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From:   Josh Don <joshdon@google.com>
Date:   Tue, 1 Nov 2022 16:39:37 -0700
Message-ID: <CABk29Nsnx=PfeLMEsD0qsnh5-QTHLT4xVB3HmBgGAqqmWnkmvg@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: favor non-idle group in tick preemption
To:     Chuyi Zhou <zhouchuyi@bytedance.com>
Cc:     peterz@infradead.org, juri.lelli@redhat.com, mingo@redhat.com,
        vincent.guittot@linaro.org, linux-kernel@vger.kernel.org,
        Abel Wu <wuyun.abel@bytedance.com>
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> > Some weirdness about this change though, is that if there is a
> > non-idle current entity, and the two next entities on the cfs_rq are
> > idle and non-idle respectively, we'll now take longer to preempt the
> > on-cpu non-idle entity, because the non-idle entity on the cfs_rq is
> > 'hidden' by the idle 'first' entity. Wakeup preemption is different
> > because we're always directly comparing the current entity with the
> > newly woken entity.
> >
> You are right, this can happen with high probability.
> This patch just compared the curr with the first entity in
> the tick, and it seems hard to consider all the other entity
> in cfs_rq.
>
> So, what specific negative effects this situation would cause?
> For example, the "hidden" non-idle entity's latency will be worse
> than before?

As Abel points out in his email, it can push out the time it'll take
to switch to the other non-idle entity. The change might boost some
benchmarks numbers, but I don't think it is conclusive enough to say
it is a generically beneficial improvement that should be integrated.

By the way, I'm curious if you modified any of the sched_idle_cpu()
and related load balancing around idle entities given that you've made
it so that idle entities can have arbitrary weight (since, as I
described in my prior email, this can otherwise cause issues there).