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

[Chen Yu]

Hello Honglei,

On 2024-09-27 at 09:54:28 +0800, Honglei Wang wrote:
> 
> 
> 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 for taking a look.

It could be possible that Peter wanted the short tasks to preempt other quickly.
If I understand correctly, when we say preemption, we usually consider two
tasks which are in the same cfs_rq(level). For example, check_preempt_wakeup_fair()
iterates the hierarchy from down-up until the current task and the wakee are in the
same level via find_matching_se(&se, &pse), then check if the wakee can preempt the
current. This should be consistent with the tick preemption in update_curr(). And
whether the short task should preempt the current is checked by
update_curr() -> did_preempt_short(), rather than checking the cfs_rq->nr_running/nr_h_running
I suppose.

Thanks,
Chenyu