Re: [PATCH] sched: Clear ttwu_pending after enqueue_task

[Chen Yu <yu.c.chen@intel.com>]

On 2022-11-01 at 11:34:04 +0100, Peter Zijlstra wrote:
> 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@linux.alibaba.com>
> > ---
> >  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);
Just curious why do we put above code inside llist_for_each_entry_safe loop?
My understanding is that once 1 task is queued, select_idle_cpu() would not
treat this rq as idle anymore because nr_running is not 0. But would this bring
overhead to write the rq->ttwu_pending multiple times, do I miss something?

thanks,
Chenyu
>  	}
>  
>  	rq_unlock_irqrestore(rq, &rf);