// SPDX-License-Identifier: GPL-2.0-only
/*
* kernel/sched/cpupri.c
*
* CPU priority management
*
* Copyright (C) 2007-2008 Novell
*
* Author: Gregory Haskins <[email protected]>
*
* 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.
*/
/*
* 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 = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
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);
}
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