linux-kernel - Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task

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Date:   Wed, 16 Jun 2021 09:44:46 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched, fair: try to prevent migration thread from
 preempting non-cfs task

On Wed, Jun 16, 2021 at 4:35 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Jun 15, 2021 at 08:15:51PM +0800, Yafang Shao wrote:
> > ---
> >  kernel/sched/fair.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 3248e24a90b0..597c7a940746 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9797,6 +9797,20 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> >                       /* Record that we found at least one task that could run on this_cpu */
> >                       env.flags &= ~LBF_ALL_PINNED;
> >
> > +                     /*
> > +                      * There may be a race between load balance starting migration
> > +                      * thread to pull the cfs running thread and the RT thread
> > +                      * waking up and preempting cfs task before migration threads
> > +                      * which then preempt the RT thread.
> > +                      * We'd better do the last minute check before starting
> > +                      * migration thread to avoid preempting latency-sensitive thread.
> > +                      */
> > +                     if (busiest->curr->sched_class != &fair_sched_class) {
> > +                             raw_spin_unlock_irqrestore(&busiest->lock,
> > +                                                        flags);
>
> This won't apply.
>
> Also, there's still a race window, you've just shrunk it, not fixed it.
> Busiest can reschedule between the mandatory rq unlock and doing the
> stopper wakeup.
>
> An actual fix might be to have the active migration done by a FIFO-1
> task, instead of stopper. The obvious down-side is that that would mean
> spawning yet another per-cpu kthread.
>

The stopper and the migration thread are different threads in the earlier days.
commit 969c79215a35 ("sched: replace migration_thread with cpu_stop")
merged them into one thread.

Regarding the priority of stopper (with highest priority) and
migration (higher than CFS, but lower than RT) , keeping them in one
single thread seems not a good way.


> How much do we care?



-- 
Thanks
Yafang