Re: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

[Hillf Danton <hdanton@sina.com>]

On 31 Mar 2023 17:26:51 +0200 Vincent Guittot <vincent.guittot@linaro.org>
> On Tue, 28 Mar 2023 at 13:06, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > @@ -4832,6 +4834,18 @@ place_entity(struct cfs_rq *cfs_rq, stru
> >                 lag = se->vlag;
> >
> >                 /*
> > +                * For latency sensitive tasks; those that have a shorter than
> > +                * average slice and do not fully consume the slice, transition
> > +                * to EEVDF placement strategy #2.
> > +                */
> > +               if (sched_feat(PLACE_FUDGE) &&
> > +                   cfs_rq->avg_slice > se->slice * cfs_rq->avg_load) {
> > +                       lag += vslice;
> > +                       if (lag > 0)
> > +                               lag = 0;
> 
> By using different lag policies for tasks, doesn't this create
> unfairness between tasks ?
> 
> I wanted to stress this situation with a simple use case but it seems
> that even without changing the slice, there is a fairness problem:
> 
> Task A always run
> Task B loops on : running 1ms then sleeping 1ms
> default nice and latency nice prio bot both
> each task should get around 50% of the time.
> 
> The fairness is ok with tip/sched/core
> but with eevdf, Task B only gets around 30%

Convincing evidence for glitch in wakeup preempt.