// SPDX-License-Identifier: GPL-2.0-only /* * kernel/sched/cpupri.c * * CPU priority management * * Copyright (C) 2007-2008 Novell * * Author: Gregory Haskins <ghaskins@novell.com> * * This code tracks the priority of each CPU so that global migration * decisions are easy to calculate. Each CPU can be in a state as follows: * * (INVALID), NORMAL, RT1, ... RT99, HIGHER * * going from the lowest priority to the highest. CPUs in the INVALID state * are not eligible for routing. The system maintains this state with * a 2 dimensional bitmap (the first for priority class, the second for CPUs * in that class). Therefore a typical application without affinity * restrictions can find a suitable CPU with O(1) complexity (e.g. two bit * searches). For tasks with affinity restrictions, the algorithm has a * worst case complexity of O(min(101, nr_domcpus)), though the scenario that * yields the worst case search is fairly contrived. */ #include "sched.h" /* * p->rt_priority p->prio newpri cpupri * * -1 -1 (CPUPRI_INVALID) * * 99 0 (CPUPRI_NORMAL) * * 1 98 98 1 * ... * 49 50 50 49 * 50 49 49 50 * ... * 99 0 0 99 * * 100 100 (CPUPRI_HIGHER) */ static int convert_prio(int prio) { int cpupri; switch (prio) { case CPUPRI_INVALID: cpupri = CPUPRI_INVALID; /* -1 */ break; case 0 ... 98: cpupri = MAX_RT_PRIO-1 - prio; /* 1 ... 99 */ break; case MAX_RT_PRIO-1: cpupri = CPUPRI_NORMAL; /* 0 */ break; case MAX_RT_PRIO: cpupri = CPUPRI_HIGHER; /* 100 */ break; } return cpupri; } static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p, struct cpumask *lowest_mask, int idx) { struct cpupri_vec *vec = &cp->pri_to_cpu[idx]; int skip = 0; if (!atomic_read(&(vec)->count)) skip = 1; /* * When looking at the vector, we need to read the counter, * do a memory barrier, then read the mask. * * Note: This is still all racy, but we can deal with it. * Ideally, we only want to look at masks that are set. * * If a mask is not set, then the only thing wrong is that we * did a little more work than necessary. * * If we read a zero count but the mask is set, because of the * memory barriers, that can only happen when the highest prio * task for a run queue has left the run queue, in which case, * it will be followed by a pull. If the task we are processing * fails to find a proper place to go, that pull request will * pull this task if the run queue is running at a lower * priority. */ smp_rmb(); /* Need to do the rmb for every iteration */ if (skip) return 0; if (cpumask_any_and(&p->cpus_mask, vec->mask) >= nr_cpu_ids) return 0; if (lowest_mask) { cpumask_and(lowest_mask, &p->cpus_mask, vec->mask); cpumask_and(lowest_mask, lowest_mask, cpu_active_mask); /* * We have to ensure that we have at least one bit * still set in the array, since the map could have * been concurrently emptied between the first and * second reads of vec->mask. If we hit this * condition, simply act as though we never hit this * priority level and continue on. */ if (cpumask_empty(lowest_mask)) return 0; } return 1; } int cpupri_find(struct cpupri *cp, struct task_struct *p, struct cpumask *lowest_mask) { return cpupri_find_fitness(cp, p, lowest_mask, NULL); } /** * cpupri_find_fitness - find the best (lowest-pri) CPU in the system * @cp: The cpupri context * @p: The task * @lowest_mask: A mask to fill in with selected CPUs (or NULL) * @fitness_fn: A pointer to a function to do custom checks whether the CPU * fits a specific criteria so that we only return those CPUs. * * Note: This function returns the recommended CPUs as calculated during the * current invocation. By the time the call returns, the CPUs may have in * fact changed priorities any number of times. While not ideal, it is not * an issue of correctness since the normal rebalancer logic will correct * any discrepancies created by racing against the uncertainty of the current * priority configuration. * * Return: (int)bool - CPUs were found */ int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p, struct cpumask *lowest_mask, bool (*fitness_fn)(struct task_struct *p, int cpu)) { int task_pri = convert_prio(p->prio); int idx, cpu; WARN_ON_ONCE(task_pri >= CPUPRI_NR_PRIORITIES); for (idx = 0; idx < task_pri; idx++) { if (!__cpupri_find(cp, p, lowest_mask, idx)) continue; if (!lowest_mask || !fitness_fn) return 1; /* Ensure the capacity of the CPUs fit the task */ for_each_cpu(cpu, lowest_mask) { if (!fitness_fn(p, cpu)) cpumask_clear_cpu(cpu, lowest_mask); } /* * If no CPU at the current priority can fit the task * continue looking */ if (cpumask_empty(lowest_mask)) continue; return 1; } /* * If we failed to find a fitting lowest_mask, kick off a new search * but without taking into account any fitness criteria this time. * * This rule favours honouring priority over fitting the task in the * correct CPU (Capacity Awareness being the only user now). * The idea is that if a higher priority task can run, then it should * run even if this ends up being on unfitting CPU. * * The cost of this trade-off is not entirely clear and will probably * be good for some workloads and bad for others. * * The main idea here is that if some CPUs were over-committed, we try * to spread which is what the scheduler traditionally did. Sys admins * must do proper RT planning to avoid overloading the system if they * really care. */ if (fitness_fn) return cpupri_find(cp, p, lowest_mask); return 0; } /** * cpupri_set - update the CPU priority setting * @cp: The cpupri context * @cpu: The target CPU * @newpri: The priority (INVALID,NORMAL,RT1-RT99,HIGHER) to assign to this CPU * * Note: Assumes cpu_rq(cpu)->lock is locked * * Returns: (void) */ void cpupri_set(struct cpupri *cp, int cpu, int newpri) { int *currpri = &cp->cpu_to_pri[cpu]; int oldpri = *currpri; int do_mb = 0; newpri = convert_prio(newpri); BUG_ON(newpri >= CPUPRI_NR_PRIORITIES); if (newpri == oldpri) return; /* * If the CPU was currently mapped to a different value, we * need to map it to the new value then remove the old value. * Note, we must add the new value first, otherwise we risk the * cpu being missed by the priority loop in cpupri_find. */ if (likely(newpri != CPUPRI_INVALID)) { struct cpupri_vec *vec = &cp->pri_to_cpu[newpri]; cpumask_set_cpu(cpu, vec->mask); /* * When adding a new vector, we update the mask first, * do a write memory barrier, and then update the count, to * make sure the vector is visible when count is set. */ smp_mb__before_atomic(); atomic_inc(&(vec)->count); do_mb = 1; } if (likely(oldpri != CPUPRI_INVALID)) { struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri]; /* * Because the order of modification of the vec->count * is important, we must make sure that the update * of the new prio is seen before we decrement the * old prio. This makes sure that the loop sees * one or the other when we raise the priority of * the run queue. We don't care about when we lower the * priority, as that will trigger an rt pull anyway. * * We only need to do a memory barrier if we updated * the new priority vec. */ if (do_mb) smp_mb__after_atomic(); /* * When removing from the vector, we decrement the counter first * do a memory barrier and then clear the mask. */ atomic_dec(&(vec)->count); smp_mb__after_atomic(); cpumask_clear_cpu(cpu, vec->mask); } *currpri = newpri; } /** * cpupri_init - initialize the cpupri structure * @cp: The cpupri context * * Return: -ENOMEM on memory allocation failure. */ int cpupri_init(struct cpupri *cp) { int i; for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) { struct cpupri_vec *vec = &cp->pri_to_cpu[i]; atomic_set(&vec->count, 0); if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL)) goto cleanup; } cp->cpu_to_pri = kzalloc_objs(int, nr_cpu_ids); if (!cp->cpu_to_pri) goto cleanup; for_each_possible_cpu(i) cp->cpu_to_pri[i] = CPUPRI_INVALID; return 0; cleanup: for (i--; i >= 0; i--) free_cpumask_var(cp->pri_to_cpu[i].mask); return -ENOMEM; } /** * cpupri_cleanup - clean up the cpupri structure * @cp: The cpupri context */ void cpupri_cleanup(struct cpupri *cp) { int i; kfree(cp->cpu_to_pri); for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) free_cpumask_var(cp->pri_to_cpu[i].mask); }
