Re: [PATCH v2] sched: Clear ttwu_pending after enqueue_task

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

On 2022-11-04 at 10:36:01 +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
> 
> We also tested on unixbench and hackbench, and saw no performance
> change.
> 
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
> ---
> v2:
> Update commit log about other benchmarks.
> Add comment in code.
> Move the code before rq_unlock. This can make ttwu_pending updated a bit
> earlier than v1 so that it can reflect the real condition more timely,
> maybe.
> 
> v1: https://lore.kernel.org/all/20221101073630.2797-1-dtcccc@linux.alibaba.com/
> ---
>  kernel/sched/core.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 87c9cdf37a26..7a04b5565389 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);
>  
> @@ -3759,6 +3752,17 @@ void sched_ttwu_pending(void *arg)
>  		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.
> +	 *
> +	 * It is ok to clear ttwu_pending when another task pending.
> +	 * We will receive IPI after local irq enabled and then enqueue it.
> +	 * Since now nr_running > 0, idle_cpu() will always get correct result.
> +	 */
> +	WRITE_ONCE(rq->ttwu_pending, 0);
>  	rq_unlock_irqrestore(rq, &rf);
>  }
>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>

thanks,
Chenyu