Re: [PATCH v2] sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr_running

[Honglei Wang]

On 2024/9/25 16:54, Chen Yu wrote:
> Commit 85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")
> introduced a mechanism that a wakee with shorter slice could preempt
> the current running task. It also lower the bar for the current task
> to be preempted, by checking the rq->nr_running instead of cfs_rq->nr_running
> when the current task has ran out of time slice. But there is a scenario
> that is problematic. Say, if there is 1 cfs task and 1 rt task, before
> 85e511df3cec, update_deadline() will not trigger a reschedule, and after
> 85e511df3cec, since rq->nr_running is 2 and resched is true, a resched_curr()
> would happen.
>
> Some workloads (like the hackbench reported by lkp) do not like
> over-scheduling. We can see that the preemption rate has been
> increased by 2.2%:
>
> 1.654e+08            +2.2%   1.69e+08        hackbench.time.involuntary_context_switches
>
> Restore its previous check criterion.
>
> Fixes: 85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202409231416.9403c2e9-oliver.sang@xxxxxxxxx
> Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> ---
> v1->v2:
>      Check cfs_rq->nr_running instead of rq->nr_running(K Prateek Nayak)
> ---
>   kernel/sched/fair.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 225b31aaee55..53a351b18740 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1247,7 +1247,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
>   
>   	account_cfs_rq_runtime(cfs_rq, delta_exec);
>   
> -	if (rq->nr_running == 1)
> +	if (cfs_rq->nr_running == 1)
>   		return;
>  
Hi Yu,

I'm wondering if commit 85e511df3cec wants to give more chances to do 
resched just in case there are 'short slices' tasks ready in the other 
cfs hierarchy. Does something like rq->cfs->nr_running == 1 make more 
sense? But maybe it helps less than 'cfs_rq->nr_running == 1' in this 
hackbench case.


Thanks,
Honglei

>   	if (resched || did_preempt_short(cfs_rq, curr)) {