Re: [PATCH] sched: Clear ttwu_pending after enqueue_task

[Peter Zijlstra]

On Tue, Nov 01, 2022 at 03:36:30PM +0800, Tianchen Ding wrote:
> We found a long tail latency in schbench whem m*t is close to nr_cpus.
> (e.g., "schbench -m 2 -t 16" on a machine with 32 cpus.)
> 
> This is because when the wakee cpu is idle, rq->ttwu_pending is cleared
> too early, and idle_cpu() will return true until the wakee task enqueued.
> This will mislead the waker when selecting idle cpu, and wake multiple
> worker threads on the same wakee cpu. This situation is enlarged by
> commit f3dd3f674555 ("sched: Remove the limitation of WF_ON_CPU on
> wakelist if wakee cpu is idle") because it tends to use wakelist.
> 
> Here is the result of "schbench -m 2 -t 16" on a VM with 32vcpu
> (Intel(R) Xeon(R) Platinum 8369B).
> 
> Latency percentiles (usec):
>                 base      base+revert_f3dd3f674555   base+this_patch
> 50.0000th:         9                            13                 9
> 75.0000th:        12                            19                12
> 90.0000th:        15                            22                15
> 95.0000th:        18                            24                17
> *99.0000th:       27                            31                24
> 99.5000th:      3364                            33                27
> 99.9000th:     12560                            36                30

Nice; but have you also ran other benchmarks and confirmed it doesn't
negatively affect those?

If so; mentioning that is very helpful. If not; best go do so :-)

> Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 87c9cdf37a26..b07de1753be5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3739,13 +3739,6 @@ void sched_ttwu_pending(void *arg)
>  	if (!llist)
>  		return;
>  
> -	/*
> -	 * rq::ttwu_pending racy indication of out-standing wakeups.
> -	 * Races such that false-negatives are possible, since they
> -	 * are shorter lived that false-positives would be.
> -	 */
> -	WRITE_ONCE(rq->ttwu_pending, 0);
> -
>  	rq_lock_irqsave(rq, &rf);
>  	update_rq_clock(rq);
>  

Could you try the below instead? Also note the comment; since you did
the work to figure out why -- best record that for posterity.

@@ -3737,6 +3730,13 @@ void sched_ttwu_pending(void *arg)
 			set_task_cpu(p, cpu_of(rq));
 
 		ttwu_do_activate(rq, p, p->sched_remote_wakeup ? WF_MIGRATED : 0, &rf);
+		/*
+		 * Must be after enqueueing at least once task such that
+		 * idle_cpu() does not observe a false-negative -- if it does,
+		 * it is possible for select_idle_siblings() to stack a number
+		 * of tasks on this CPU during that window.
+		 */
+		WRITE_ONCE(rq->ttwu_pending, 0);
 	}
 
 	rq_unlock_irqrestore(rq, &rf);