linux-kernel - Re: [PATCH] sched/fair: Allow non-idle task to preempt idle task directly

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Date:   Mon, 4 Apr 2022 12:16:20 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     "zhangsong (J)" <zhangsong34@...wei.com>
Cc:     peterz@...radead.org, mingo@...hat.com, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        zhangsong <zhangsong34@...il.com>
Subject: Re: [PATCH] sched/fair: Allow non-idle task to preempt idle task directly

On Sat, 2 Apr 2022 at 05:33, zhangsong (J) <zhangsong34@...wei.com> wrote:
>
>
> 在 2022/4/1 21:09, Vincent Guittot 写道:
> > On Fri, 1 Apr 2022 at 11:13, zhangsong <zhangsong34@...wei.com> wrote:
> >> From: zhangsong <zhangsong34@...il.com>
> >>
> >> In check_preempt_tick(), the sched idle task may exectue at least
> >> `sysctl_sched_min_granularity` time but any other cfs tasks cannot
> >> preempt it. So it is nessesary to ignore the `sysctl_sched_min_granularity`
> >> resctriction for sched idle task preemption.
> > Could you explain why you need to remove this condition for sched_idle ?
> > sched_idle tasks are already preempted at wakeup by others. And they
> > run while others are runnable only if they has not run for a very long
> > time compares to other. The ideal_runtime of a sched_idle task is
> > capped to 750us min to ensure a minimum progress. But this will happen
> > not more than once  every 256ms and most probably even less often.
>
> Thanks for your reply！I think that sched idle task is treated offline
> task, and sched normal task is treated online task. To reduce latency of
> online tasks and the interference from offline tasks, it is no need to
> let offline task occupy any CPU time.

This doesn't explain why you want to do this. What particular problem
are you facing that needs this change ?
Not sure what you mean by offline task. sched_idle tasks are cfs tasks
with very low weight, preempted at wakeup and which don't preempt
others. Nevertheless, they need to make progress to not starve the
system so there is a need to let sched_idle task to make progress from
time to time so we ensure them 0.3% of cpu bandwidth.

>
> >
> >> Signed-off-by: zhangsong <zhangsong34@...il.com>
> >> ---
> >>   kernel/sched/fair.c | 10 +++++++++-
> >>   1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index d4bd299d6..edcb33440 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -4477,6 +4477,15 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >>          struct sched_entity *se;
> >>          s64 delta;
> >>
> >> +       se = __pick_first_entity(cfs_rq);
> >> +
> >> +       if ((cfs_rq->last && se_is_idle(cfs_rq->last) - se_is_idle(curr) < 0) ||
> >> +           (cfs_rq->next && se_is_idle(cfs_rq->last) - se_is_idle(curr) < 0) ||
> >> +           se_is_idle(se) - se_is_idle(curr) < 0) {
> >> +               resched_curr(rq_of(cfs_rq));
> >> +               return;
> > Why all these complex conditions ?
> > if (se_is_idle(curr)) should be enough
> >
> I think that if se/next/last is not idle and curr is idle, current
> cfs_rq should resched and curr can be preempt by others.
> >> +       }
> >> +
> >>          ideal_runtime = sched_slice(cfs_rq, curr);
> >>          delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
> >>          if (delta_exec > ideal_runtime) {
> >> @@ -4497,7 +4506,6 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >>          if (delta_exec < sysctl_sched_min_granularity)
> >>                  return;
> >>
> >> -       se = __pick_first_entity(cfs_rq);
> >>          delta = curr->vruntime - se->vruntime;
> >>
> >>          if (delta < 0)
> >> --
> >> 2.27.0
> >>
> > .