/*      $NetBSD: kern_synch.c,v 1.366 2023/11/22 13:18:48 riastradh Exp $       */

/*-
 * Copyright (c) 1999, 2000, 2004, 2006, 2007, 2008, 2009, 2019, 2020, 2023
 *    The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe of the Numerical Aerospace Simulation Facility,
 * NASA Ames Research Center, by Charles M. Hannum, Andrew Doran and
 * Daniel Sieger.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*-
 * Copyright (c) 1982, 1986, 1990, 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)kern_synch.c        8.9 (Berkeley) 5/19/95
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_synch.c,v 1.366 2023/11/22 13:18:48 riastradh Exp $");

#include "opt_kstack.h"
#include "opt_ddb.h"
#include "opt_dtrace.h"

#define __MUTEX_PRIVATE

#include <sys/param.h>

#include <sys/atomic.h>
#include <sys/cpu.h>
#include <sys/dtrace_bsd.h>
#include <sys/evcnt.h>
#include <sys/intr.h>
#include <sys/kernel.h>
#include <sys/lockdebug.h>
#include <sys/lwpctl.h>
#include <sys/proc.h>
#include <sys/pserialize.h>
#include <sys/resource.h>
#include <sys/resourcevar.h>
#include <sys/rwlock.h>
#include <sys/sched.h>
#include <sys/sleepq.h>
#include <sys/syncobj.h>
#include <sys/syscall_stats.h>
#include <sys/syslog.h>
#include <sys/systm.h>

#include <uvm/uvm_extern.h>

#include <dev/lockstat.h>

int                             dtrace_vtime_active=0;
dtrace_vtime_switch_func_t      dtrace_vtime_switch_func;

#ifdef DDB
#include <ddb/ddb.h>
#endif

static void     sched_unsleep(struct lwp *, bool);
static void     sched_changepri(struct lwp *, pri_t);
static void     sched_lendpri(struct lwp *, pri_t);

syncobj_t sleep_syncobj = {
        .sobj_name      = "sleep",
        .sobj_flag      = SOBJ_SLEEPQ_SORTED,
        .sobj_boostpri  = PRI_KERNEL,
        .sobj_unsleep   = sleepq_unsleep,
        .sobj_changepri = sleepq_changepri,
        .sobj_lendpri   = sleepq_lendpri,
        .sobj_owner     = syncobj_noowner,
};

syncobj_t sched_syncobj = {
        .sobj_name      = "sched",
        .sobj_flag      = SOBJ_SLEEPQ_SORTED,
        .sobj_boostpri  = PRI_USER,
        .sobj_unsleep   = sched_unsleep,
        .sobj_changepri = sched_changepri,
        .sobj_lendpri   = sched_lendpri,
        .sobj_owner     = syncobj_noowner,
};

syncobj_t kpause_syncobj = {
        .sobj_name      = "kpause",
        .sobj_flag      = SOBJ_SLEEPQ_NULL,
        .sobj_boostpri  = PRI_KERNEL,
        .sobj_unsleep   = sleepq_unsleep,
        .sobj_changepri = sleepq_changepri,
        .sobj_lendpri   = sleepq_lendpri,
        .sobj_owner     = syncobj_noowner,
};

/* "Lightning bolt": once a second sleep address. */
kcondvar_t              lbolt                   __cacheline_aligned;

u_int                   sched_pstats_ticks      __cacheline_aligned;

/* Preemption event counters. */
static struct evcnt     kpreempt_ev_crit        __cacheline_aligned;
static struct evcnt     kpreempt_ev_klock       __cacheline_aligned;
static struct evcnt     kpreempt_ev_immed       __cacheline_aligned;

void
synch_init(void)
{

        cv_init(&lbolt, "lbolt");

        evcnt_attach_dynamic(&kpreempt_ev_crit, EVCNT_TYPE_MISC, NULL,
           "kpreempt", "defer: critical section");
        evcnt_attach_dynamic(&kpreempt_ev_klock, EVCNT_TYPE_MISC, NULL,
           "kpreempt", "defer: kernel_lock");
        evcnt_attach_dynamic(&kpreempt_ev_immed, EVCNT_TYPE_MISC, NULL,
           "kpreempt", "immediate");
}

/*
 * OBSOLETE INTERFACE
 *
 * General sleep call.  Suspends the current LWP until a wakeup is
 * performed on the specified identifier.  The LWP will then be made
 * runnable with the specified priority.  Sleeps at most timo/hz seconds (0
 * means no timeout).  If pri includes PCATCH flag, signals are checked
 * before and after sleeping, else signals are not checked.  Returns 0 if
 * awakened, EWOULDBLOCK if the timeout expires.  If PCATCH is set and a
 * signal needs to be delivered, ERESTART is returned if the current system
 * call should be restarted if possible, and EINTR is returned if the system
 * call should be interrupted by the signal (return EINTR).
 */
int
tsleep(wchan_t ident, pri_t priority, const char *wmesg, int timo)
{
        struct lwp *l = curlwp;
        sleepq_t *sq;
        kmutex_t *mp;
        bool catch_p;
        int nlocks;

        KASSERT((l->l_pflag & LP_INTR) == 0);
        KASSERT(ident != &lbolt);
        //KASSERT(KERNEL_LOCKED_P());

        if (sleepq_dontsleep(l)) {
                (void)sleepq_abort(NULL, 0);
                return 0;
        }

        catch_p = priority & PCATCH;
        sq = sleeptab_lookup(&sleeptab, ident, &mp);
        nlocks = sleepq_enter(sq, l, mp);
        sleepq_enqueue(sq, ident, wmesg, &sleep_syncobj, catch_p);
        return sleepq_block(timo, catch_p, &sleep_syncobj, nlocks);
}

int
mtsleep(wchan_t ident, pri_t priority, const char *wmesg, int timo,
        kmutex_t *mtx)
{
        struct lwp *l = curlwp;
        sleepq_t *sq;
        kmutex_t *mp;
        bool catch_p;
        int error, nlocks;

        KASSERT((l->l_pflag & LP_INTR) == 0);
        KASSERT(ident != &lbolt);

        if (sleepq_dontsleep(l)) {
                (void)sleepq_abort(mtx, (priority & PNORELOCK) != 0);
                return 0;
        }

        catch_p = priority & PCATCH;
        sq = sleeptab_lookup(&sleeptab, ident, &mp);
        nlocks = sleepq_enter(sq, l, mp);
        sleepq_enqueue(sq, ident, wmesg, &sleep_syncobj, catch_p);
        mutex_exit(mtx);
        error = sleepq_block(timo, catch_p, &sleep_syncobj, nlocks);

        if ((priority & PNORELOCK) == 0)
                mutex_enter(mtx);

        return error;
}

/*
 * General sleep call for situations where a wake-up is not expected.
 */
int
kpause(const char *wmesg, bool intr, int timo, kmutex_t *mtx)
{
        struct lwp *l = curlwp;
        int error, nlocks;

        KASSERTMSG(timo != 0 || intr, "wmesg=%s intr=%s timo=%d mtx=%p",
            wmesg, intr ? "true" : "false", timo, mtx);

        if (sleepq_dontsleep(l))
                return sleepq_abort(NULL, 0);

        if (mtx != NULL)
                mutex_exit(mtx);
        nlocks = sleepq_enter(NULL, l, NULL);
        sleepq_enqueue(NULL, l, wmesg, &kpause_syncobj, intr);
        error = sleepq_block(timo, intr, &kpause_syncobj, nlocks);
        if (mtx != NULL)
                mutex_enter(mtx);

        return error;
}

/*
 * OBSOLETE INTERFACE
 *
 * Make all LWPs sleeping on the specified identifier runnable.
 */
void
wakeup(wchan_t ident)
{
        sleepq_t *sq;
        kmutex_t *mp;

        if (__predict_false(cold))
                return;

        sq = sleeptab_lookup(&sleeptab, ident, &mp);
        sleepq_wake(sq, ident, (u_int)-1, mp);
}

/*
 * General yield call.  Puts the current LWP back on its run queue and
 * performs a context switch.
 */
void
yield(void)
{
        struct lwp *l = curlwp;
        int nlocks;

        KERNEL_UNLOCK_ALL(l, &nlocks);
        lwp_lock(l);

        KASSERT(lwp_locked(l, l->l_cpu->ci_schedstate.spc_lwplock));
        KASSERT(l->l_stat == LSONPROC);

        spc_lock(l->l_cpu);
        mi_switch(l);
        KERNEL_LOCK(nlocks, l);
}

/*
 * General preemption call.  Puts the current LWP back on its run queue
 * and performs an involuntary context switch.  Different from yield()
 * in that:
 *
 * - It's counted differently (involuntary vs. voluntary).
 * - Realtime threads go to the head of their runqueue vs. tail for yield().
 */
void
preempt(void)
{
        struct lwp *l = curlwp;
        int nlocks;

        KERNEL_UNLOCK_ALL(l, &nlocks);
        lwp_lock(l);

        KASSERT(lwp_locked(l, l->l_cpu->ci_schedstate.spc_lwplock));
        KASSERT(l->l_stat == LSONPROC);

        spc_lock(l->l_cpu);
        l->l_pflag |= LP_PREEMPTING;
        mi_switch(l);
        KERNEL_LOCK(nlocks, l);
}

/*
 * Return true if the current LWP should yield the processor.  Intended to
 * be used by long-running code in kernel.
 */
inline bool
preempt_needed(void)
{
        lwp_t *l = curlwp;
        int needed;

        KPREEMPT_DISABLE(l);
        needed = l->l_cpu->ci_want_resched;
        KPREEMPT_ENABLE(l);

        return (needed != 0);
}

/*
 * A breathing point for long running code in kernel.
 */
void
preempt_point(void)
{

        if (__predict_false(preempt_needed())) {
                preempt();
        }
}

/*
 * Handle a request made by another agent to preempt the current LWP
 * in-kernel.  Usually called when l_dopreempt may be non-zero.
 *
 * Character addresses for lockstat only.
 */
static char     kpreempt_is_disabled;
static char     kernel_lock_held;
static char     is_softint_lwp;
static char     spl_is_raised;

bool
kpreempt(uintptr_t where)
{
        uintptr_t failed;
        lwp_t *l;
        int s, dop, lsflag;

        l = curlwp;
        failed = 0;
        while ((dop = l->l_dopreempt) != 0) {
                if (l->l_stat != LSONPROC) {
                        /*
                         * About to block (or die), let it happen.
                         * Doesn't really count as "preemption has
                         * been blocked", since we're going to
                         * context switch.
                         */
                        atomic_swap_uint(&l->l_dopreempt, 0);
                        return true;
                }
                KASSERT((l->l_flag & LW_IDLE) == 0);
                if (__predict_false(l->l_nopreempt != 0)) {
                        /* LWP holds preemption disabled, explicitly. */
                        if ((dop & DOPREEMPT_COUNTED) == 0) {
                                kpreempt_ev_crit.ev_count++;
                        }
                        failed = (uintptr_t)&kpreempt_is_disabled;
                        break;
                }
                if (__predict_false((l->l_pflag & LP_INTR) != 0)) {
                        /* Can't preempt soft interrupts yet. */
                        atomic_swap_uint(&l->l_dopreempt, 0);
                        failed = (uintptr_t)&is_softint_lwp;
                        break;
                }
                s = splsched();
                if (__predict_false(l->l_blcnt != 0 ||
                    curcpu()->ci_biglock_wanted != NULL)) {
                        /* Hold or want kernel_lock, code is not MT safe. */
                        splx(s);
                        if ((dop & DOPREEMPT_COUNTED) == 0) {
                                kpreempt_ev_klock.ev_count++;
                        }
                        failed = (uintptr_t)&kernel_lock_held;
                        break;
                }
                if (__predict_false(!cpu_kpreempt_enter(where, s))) {
                        /*
                         * It may be that the IPL is too high.
                         * kpreempt_enter() can schedule an
                         * interrupt to retry later.
                         */
                        splx(s);
                        failed = (uintptr_t)&spl_is_raised;
                        break;
                }
                /* Do it! */
                if (__predict_true((dop & DOPREEMPT_COUNTED) == 0)) {
                        kpreempt_ev_immed.ev_count++;
                }
                lwp_lock(l);
                l->l_pflag |= LP_PREEMPTING;
                spc_lock(l->l_cpu);
                mi_switch(l);
                l->l_nopreempt++;
                splx(s);

                /* Take care of any MD cleanup. */
                cpu_kpreempt_exit(where);
                l->l_nopreempt--;
        }

        if (__predict_true(!failed)) {
                return false;
        }

        /* Record preemption failure for reporting via lockstat. */
        atomic_or_uint(&l->l_dopreempt, DOPREEMPT_COUNTED);
        lsflag = 0;
        LOCKSTAT_ENTER(lsflag);
        if (__predict_false(lsflag)) {
                if (where == 0) {
                        where = (uintptr_t)__builtin_return_address(0);
                }
                /* Preemption is on, might recurse, so make it atomic. */
                if (atomic_cas_ptr_ni((void *)&l->l_pfailaddr, NULL,
                    (void *)where) == NULL) {
                        LOCKSTAT_START_TIMER(lsflag, l->l_pfailtime);
                        l->l_pfaillock = failed;
                }
        }
        LOCKSTAT_EXIT(lsflag);
        return true;
}

/*
 * Return true if preemption is explicitly disabled.
 */
bool
kpreempt_disabled(void)
{
        const lwp_t *l = curlwp;

        return l->l_nopreempt != 0 || l->l_stat == LSZOMB ||
            (l->l_flag & LW_IDLE) != 0 || (l->l_pflag & LP_INTR) != 0 ||
            cpu_kpreempt_disabled();
}

/*
 * Disable kernel preemption.
 */
void
kpreempt_disable(void)
{

        KPREEMPT_DISABLE(curlwp);
}

/*
 * Reenable kernel preemption.
 */
void
kpreempt_enable(void)
{

        KPREEMPT_ENABLE(curlwp);
}

/*
 * Compute the amount of time during which the current lwp was running.
 *
 * - update l_rtime unless it's an idle lwp.
 */

void
updatertime(lwp_t *l, const struct bintime *now)
{
        static bool backwards = false;

        if (__predict_false(l->l_flag & LW_IDLE))
                return;

        if (__predict_false(bintimecmp(now, &l->l_stime, <)) && !backwards) {
                char caller[128];

#ifdef DDB
                db_symstr(caller, sizeof(caller),
                    (db_expr_t)(intptr_t)__builtin_return_address(0),
                    DB_STGY_PROC);
#else
                snprintf(caller, sizeof(caller), "%p",
                    __builtin_return_address(0));
#endif
                backwards = true;
                printf("WARNING: lwp %ld (%s%s%s) flags 0x%x:"
                    " timecounter went backwards"
                    " from (%jd + 0x%016"PRIx64"/2^64) sec"
                    " to (%jd + 0x%016"PRIx64"/2^64) sec"
                    " in %s\n",
                    (long)l->l_lid,
                    l->l_proc->p_comm,
                    l->l_name ? " " : "",
                    l->l_name ? l->l_name : "",
                    l->l_pflag,
                    (intmax_t)l->l_stime.sec, l->l_stime.frac,
                    (intmax_t)now->sec, now->frac,
                    caller);
        }

        /* rtime += now - stime */
        bintime_add(&l->l_rtime, now);
        bintime_sub(&l->l_rtime, &l->l_stime);
}

/*
 * Select next LWP from the current CPU to run..
 */
static inline lwp_t *
nextlwp(struct cpu_info *ci, struct schedstate_percpu *spc)
{
        lwp_t *newl;

        /*
         * Let sched_nextlwp() select the LWP to run the CPU next.
         * If no LWP is runnable, select the idle LWP.
         * 
         * On arrival here LWPs on a run queue are locked by spc_mutex which
         * is currently held.  Idle LWPs are always locked by spc_lwplock,
         * which may or may not be held here.  On exit from this code block,
         * in all cases newl is locked by spc_lwplock.
         */
        newl = sched_nextlwp();
        if (newl != NULL) {
                sched_dequeue(newl);
                KASSERT(lwp_locked(newl, spc->spc_mutex));
                KASSERT(newl->l_cpu == ci);
                newl->l_stat = LSONPROC;
                newl->l_pflag |= LP_RUNNING;
                newl->l_boostpri = PRI_NONE;
                spc->spc_curpriority = lwp_eprio(newl);
                spc->spc_flags &= ~(SPCF_SWITCHCLEAR | SPCF_IDLE);
                lwp_setlock(newl, spc->spc_lwplock);
        } else {
                /*
                 * The idle LWP does not get set to LSONPROC, because
                 * otherwise it screws up the output from top(1) etc.
                 */
                newl = ci->ci_data.cpu_idlelwp;
                newl->l_pflag |= LP_RUNNING;
                spc->spc_curpriority = PRI_IDLE;
                spc->spc_flags = (spc->spc_flags & ~SPCF_SWITCHCLEAR) |
                    SPCF_IDLE;
        }

        /*
         * Only clear want_resched if there are no pending (slow) software
         * interrupts.  We can do this without an atomic, because no new
         * LWPs can appear in the queue due to our hold on spc_mutex, and
         * the update to ci_want_resched will become globally visible before
         * the release of spc_mutex becomes globally visible.
         */
        if (ci->ci_data.cpu_softints == 0)
                ci->ci_want_resched = 0;

        return newl;
}

/*
 * The machine independent parts of context switch.
 *
 * NOTE: l->l_cpu is not changed in this routine, because an LWP never
 * changes its own l_cpu (that would screw up curcpu on many ports and could
 * cause all kinds of other evil stuff).  l_cpu is always changed by some
 * other actor, when it's known the LWP is not running (the LP_RUNNING flag
 * is checked under lock).
 */
void
mi_switch(lwp_t *l)
{
        struct cpu_info *ci;
        struct schedstate_percpu *spc;
        struct lwp *newl;
        kmutex_t *lock;
        int oldspl;
        struct bintime bt;
        bool returning;

        KASSERT(lwp_locked(l, NULL));
        KASSERT(kpreempt_disabled());
        KASSERT(mutex_owned(curcpu()->ci_schedstate.spc_mutex));
        KASSERTMSG(l->l_blcnt == 0, "kernel_lock leaked");

        kstack_check_magic(l);

        binuptime(&bt);

        KASSERTMSG(l == curlwp, "l %p curlwp %p", l, curlwp);
        KASSERT((l->l_pflag & LP_RUNNING) != 0);
        KASSERT(l->l_cpu == curcpu() || l->l_stat == LSRUN);
        ci = curcpu();
        spc = &ci->ci_schedstate;
        returning = false;
        newl = NULL;

        /*
         * If we have been asked to switch to a specific LWP, then there
         * is no need to inspect the run queues.  If a soft interrupt is
         * blocking, then return to the interrupted thread without adjusting
         * VM context or its start time: neither have been changed in order
         * to take the interrupt.
         */
        if (l->l_switchto != NULL) {
                if ((l->l_pflag & LP_INTR) != 0) {
                        returning = true;
                        softint_block(l);
                        if ((l->l_pflag & LP_TIMEINTR) != 0)
                                updatertime(l, &bt);
                }
                newl = l->l_switchto;
                l->l_switchto = NULL;
        }
#ifndef __HAVE_FAST_SOFTINTS
        else if (ci->ci_data.cpu_softints != 0) {
                /* There are pending soft interrupts, so pick one. */
                newl = softint_picklwp();
                newl->l_stat = LSONPROC;
                newl->l_pflag |= LP_RUNNING;
        }
#endif  /* !__HAVE_FAST_SOFTINTS */

        /*
         * If on the CPU and we have gotten this far, then we must yield.
         */
        if (l->l_stat == LSONPROC && l != newl) {
                KASSERT(lwp_locked(l, spc->spc_lwplock));
                KASSERT((l->l_flag & LW_IDLE) == 0);
                l->l_stat = LSRUN;
                lwp_setlock(l, spc->spc_mutex);
                sched_enqueue(l);
                sched_preempted(l);

                /*
                 * Handle migration.  Note that "migrating LWP" may
                 * be reset here, if interrupt/preemption happens
                 * early in idle LWP.
                 */
                if (l->l_target_cpu != NULL && (l->l_pflag & LP_BOUND) == 0) {
                        KASSERT((l->l_pflag & LP_INTR) == 0);
                        spc->spc_migrating = l;
                }
        }

        /* Pick new LWP to run. */
        if (newl == NULL) {
                newl = nextlwp(ci, spc);
        }

        /* Items that must be updated with the CPU locked. */
        if (!returning) {
                /* Count time spent in current system call */
                SYSCALL_TIME_SLEEP(l);

                updatertime(l, &bt);

                /* Update the new LWP's start time. */
                newl->l_stime = bt;

                /*
                 * ci_curlwp changes when a fast soft interrupt occurs.
                 * We use ci_onproc to keep track of which kernel or
                 * user thread is running 'underneath' the software
                 * interrupt.  This is important for time accounting,
                 * itimers and forcing user threads to preempt (aston).
                 */
                ci->ci_onproc = newl;
        }

        /*
         * Preemption related tasks.  Must be done holding spc_mutex.  Clear
         * l_dopreempt without an atomic - it's only ever set non-zero by
         * sched_resched_cpu() which also holds spc_mutex, and only ever
         * cleared by the LWP itself (us) with atomics when not under lock.
         */
        l->l_dopreempt = 0;
        if (__predict_false(l->l_pfailaddr != 0)) {
                LOCKSTAT_FLAG(lsflag);
                LOCKSTAT_ENTER(lsflag);
                LOCKSTAT_STOP_TIMER(lsflag, l->l_pfailtime);
                LOCKSTAT_EVENT_RA(lsflag, l->l_pfaillock, LB_NOPREEMPT|LB_SPIN,
                    1, l->l_pfailtime, l->l_pfailaddr);
                LOCKSTAT_EXIT(lsflag);
                l->l_pfailtime = 0;
                l->l_pfaillock = 0;
                l->l_pfailaddr = 0;
        }

        if (l != newl) {
                struct lwp *prevlwp;

                /* Release all locks, but leave the current LWP locked */
                if (l->l_mutex == spc->spc_mutex) {
                        /*
                         * Drop spc_lwplock, if the current LWP has been moved
                         * to the run queue (it is now locked by spc_mutex).
                         */
                        mutex_spin_exit(spc->spc_lwplock);
                } else {
                        /*
                         * Otherwise, drop the spc_mutex, we are done with the
                         * run queues.
                         */
                        mutex_spin_exit(spc->spc_mutex);
                }

                /* We're down to only one lock, so do debug checks. */
                LOCKDEBUG_BARRIER(l->l_mutex, 1);

                /* Count the context switch. */
                CPU_COUNT(CPU_COUNT_NSWTCH, 1);
                if ((l->l_pflag & LP_PREEMPTING) != 0) {
                        l->l_ru.ru_nivcsw++;
                        l->l_pflag &= ~LP_PREEMPTING;
                } else {
                        l->l_ru.ru_nvcsw++;
                }

                /*
                 * Increase the count of spin-mutexes before the release
                 * of the last lock - we must remain at IPL_SCHED after
                 * releasing the lock.
                 */
                KASSERTMSG(ci->ci_mtx_count == -1,
                    "%s: cpu%u: ci_mtx_count (%d) != -1 "
                    "(block with spin-mutex held)",
                     __func__, cpu_index(ci), ci->ci_mtx_count);
                oldspl = MUTEX_SPIN_OLDSPL(ci);
                ci->ci_mtx_count = -2;

                /* Update status for lwpctl, if present. */
                if (l->l_lwpctl != NULL) {
                        l->l_lwpctl->lc_curcpu = (l->l_stat == LSZOMB ?
                            LWPCTL_CPU_EXITED : LWPCTL_CPU_NONE);
                }

                /*
                 * If curlwp is a soft interrupt LWP, there's nobody on the
                 * other side to unlock - we're returning into an assembly
                 * trampoline.  Unlock now.  This is safe because this is a
                 * kernel LWP and is bound to current CPU: the worst anyone
                 * else will do to it, is to put it back onto this CPU's run
                 * queue (and the CPU is busy here right now!).
                 */
                if (returning) {
                        /* Keep IPL_SCHED after this; MD code will fix up. */
                        l->l_pflag &= ~LP_RUNNING;
                        lwp_unlock(l);
                } else {
                        /* A normal LWP: save old VM context. */
                        pmap_deactivate(l);
                }

                /*
                 * If DTrace has set the active vtime enum to anything
                 * other than INACTIVE (0), then it should have set the
                 * function to call.
                 */
                if (__predict_false(dtrace_vtime_active)) {
                        (*dtrace_vtime_switch_func)(newl);
                }

                /*
                 * We must ensure not to come here from inside a read section.
                 */
                KASSERT(pserialize_not_in_read_section());

                /* Switch to the new LWP.. */
#ifdef MULTIPROCESSOR
                KASSERT(curlwp == ci->ci_curlwp);
#endif
                KASSERTMSG(l == curlwp, "l %p curlwp %p", l, curlwp);
                prevlwp = cpu_switchto(l, newl, returning);
                ci = curcpu();
#ifdef MULTIPROCESSOR
                KASSERT(curlwp == ci->ci_curlwp);
#endif
                KASSERTMSG(l == curlwp, "l %p curlwp %p prevlwp %p",
                    l, curlwp, prevlwp);
                KASSERT(prevlwp != NULL);
                KASSERT(l->l_cpu == ci);
                KASSERT(ci->ci_mtx_count == -2);

                /*
                 * Immediately mark the previous LWP as no longer running
                 * and unlock (to keep lock wait times short as possible).
                 * We'll still be at IPL_SCHED afterwards.  If a zombie,
                 * don't touch after clearing LP_RUNNING as it could be
                 * reaped by another CPU.  Issue a memory barrier to ensure
                 * this.
                 *
                 * atomic_store_release matches atomic_load_acquire in
                 * lwp_free.
                 */
                KASSERT((prevlwp->l_pflag & LP_RUNNING) != 0);
                lock = prevlwp->l_mutex;
                if (__predict_false(prevlwp->l_stat == LSZOMB)) {
                        atomic_store_release(&prevlwp->l_pflag,
                            prevlwp->l_pflag & ~LP_RUNNING);
                } else {
                        prevlwp->l_pflag &= ~LP_RUNNING;
                }
                mutex_spin_exit(lock);

                /*
                 * Switched away - we have new curlwp.
                 * Restore VM context and IPL.
                 */
                pmap_activate(l);
                pcu_switchpoint(l);

                /* Update status for lwpctl, if present. */
                if (l->l_lwpctl != NULL) {
                        l->l_lwpctl->lc_curcpu = (int)cpu_index(ci);
                        l->l_lwpctl->lc_pctr++;
                }

                /*
                 * Normalize the spin mutex count and restore the previous
                 * SPL.  Note that, unless the caller disabled preemption,
                 * we can be preempted at any time after this splx().
                 */
                KASSERT(l->l_cpu == ci);
                KASSERT(ci->ci_mtx_count == -1);
                ci->ci_mtx_count = 0;
                splx(oldspl);
        } else {
                /* Nothing to do - just unlock and return. */
                mutex_spin_exit(spc->spc_mutex);
                l->l_pflag &= ~LP_PREEMPTING;
                lwp_unlock(l);
        }

        KASSERT(l == curlwp);
        KASSERT(l->l_stat == LSONPROC || (l->l_flag & LW_IDLE) != 0); 

        SYSCALL_TIME_WAKEUP(l);
        LOCKDEBUG_BARRIER(NULL, 1);
}

/*
 * setrunnable: change LWP state to be runnable, placing it on the run queue.
 *
 * Call with the process and LWP locked.  Will return with the LWP unlocked.
 */
void
setrunnable(struct lwp *l)
{
        struct proc *p = l->l_proc;
        struct cpu_info *ci;
        kmutex_t *oldlock;

        KASSERT((l->l_flag & LW_IDLE) == 0);
        KASSERT((l->l_flag & LW_DBGSUSPEND) == 0);
        KASSERT(mutex_owned(p->p_lock));
        KASSERT(lwp_locked(l, NULL));
        KASSERT(l->l_mutex != l->l_cpu->ci_schedstate.spc_mutex);

        switch (l->l_stat) {
        case LSSTOP:
                /*
                 * If we're being traced (possibly because someone attached us
                 * while we were stopped), check for a signal from the debugger.
                 */
                if ((p->p_slflag & PSL_TRACED) != 0 && p->p_xsig != 0)
                        signotify(l);
                p->p_nrlwps++;
                break;
        case LSSUSPENDED:
                KASSERT(lwp_locked(l, l->l_cpu->ci_schedstate.spc_lwplock));
                l->l_flag &= ~LW_WSUSPEND;
                p->p_nrlwps++;
                cv_broadcast(&p->p_lwpcv);
                break;
        case LSSLEEP:
                KASSERT(l->l_wchan != NULL);
                break;
        case LSIDL:
                KASSERT(lwp_locked(l, l->l_cpu->ci_schedstate.spc_lwplock));
                break;
        default:
                panic("setrunnable: lwp %p state was %d", l, l->l_stat);
        }

        /*
         * If the LWP was sleeping, start it again.
         */
        if (l->l_wchan != NULL) {
                l->l_stat = LSSLEEP;
                /* lwp_unsleep() will release the lock. */
                lwp_unsleep(l, true);
                return;
        }

        /*
         * If the LWP is still on the CPU, mark it as LSONPROC.  It may be
         * about to call mi_switch(), in which case it will yield.
         */
        if ((l->l_pflag & LP_RUNNING) != 0) {
                l->l_stat = LSONPROC;
                l->l_slptime = 0;
                lwp_unlock(l);
                return;
        }

        /*
         * Look for a CPU to run.
         * Set the LWP runnable.
         */
        ci = sched_takecpu(l);
        l->l_cpu = ci;
        spc_lock(ci);
        oldlock = lwp_setlock(l, l->l_cpu->ci_schedstate.spc_mutex);
        sched_setrunnable(l);
        l->l_stat = LSRUN;
        l->l_slptime = 0;
        sched_enqueue(l);
        sched_resched_lwp(l, true);
        /* SPC & LWP now unlocked. */
        mutex_spin_exit(oldlock);
}

/*
 * suspendsched:
 *
 *      Convert all non-LW_SYSTEM LSSLEEP or LSRUN LWPs to LSSUSPENDED. 
 */
void
suspendsched(void)
{
        CPU_INFO_ITERATOR cii;
        struct cpu_info *ci;
        struct lwp *l;
        struct proc *p;

        /*
         * We do this by process in order not to violate the locking rules.
         */
        mutex_enter(&proc_lock);
        PROCLIST_FOREACH(p, &allproc) {
                mutex_enter(p->p_lock);
                if ((p->p_flag & PK_SYSTEM) != 0) {
                        mutex_exit(p->p_lock);
                        continue;
                }

                if (p->p_stat != SSTOP) {
                        if (p->p_stat != SZOMB && p->p_stat != SDEAD) {
                                p->p_pptr->p_nstopchild++;
                                p->p_waited = 0;
                        }
                        p->p_stat = SSTOP;
                }

                LIST_FOREACH(l, &p->p_lwps, l_sibling) {
                        if (l == curlwp)
                                continue;

                        lwp_lock(l);

                        /*
                         * Set L_WREBOOT so that the LWP will suspend itself
                         * when it tries to return to user mode.  We want to
                         * try and get to get as many LWPs as possible to
                         * the user / kernel boundary, so that they will
                         * release any locks that they hold.
                         */
                        l->l_flag |= (LW_WREBOOT | LW_WSUSPEND);

                        if (l->l_stat == LSSLEEP &&
                            (l->l_flag & LW_SINTR) != 0) {
                                /* setrunnable() will release the lock. */
                                setrunnable(l);
                                continue;
                        }

                        lwp_unlock(l);
                }

                mutex_exit(p->p_lock);
        }
        mutex_exit(&proc_lock);

        /*
         * Kick all CPUs to make them preempt any LWPs running in user mode. 
         * They'll trap into the kernel and suspend themselves in userret(). 
         *
         * Unusually, we don't hold any other scheduler object locked, which
         * would keep preemption off for sched_resched_cpu(), so disable it
         * explicitly.
         */
        kpreempt_disable();
        for (CPU_INFO_FOREACH(cii, ci)) {
                spc_lock(ci);
                sched_resched_cpu(ci, PRI_KERNEL, true);
                /* spc now unlocked */
        }
        kpreempt_enable();
}

/*
 * sched_unsleep:
 *
 *      The is called when the LWP has not been awoken normally but instead
 *      interrupted: for example, if the sleep timed out.  Because of this,
 *      it's not a valid action for running or idle LWPs.
 */
static void
sched_unsleep(struct lwp *l, bool cleanup)
{

        lwp_unlock(l);
        panic("sched_unsleep");
}

static void
sched_changepri(struct lwp *l, pri_t pri)
{
        struct schedstate_percpu *spc;
        struct cpu_info *ci;

        KASSERT(lwp_locked(l, NULL));

        ci = l->l_cpu;
        spc = &ci->ci_schedstate;

        if (l->l_stat == LSRUN) {
                KASSERT(lwp_locked(l, spc->spc_mutex));
                sched_dequeue(l);
                l->l_priority = pri;
                sched_enqueue(l);
                sched_resched_lwp(l, false);
        } else if (l->l_stat == LSONPROC && l->l_class != SCHED_OTHER) {
                /* On priority drop, only evict realtime LWPs. */
                KASSERT(lwp_locked(l, spc->spc_lwplock));
                l->l_priority = pri;
                spc_lock(ci);
                sched_resched_cpu(ci, spc->spc_maxpriority, true);
                /* spc now unlocked */
        } else {
                l->l_priority = pri;
        }
}

static void
sched_lendpri(struct lwp *l, pri_t pri)
{
        struct schedstate_percpu *spc;
        struct cpu_info *ci;

        KASSERT(lwp_locked(l, NULL));

        ci = l->l_cpu;
        spc = &ci->ci_schedstate;

        if (l->l_stat == LSRUN) {
                KASSERT(lwp_locked(l, spc->spc_mutex));
                sched_dequeue(l);
                l->l_inheritedprio = pri;
                l->l_auxprio = MAX(l->l_inheritedprio, l->l_protectprio);
                sched_enqueue(l);
                sched_resched_lwp(l, false);
        } else if (l->l_stat == LSONPROC && l->l_class != SCHED_OTHER) {
                /* On priority drop, only evict realtime LWPs. */
                KASSERT(lwp_locked(l, spc->spc_lwplock));
                l->l_inheritedprio = pri;
                l->l_auxprio = MAX(l->l_inheritedprio, l->l_protectprio);
                spc_lock(ci);
                sched_resched_cpu(ci, spc->spc_maxpriority, true);
                /* spc now unlocked */
        } else {
                l->l_inheritedprio = pri;
                l->l_auxprio = MAX(l->l_inheritedprio, l->l_protectprio);
        }
}

struct lwp *
syncobj_noowner(wchan_t wchan)
{

        return NULL;
}

/* Decay 95% of proc::p_pctcpu in 60 seconds, ccpu = exp(-1/20) */
const fixpt_t ccpu = 0.95122942450071400909 * FSCALE;

/*
 * Constants for averages over 1, 5 and 15 minutes when sampling at
 * 5 second intervals.
 */
static const fixpt_t cexp[ ] = {
        0.9200444146293232 * FSCALE,    /* exp(-1/12) */
        0.9834714538216174 * FSCALE,    /* exp(-1/60) */
        0.9944598480048967 * FSCALE,    /* exp(-1/180) */
};

/*
 * sched_pstats:
 *
 * => Update process statistics and check CPU resource allocation.
 * => Call scheduler-specific hook to eventually adjust LWP priorities.
 * => Compute load average of a quantity on 1, 5 and 15 minute intervals.
 */
void
sched_pstats(void)
{
        struct loadavg *avg = &averunnable;
        const int clkhz = (stathz != 0 ? stathz : hz);
        static bool backwardslwp = false;
        static bool backwardsproc = false;
        static u_int lavg_count = 0;
        struct proc *p;
        int nrun;

        sched_pstats_ticks++;
        if (++lavg_count >= 5) {
                lavg_count = 0;
                nrun = 0;
        }
        mutex_enter(&proc_lock);
        PROCLIST_FOREACH(p, &allproc) {
                struct lwp *l;
                struct rlimit *rlim;
                time_t runtm;
                int sig;

                /* Increment sleep time (if sleeping), ignore overflow. */
                mutex_enter(p->p_lock);
                runtm = p->p_rtime.sec;
                LIST_FOREACH(l, &p->p_lwps, l_sibling) {
                        fixpt_t lpctcpu;
                        u_int lcpticks;

                        if (__predict_false((l->l_flag & LW_IDLE) != 0))
                                continue;
                        lwp_lock(l);
                        if (__predict_false(l->l_rtime.sec < 0) &&
                            !backwardslwp) {
                                backwardslwp = true;
                                printf("WARNING: lwp %ld (%s%s%s): "
                                    "negative runtime: "
                                    "(%jd + 0x%016"PRIx64"/2^64) sec\n",
                                    (long)l->l_lid,
                                    l->l_proc->p_comm,
                                    l->l_name ? " " : "",
                                    l->l_name ? l->l_name : "",
                                    (intmax_t)l->l_rtime.sec,
                                    l->l_rtime.frac);
                        }
                        runtm += l->l_rtime.sec;
                        l->l_swtime++;
                        sched_lwp_stats(l);

                        /* For load average calculation. */
                        if (__predict_false(lavg_count == 0) &&
                            (l->l_flag & (LW_SINTR | LW_SYSTEM)) == 0) {
                                switch (l->l_stat) {
                                case LSSLEEP:
                                        if (l->l_slptime > 1) {
                                                break;
                                        }
                                        /* FALLTHROUGH */
                                case LSRUN:
                                case LSONPROC:
                                case LSIDL:
                                        nrun++;
                                }
                        }
                        lwp_unlock(l);

                        l->l_pctcpu = (l->l_pctcpu * ccpu) >> FSHIFT;
                        if (l->l_slptime != 0)
                                continue;

                        lpctcpu = l->l_pctcpu;
                        lcpticks = atomic_swap_uint(&l->l_cpticks, 0);
                        lpctcpu += ((FSCALE - ccpu) *
                            (lcpticks * FSCALE / clkhz)) >> FSHIFT;
                        l->l_pctcpu = lpctcpu;
                }
                /* Calculating p_pctcpu only for ps(1) */
                p->p_pctcpu = (p->p_pctcpu * ccpu) >> FSHIFT;

                if (__predict_false(runtm < 0)) {
                        if (!backwardsproc) {
                                backwardsproc = true;
                                printf("WARNING: pid %ld (%s): "
                                    "negative runtime; "
                                    "monotonic clock has gone backwards\n",
                                    (long)p->p_pid, p->p_comm);
                        }
                        mutex_exit(p->p_lock);
                        continue;
                }

                /*
                 * Check if the process exceeds its CPU resource allocation.
                 * If over the hard limit, kill it with SIGKILL.
                 * If over the soft limit, send SIGXCPU and raise
                 * the soft limit a little.
                 */
                rlim = &p->p_rlimit[RLIMIT_CPU];
                sig = 0;
                if (__predict_false(runtm >= rlim->rlim_cur)) {
                        if (runtm >= rlim->rlim_max) {
                                sig = SIGKILL;
                                log(LOG_NOTICE,
                                    "pid %d, command %s, is killed: %s\n",
                                    p->p_pid, p->p_comm, "exceeded RLIMIT_CPU");
                                uprintf("pid %d, command %s, is killed: %s\n",
                                    p->p_pid, p->p_comm, "exceeded RLIMIT_CPU");
                        } else {
                                sig = SIGXCPU;
                                if (rlim->rlim_cur < rlim->rlim_max)
                                        rlim->rlim_cur += 5;
                        }
                }
                mutex_exit(p->p_lock);
                if (__predict_false(sig)) {
                        KASSERT((p->p_flag & PK_SYSTEM) == 0);
                        psignal(p, sig);
                }
        }

        /* Load average calculation. */
        if (__predict_false(lavg_count == 0)) {
                int i;
                CTASSERT(__arraycount(cexp) == __arraycount(avg->ldavg));
                for (i = 0; i < __arraycount(cexp); i++) {
                        avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
                            nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
                }
        }

        /* Lightning bolt. */
        cv_broadcast(&lbolt);

        mutex_exit(&proc_lock);
}