leafidx, startidx;
        hrtime_t        stealtime;
        lgrp_id_t       local_id;

        maxpri = -1;
        tcp = NULL;

        kpq = &cp->cpu_part->cp_kp_queue;
        while (kpq->disp_maxrunpri >= 0) {
                /*
                 * Try to take a thread from the kp_queue.
                 */
                tp = (disp_getbest(kpq));
                if (tp)
                        return (disp_ratify(tp, kpq));
        }

        kpreempt_disable();             /* protect the cpu_active list */

        /*
         * Try to find something to do on another CPU's run queue.
         * Loop through all other CPUs looking for the one with the highest
         * priority unbound thread.
         *
         * On NUMA machines, the partition's CPUs are consulted in order of
         * distance from the current CPU. This way, the first available
         * work found is also the closest, and will suffer the least
         * from being migrated.
         */
        lpl = lpl_leaf = cp->cpu_lpl;
        local_id = lpl_leaf->lpl_lgrpid;
        leafidx = startidx = 0;

        /*
         * This loop traverses the lpl hierarchy. Higher level lpls represent
         * broader levels of locality
         */
        do {
                /* This loop iterates over the lpl's leaves */
                do {
                        if (lpl_leaf != cp->cpu_lpl)
                                ocp = lpl_leaf->lpl_cpus;
                        else
                                ocp = cp->cpu_next_lpl;

                        /* This loop iterates over the CPUs in the leaf */
                        ocp_start = ocp;
                        do {
                                pri_t pri;

                                ASSERT(CPU_ACTIVE(ocp));

                                /*
                                 * End our stroll around this lpl if:
                                 *
                                 * - Something became runnable on the local
                                 *   queue...which also ends our stroll around
                                 *   the partition.
                                 *
                                 * - We happen across another idle CPU.
                                 *   Since it is patrolling the next portion
                                 *   of the lpl's list (assuming it's not
                                 *   halted, or busy servicing an interrupt),
                                 *   move to the next higher level of locality.
                                 */
                                if (cp->cpu_disp->disp_nrunnable != 0) {
                                        kpreempt_enable();
                                        return (NULL);
                                }
                                if (ocp->cpu_dispatch_pri == -1) {
                                        if (ocp->cpu_disp_flags &
                                            CPU_DISP_HALTED ||
                                            ocp->cpu_intr_actv != 0)
                                                continue;
                                        else
                                                goto next_level;
                                }

                                /*
                                 * If there's only one thread and the CPU
                                 * is in the middle of a context switch,
                                 * or it's currently running the idle thread,
                                 * don't steal it.
                                 */
                                if ((ocp->cpu_disp_flags &
                                    CPU_DISP_DONTSTEAL) &&
                                    ocp->cpu_disp->disp_nrunnable == 1)
                                        continue;

                                pri = ocp->cpu_disp->disp_max_unbound_pri;
                                if (pri > maxpri) {
                                        /*
                                         * Don't steal threads that we attempted
                                         * to steal recently until they're ready
                                         * to be stolen again.
                                         */
                                        stealtime = ocp->cpu_disp->disp_steal;
                                        if (stealtime == 0 ||
                                            stealtime - gethrtime() <= 0) {
                                                maxpri = pri;
                                                tcp = ocp;
                                        } else {
                                                /*
                                                 * Don't update tcp, just set
                                                 * the retval to T_DONTSTEAL, so
                                                 * that if no acceptable CPUs
                                                 * are found the return value
                                                 * will be T_DONTSTEAL rather
                                                 * then NULL.
                                                 */
                                                retval = T_DONTSTEAL;
                                        }
                                }
                        } while ((ocp = ocp->cpu_next_lpl) != ocp_start);

                        /*
                         * Iterate to the next leaf lpl in the resource set
                         * at this level of locality. If we hit the end of
                         * the set, wrap back around to the beginning.
                         *
                         * Note: This iteration is NULL terminated for a reason
                         * see lpl_topo_bootstrap() in lgrp.c for details.
                         */
                        if ((lpl_leaf = lpl->lpl_rset[++leafidx]) == NULL) {
                                leafidx = 0;
                                lpl_leaf = lpl->lpl_rset[leafidx];
                        }
                } while (leafidx != startidx);

next_level:
                /*
                 * Expand the search to include farther away CPUs (next
                 * locality level). The closer CPUs that have already been
                 * checked will be checked again. In doing so, idle CPUs
                 * will tend to be more aggresive about stealing from CPUs
                 * that are closer (since the closer CPUs will be considered
                 * more often).
                 * Begin at this level with the CPUs local leaf lpl.
                 */
                if ((lpl = lpl->lpl_parent) != NULL) {
                        leafidx = startidx = lpl->lpl_id2rset[local_id];
                        lpl_leaf = lpl->lpl_rset[leafidx];
                }
        } while (!tcp && lpl);

        kpreempt_enable();

        /*
         * If another queue looks good, and there is still nothing on
         * the local queue, try to transfer one or more threads
         * from it to our queue.
         */
        if (tcp && cp->cpu_disp->disp_nrunnable == 0) {
                tp = disp_getbest(tcp->cpu_disp);
                if (tp == NULL || tp == T_DONTSTEAL)
                        return (tp);
                return (disp_ratify(tp, kpq));
        }
        return (retval);
}


/*
 * disp_fix_unbound_pri()
 *      Determines the maximum priority of unbound threads on the queue.
 *      The priority is kept for the queue, but is only increased, never
 *      reduced unless some CPU is looking for something on that queue.
 *
 *      The priority argument is the known upper limit.
 *
 *      Perhaps this should be kept accurately, but that probably means
 *      separate bitmaps for bound and unbound threads.  Since only idled
 *      CPUs will have to do this recalculation, it seems better this way.
 */
static void
disp_fix_unbound_pri(disp_t *dp, pri_t pri)
{
        kthread_t       *tp;
        dispq_t         *dq;
        ulong_t         *dqactmap = dp->disp_qactmap;
        ulong_t         mapword;
        int             wx;

        ASSERT(DISP_LOCK_HELD(&dp->disp_lock));

        ASSERT(pri >= 0);                       /* checked by caller */

        /*
         * Start the search at the next lowest priority below the supplied
         * priority.  This depends on the bitmap implementation.
         */
        do {
                wx = pri >> BT_ULSHIFT;         /* index of word in map */

                /*
                 * Form mask for all lower priorities in the word.
                 */
                mapword = dqactmap[wx] & (BT_BIW(pri) - 1);

                /*
                 * Get next lower active priority.
                 */
                if (mapword != 0) {
                        pri = (wx << BT_ULSHIFT) + highbit(mapword) - 1;
                } else if (wx > 0) {
                        pri = bt_gethighbit(dqactmap, wx - 1); /* sign extend */
                        if (pri < 0)
                                break;
                } else {
                        pri = -1;
                        break;
                }

                /*
                 * Search the queue for unbound, runnable threads.
                 */
                dq = &dp->disp_q[pri];
                tp = dq->dq_first;

                while (tp && (tp->t_bound_cpu || tp->t_weakbound_cpu)) {
                        tp = tp->t_link;
                }

                /*
                 * If a thread was found, set the priority and return.
                 */
        } while (tp == NULL);

        /*
         * pri holds the maximum unbound thread priority or -1.
         */
        if (dp->disp_max_unbound_pri != pri)
                dp->disp_max_unbound_pri = pri;
}

/*
 * disp_adjust_unbound_pri() - thread is becoming unbound, so we should
 *      check if the CPU to which is was previously bound should have
 *      its disp_max_unbound_pri increased.
 */
void
disp_adjust_unbound_pri(kthread_t *tp)
{
        disp_t *dp;
        pri_t tpri;

        ASSERT(THREAD_LOCK_HELD(tp));

        /*
         * Don't do anything if the thread is not bound, or
         * currently not runnable or swapped out.
         */
        if (tp->t_bound_cpu == NULL ||
            tp->t_state != TS_RUN ||
            tp->t_schedflag & TS_ON_SWAPQ)
                return;

        tpri = DISP_PRIO(tp);
        dp = tp->t_bound_cpu->cpu_disp;
        ASSERT(tpri >= 0 && tpri < dp->disp_npri);
        if (tpri > dp->disp_max_unbound_pri)
                dp->disp_max_unbound_pri = tpri;
}

/*
 * disp_getbest()
 *   De-queue the highest priority unbound runnable thread.
 *   Returns with the thread unlocked and onproc but at splhigh (like disp()).
 *   Returns NULL if nothing found.
 *   Returns T_DONTSTEAL if the thread was not stealable.
 *   so that the caller will try again later.
 *
 *   Passed a pointer to a dispatch queue not associated with this CPU, and
 *   its type.
 */
static kthread_t *
disp_getbest(disp_t *dp)
{
        kthread_t       *tp;
        dispq_t         *dq;
        pri_t           pri;
        cpu_t           *cp, *tcp;
        boolean_t       allbound;

        disp_lock_enter(&dp->disp_lock);

        /*
         * If there is nothing to run, or the CPU is in the middle of a
         * context switch of the only thread, return NULL.
         */
        tcp = dp->disp_cpu;
        cp = CPU;
        pri = dp->disp_max_unbound_pri;
        if (pri == -1 ||
            (tcp != NULL && (tcp->cpu_disp_flags & CPU_DISP_DONTSTEAL) &&
            tcp->cpu_disp->disp_nrunnable == 1)) {
                disp_lock_exit_nopreempt(&dp->disp_lock);
                return (NULL);
        }

        dq = &dp->disp_q[pri];


        /*
         * Assume that all threads are bound on this queue, and change it
         * later when we find out that it is not the case.
         */
        allbound = B_TRUE;
        for (tp = dq->dq_first; tp != NULL; tp = tp->t_link) {
                hrtime_t now, nosteal, rqtime;

                /*
                 * Skip over bound threads which could be here even
                 * though disp_max_unbound_pri indicated this level.
                 */
                if (tp->t_bound_cpu || tp->t_weakbound_cpu)
                        continue;

                /*
                 * We've got some unbound threads on this queue, so turn
                 * the allbound flag off now.
                 */
                allbound = B_FALSE;

                /*
                 * The thread is a candidate for stealing from its run queue. We
                 * don't want to steal threads that became runnable just a
                 * moment ago. This improves CPU affinity for threads that get
                 * preempted for short periods of time and go back on the run
                 * queue.
                 *
                 * We want to let it stay on its run queue if it was only placed
                 * there recently and it was running on the same CPU before that
                 * to preserve its cache investment. For the thread to remain on
                 * its run queue, ALL of the following conditions must be
                 * satisfied:
                 *
                 * - the disp queue should not be the kernel preemption queue
                 * - delayed idle stealing should not be disabled
                 * - nosteal_nsec should be non-zero
                 * - it should run with user priority
                 * - it should be on the run queue of the CPU where it was
                 *   running before being placed on the run queue
                 * - it should be the only thread on the run queue (to prevent
                 *   extra scheduling latency for other threads)
                 * - it should sit on the run queue for less than per-chip
                 *   nosteal interval or global nosteal interval
                 * - in case of CPUs with shared cache it should sit in a run
                 *   queue of a CPU from a different chip
                 *
                 * The checks are arranged so that the ones that are faster are
                 * placed earlier.
                 */
                if (tcp == NULL ||
                    pri >= minclsyspri ||
                    tp->t_cpu != tcp)
                        break;

                /*
                 * Steal immediately if, due to CMT processor architecture
                 * migraiton between cp and tcp would incur no performance
                 * penalty.
                 */
                if (pg_cmt_can_migrate(cp, tcp))
                        break;

                nosteal = nosteal_nsec;
                if (nosteal == 0)
                        break;

                /*
                 * Calculate time spent sitting on run queue
                 */
                now = gethrtime_unscaled();
                rqtime = now - tp->t_waitrq;
                scalehrtime(&rqtime);

                /*
                 * Steal immediately if the time spent on this run queue is more
                 * than allowed nosteal delay.
                 *
                 * Negative rqtime check is needed here to avoid infinite
                 * stealing delays caused by unlikely but not impossible
                 * drifts between CPU times on different CPUs.
                 */
                if (rqtime > nosteal || rqtime < 0)
                        break;

                DTRACE_PROBE4(nosteal, kthread_t *, tp,
                    cpu_t *, tcp, cpu_t *, cp, hrtime_t, rqtime);
                scalehrtime(&now);
                /*
                 * Calculate when this thread becomes stealable
                 */
                now += (nosteal - rqtime);

                /*
                 * Calculate time when some thread becomes stealable
                 */
                if (now < dp->disp_steal)
                        dp->disp_steal = now;
        }

        /*
         * If there were no unbound threads on this queue, find the queue
         * where they are and then return later. The value of
         * disp_max_unbound_pri is not always accurate because it isn't
         * reduced until another idle CPU looks for work.
         */
        if (allbound)
                disp_fix_unbound_pri(dp, pri);

        /*
         * If we reached the end of the queue and found no unbound threads
         * then return NULL so that other CPUs will be considered.  If there
         * are unbound threads but they cannot yet be stolen, then
         * return T_DONTSTEAL and try again later.
         */
        if (tp == NULL) {
                disp_lock_exit_nopreempt(&dp->disp_lock);
                return (allbound ? NULL : T_DONTSTEAL);
        }

        /*
         * Found a runnable, unbound thread, so remove it from queue.
         * dispdeq() requires that we have the thread locked, and we do,
         * by virtue of holding the dispatch queue lock.  dispdeq() will
         * put the thread in transition state, thereby dropping the dispq
         * lock.
         */

#ifdef DEBUG
        {
                int     thread_was_on_queue;

                thread_was_on_queue = dispdeq(tp);      /* drops disp_lock */
                ASSERT(thread_was_on_queue);
        }

#else /* DEBUG */
        (void) dispdeq(tp);                     /* drops disp_lock */
#endif /* DEBUG */

        /*
         * Reset the disp_queue steal time - we do not know what is the smallest
         * value across the queue is.
         */
        dp->disp_steal = 0;

        tp->t_schedflag |= TS_DONT_SWAP;

        /*
         * Setup thread to run on the current CPU.
         */
        tp->t_disp_queue = cp->cpu_disp;

        cp->cpu_dispthread = tp;                /* protected by spl only */
        cp->cpu_dispatch_pri = pri;

        /*
         * There can be a memory synchronization race between disp_getbest()
         * and disp_ratify() vs cpu_resched() where cpu_resched() is trying
         * to preempt the current thread to run the enqueued thread while
         * disp_getbest() and disp_ratify() are changing the current thread
         * to the stolen thread. This may lead to a situation where
         * cpu_resched() tries to preempt the wrong thread and the
         * stolen thread continues to run on the CPU which has been tagged
         * for preemption.
         * Later the clock thread gets enqueued but doesn't get to run on the
         * CPU causing the system to hang.
         *
         * To avoid this, grabbing and dropping the disp_lock (which does
         * a memory barrier) is needed to synchronize the execution of
         * cpu_resched() with disp_getbest() and disp_ratify() and
         * synchronize the memory read and written by cpu_resched(),
         * disp_getbest(), and disp_ratify() with each other.
         *  (see CR#6482861 for more details).
         */
        disp_lock_enter_high(&cp->cpu_disp->disp_lock);
        disp_lock_exit_high(&cp->cpu_disp->disp_lock);

        ASSERT(pri == DISP_PRIO(tp));

        DTRACE_PROBE3(steal, kthread_t *, tp, cpu_t *, tcp, cpu_t *, cp);

        thread_onproc(tp, cp);                  /* set t_state to TS_ONPROC */

        /*
         * Return with spl high so that swtch() won't need to raise it.
         * The disp_lock was dropped by dispdeq().
         */

        return (tp);
}

/*
 * disp_bound_common() - common routine for higher level functions
 *      that check for bound threads under certain conditions.
 *      If 'threadlistsafe' is set then there is no need to acquire
 *      pidlock to stop the thread list from changing (eg, if
 *      disp_bound_* is called with cpus paused).
 */
static int
disp_bound_common(cpu_t *cp, int threadlistsafe, int flag)
{
        int             found = 0;
        kthread_t       *tp;

        ASSERT(flag);

        if (!threadlistsafe)
                mutex_enter(&pidlock);
        tp = curthread;         /* faster than allthreads */
        do {
                if (tp->t_state != TS_FREE) {
                        /*
                         * If an interrupt thread is busy, but the
                         * caller doesn't care (i.e. BOUND_INTR is off),
                         * then just ignore it and continue through.
                         */
                        if ((tp->t_flag & T_INTR_THREAD) &&
                            !(flag & BOUND_INTR))
                                continue;

                        /*
                         * Skip the idle thread for the CPU
                         * we're about to set offline.
                         */
                        if (tp == cp->cpu_idle_thread)
                                continue;

                        /*
                         * Skip the pause thread for the CPU
                         * we're about to set offline.
                         */
                        if (tp == cp->cpu_pause_thread)
                                continue;

                        if ((flag & BOUND_CPU) &&
                            (tp->t_bound_cpu == cp ||
                            tp->t_bind_cpu == cp->cpu_id ||
                            tp->t_weakbound_cpu == cp)) {
                                found = 1;
                                break;
                        }

                        if ((flag & BOUND_PARTITION) &&
                            (tp->t_cpupart == cp->cpu_part)) {
                                found = 1;
                                break;
                        }
                }
        } while ((tp = tp->t_next) != curthread && found == 0);
        if (!threadlistsafe)
                mutex_exit(&pidlock);
        return (found);
}

/*
 * disp_bound_threads - return nonzero if threads are bound to the processor.
 *      Called infrequently.  Keep this simple.
 *      Includes threads that are asleep or stopped but not onproc.
 */
int
disp_bound_threads(cpu_t *cp, int threadlistsafe)
{
        return (disp_bound_common(cp, threadlistsafe, BOUND_CPU));
}

/*
 * disp_bound_anythreads - return nonzero if _any_ threads are bound
 * to the given processor, including interrupt threads.
 */
int
disp_bound_anythreads(cpu_t *cp, int threadlistsafe)
{
        return (disp_bound_common(cp, threadlistsafe, BOUND_CPU | BOUND_INTR));
}

/*
 * disp_bound_partition - return nonzero if threads are bound to the same
 * partition as the processor.
 *      Called infrequently.  Keep this simple.
 *      Includes threads that are asleep or stopped but not onproc.
 */
int
disp_bound_partition(cpu_t *cp, int threadlistsafe)
{
        return (disp_bound_common(cp, threadlistsafe, BOUND_PARTITION));
}

/*
 * disp_cpu_inactive - make a CPU inactive by moving all of its unbound
 * threads to other CPUs.
 */
void
disp_cpu_inactive(cpu_t *cp)
{
        kthread_t       *tp;
        disp_t          *dp = cp->cpu_disp;
        dispq_t         *dq;
        pri_t           pri;
        int             wasonq;

        disp_lock_enter(&dp->disp_lock);
        while ((pri = dp->disp_max_unbound_pri) != -1) {
                dq = &dp->disp_q[pri];
                tp = dq->dq_first;

                /*
                 * Skip over bound threads.
                 */
                while (tp != NULL && tp->t_bound_cpu != NULL) {
                        tp = tp->t_link;
                }

                if (tp == NULL) {
                        /* disp_max_unbound_pri must be inaccurate, so fix it */
                        disp_fix_unbound_pri(dp, pri);
                        continue;
                }

                wasonq = dispdeq(tp);           /* drops disp_lock */
                ASSERT(wasonq);
                ASSERT(tp->t_weakbound_cpu == NULL);

                setbackdq(tp);
                /*
                 * Called from cpu_offline:
                 *
                 * cp has already been removed from the list of active cpus
                 * and tp->t_cpu has been changed so there is no risk of
                 * tp ending up back on cp.
                 *
                 * Called from cpupart_move_cpu:
                 *
                 * The cpu has moved to a new cpupart.  Any threads that
                 * were on it's dispatch queues before the move remain
                 * in the old partition and can't run in the new partition.
                 */
                ASSERT(tp->t_cpu != cp);
                thread_unlock(tp);

                disp_lock_enter(&dp->disp_lock);
        }
        disp_lock_exit(&dp->disp_lock);
}

/*
 * Return a score rating this CPU for running this thread: lower is better.
 *
 * If curthread is looking for a new CPU, then we ignore cpu_dispatch_pri for
 * curcpu (as that's our own priority).
 *
 * If a cpu is the target of an offline request, then try to avoid it.
 *
 * Otherwise we'll use double the effective dispatcher priority for the CPU.
 *
 * We do this so smt_adjust_cpu_score() can increment the score if needed,
 * without ending up over-riding a dispatcher priority.
 */
static pri_t
cpu_score(cpu_t *cp, kthread_t *tp)
{
        pri_t score;

        if (tp == curthread && cp == curthread->t_cpu)
                score = 2 * CPU_IDLE_PRI;
        else if (cp == cpu_inmotion)
                score = SHRT_MAX;
        else
                score = 2 * cp->cpu_dispatch_pri;

        if (2 * cp->cpu_disp->disp_maxrunpri > score)
                score = 2 * cp->cpu_disp->disp_maxrunpri;
        if (2 * cp->cpu_chosen_level > score)
                score = 2 * cp->cpu_chosen_level;

        return (smt_adjust_cpu_score(tp, cp, score));
}

/*
 * disp_lowpri_cpu - find a suitable CPU to run the given thread.
 *
 * We are looking for a CPU with an effective dispatch priority lower than the
 * thread's, so that the thread will run immediately rather than be enqueued.
 * For NUMA locality, we prefer "home" CPUs within the thread's ->t_lpl group.
 * If we don't find an available CPU there, we will expand our search to include
 * wider locality levels. (Note these groups are already divided by CPU
 * partition.)
 *
 * If the thread cannot immediately run on *any* CPU, we'll enqueue ourselves on
 * the best home CPU we found.
 *
 * The hint passed in is used as a starting point so we don't favor CPU 0 or any
 * other CPU.  The caller should pass in the most recently used CPU for the
 * thread; it's of course possible that this CPU isn't in the home lgroup.
 *
 * This function must be called at either high SPL, or with preemption disabled,
 * so that the "hint" CPU cannot be removed from the online CPU list while we
 * are traversing it.
 */
cpu_t *
disp_lowpri_cpu(cpu_t *hint, kthread_t *tp, pri_t tpri)
{
        cpu_t   *bestcpu;
        cpu_t   *besthomecpu;
        cpu_t   *cp, *cpstart;

        klgrpset_t      done;

        lpl_t           *lpl_iter, *lpl_leaf;

        ASSERT(hint != NULL);
        ASSERT(tp->t_lpl->lpl_ncpu > 0);

        bestcpu = besthomecpu = NULL;
        klgrpset_clear(done);

        lpl_iter = tp->t_lpl;

        do {
                pri_t best = SHRT_MAX;
                klgrpset_t cur_set;

                klgrpset_clear(cur_set);

                for (int i = 0; i < lpl_iter->lpl_nrset; i++) {
                        lpl_leaf = lpl_iter->lpl_rset[i];
                        if (klgrpset_ismember(done, lpl_leaf->lpl_lgrpid))
                                continue;

                        klgrpset_add(cur_set, lpl_leaf->lpl_lgrpid);

                        if (hint->cpu_lpl == lpl_leaf)
                                cp = cpstart = hint;
                        else
                                cp = cpstart = lpl_leaf->lpl_cpus;

                        do {
                                pri_t score = cpu_score(cp, tp);

                                if (score < best) {
                                        best = score;
                                        bestcpu = cp;

                                        /* An idle CPU: we're done. */
                                        if (score / 2 == CPU_IDLE_PRI)
                                                goto out;
                                }
                        } while ((cp = cp->cpu_next_lpl) != cpstart);
                }

                if (bestcpu != NULL && tpri > (best / 2))
                        goto out;

                if (besthomecpu == NULL)
                        besthomecpu = bestcpu;

                /*
                 * Add the lgrps we just considered to the "done" set
                 */
                klgrpset_or(done, cur_set);

        } while ((lpl_iter = lpl_iter->lpl_parent) != NULL);

        /*
         * The specified priority isn't high enough to run immediately
         * anywhere, so just return the best CPU from the home lgroup.
         */
        bestcpu = besthomecpu;

out:
        ASSERT((bestcpu->cpu_flags & CPU_QUIESCED) == 0);
        return (bestcpu);
}

/*
 * This routine provides the generic idle cpu function for all processors.
 * If a processor has some specific code to execute when idle (say, to stop
 * the pipeline and save power) then that routine should be defined in the
 * processors specific code (module_xx.c) and the global variable idle_cpu
 * set to that function.
 */
static void
generic_idle_cpu(void)
{
}

/*ARGSUSED*/
static void
generic_enq_thread(cpu_t *cpu, int bound)
{
}

cpu_t *
disp_choose_best_cpu(void)
{
        kthread_t *t = curthread;
        cpu_t *curcpu = CPU;

        ASSERT(t->t_preempt > 0);
        ASSERT(t->t_state == TS_ONPROC);
        ASSERT(t->t_schedflag & TS_VCPU);

        if (smt_should_run(t, curcpu))
                return (curcpu);

        return (disp_lowpri_cpu(curcpu, t, t->t_pri));
}