linux-kernel - Re: [RFC PATCH v3 2/3] sched: Avoid placing RT threads on cores handling long softirqs

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Date:   Wed, 5 Oct 2022 16:31:25 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     John Stultz <jstultz@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Connor O'Brien <connoro@...gle.com>,
        John Dias <joaodias@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        John Kacur <jkacur@...hat.com>,
        Chris Redpath <chris.redpath@....com>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        "J . Avila" <elavila@...gle.com>
Subject: Re: [RFC PATCH v3 2/3] sched: Avoid placing RT threads on cores
 handling long softirqs

On 10/03/22 09:55, John Stultz wrote:

[...]

> > >               int target = find_lowest_rq(p);
> > > @@ -1656,11 +1699,14 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
> > >                       goto out_unlock;
> > >
> > >               /*
> > > -              * Don't bother moving it if the destination CPU is
> > > +              * If cpu is non-preemptible, prefer remote cpu
> > > +              * even if it's running a higher-prio task.
> > > +              * Otherwise: Don't bother moving it if the destination CPU is
> > >                * not running a lower priority task.
> > >                */
> > >               if (target != -1 &&
> > > -                 p->prio < cpu_rq(target)->rt.highest_prio.curr)
> > > +                 (may_not_preempt ||
> > > +                  p->prio < cpu_rq(target)->rt.highest_prio.curr))
> > >                       cpu = target;
> >
> > I'm not sure this makes sense. You assume a higher priority task will cause
> > less delay than softirqs. Which I think is an optimistic assumption?
> >
> > I think we should just mimic the same fallback behavior when we fail to find
> > a CPU that fits the capacity requirement. Keeps things more consistent IMO.
> 
> This sounds reasonable.  I do fret that long-running rt tasks are less
> common then the long running softirqs, so this may have an impact to
> the effectiveness of the patch, but I also suspect it's even more rare
> to have all the other cpus busy with rt tasks, so its probably very
> unlikely.

Yes. I think it is a hard problem to hit as all these other RT tasks must be
higher priority. So if this ever happens, then one should question if the
priority is set correctly for the audio threads first. Or why there are so many
higher priority tasks running for so long.


Thanks!

--
Qais Yousef