linux-kernel - Re: [External] Re: Subject: [PATCH] sched/fair: prioritize normal task over sched

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Date:   Mon, 14 Mar 2022 19:21:40 -0700
From:   Josh Don <joshdon@...gle.com>
To:     chenying <chenying.kernel@...edance.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Benjamin Segall <bsegall@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        duanxiongchun@...edance.com, zhouchengming@...edance.com,
        songmuchun@...edance.com, zhengqi.arch@...edance.com,
        zhoufeng.zf@...edance.com, ligang.bdlg@...edance.com
Subject: Re: [External] Re: Subject: [PATCH] sched/fair: prioritize normal
 task over sched_idle task with vruntime offset

On Mon, Mar 14, 2022 at 7:05 PM chenying <chenying.kernel@...edance.com> wrote:
>
> 在 2022/3/15 8:30, Josh Don 写道:
> > On Sun, Mar 13, 2022 at 3:07 AM chenying <chenying.kernel@...edance.com> wrote:
> >>
> >> If I set the sched_idle_vruntime_offset to a relatively small value
> >> (e.g. 10 minutes), can this issues be avoided?
> >
> > That's still long enough to cause lockups.
> >
> > Is the issue that you have a large number of sched_idle entities, and
> > the occasional latency sensitive thing that wakes up for a short
> > duration? Have you considered approaching this from the other
> > direction (ie. if we have a latency sensitive thing wake onto a cpu
> > running only sched idle stuff, we could change entity placement to
> > position the latency sensitive thing further left on the timeline,
> > akin to !GENTLE_FAIR_SLEEPERS).
>
> I think this may not guarantee that latency sensitive tasks are always
> to the left of idle tasks. And it may get complicated if a
> latency-sensitive task is woken up onto a cpu which there are already
> multiple latency-sensitive tasks and sched_idle tasks.

If you're waking onto a cpu with lots of latency-sensitive tasks
already, you're already outside the bounds of being able to guarantee
the latency tails you're after (given that the default
idle_min_granularity and idle weight aren't giving you the performance
at the tails that you want right now). It would be helpful to get a
clearer statement as to the problem you're trying to solve.

Perhaps Vincent's recent patch series adding latency support to CFS
("Add latency_nice priority") would be of interest?