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sched/core: Optimize ttwu() spinning on p->on_cpu

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Both Rik and Mel reported seeing ttwu() spend significant time on:

  smp_cond_load_acquire(&p->on_cpu, !VAL);

Attempt to avoid this by queueing the wakeup on the CPU that owns the
p->on_cpu value. This will then allow the ttwu() to complete without
further waiting.

Since we run schedule() with interrupts disabled, the IPI is
guaranteed to happen after p->on_cpu is cleared, this is what makes it
safe to queue early.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Jirka Hladky <jhladky@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: valentin.schneider@arm.com
Cc: Hillf Danton <hdanton@sina.com>
Cc: Rik van Riel <riel@surriel.com>
Link: https://lore.kernel.org/r/20200524202956.27665-2-mgorman@techsingularity.net
This commit is contained in:
Peter Zijlstra 2020-05-24 21:29:55 +01:00 committed by Ingo Molnar
parent d505b8af58
commit c6e7bd7afa
1 changed files with 31 additions and 21 deletions
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52
kernel/sched/core.c
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@ -2312,7 +2312,7 @@ static void wake_csd_func(void *info)
sched_ttwu_pending();
}
static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
static void __ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
{
struct rq *rq = cpu_rq(cpu);
@ -2354,6 +2354,17 @@ bool cpus_share_cache(int this_cpu, int that_cpu)
{
return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu);
}
static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
{
if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
sched_clock_cpu(cpu); /* Sync clocks across CPUs */
__ttwu_queue_remote(p, cpu, wake_flags);
return true;
}
return false;
}
#endif /* CONFIG_SMP */
static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
@ -2362,11 +2373,8 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
struct rq_flags rf;
#if defined(CONFIG_SMP)
if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
sched_clock_cpu(cpu); /* Sync clocks across CPUs */
ttwu_queue_remote(p, cpu, wake_flags);
if (ttwu_queue_remote(p, cpu, wake_flags))
return;
}
#endif
rq_lock(rq, &rf);
@ -2548,7 +2556,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
if (p->on_rq && ttwu_remote(p, wake_flags))
goto unlock;
if (p->in_iowait) {
delayacct_blkio_end(p);
atomic_dec(&task_rq(p)->nr_iowait);
}
#ifdef CONFIG_SMP
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
/*
* Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be
* possible to, falsely, observe p->on_cpu == 0.
@ -2570,6 +2586,16 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
*/
smp_rmb();
/*
* If the owning (remote) CPU is still in the middle of schedule() with
* this task as prev, considering queueing p on the remote CPUs wake_list
* which potentially sends an IPI instead of spinning on p->on_cpu to
* let the waker make forward progress. This is safe because IRQs are
* disabled and the IPI will deliver after on_cpu is cleared.
*/
if (READ_ONCE(p->on_cpu) && ttwu_queue_remote(p, cpu, wake_flags))
goto unlock;
/*
* If the owning (remote) CPU is still in the middle of schedule() with
* this task as prev, wait until its done referencing the task.
@ -2581,28 +2607,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
*/
smp_cond_load_acquire(&p->on_cpu, !VAL);
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
if (p->in_iowait) {
delayacct_blkio_end(p);
atomic_dec(&task_rq(p)->nr_iowait);
}
cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
if (task_cpu(p) != cpu) {
wake_flags |= WF_MIGRATED;
psi_ttwu_dequeue(p);
set_task_cpu(p, cpu);
}
#else /* CONFIG_SMP */
if (p->in_iowait) {
delayacct_blkio_end(p);
atomic_dec(&task_rq(p)->nr_iowait);
}
#endif /* CONFIG_SMP */
ttwu_queue(p, cpu, wake_flags);