From: Vincent Guittot <vincent.guittot@linaro.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Yafang Shao <laoar.shao@gmail.com>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task
Date: Tue, 15 Jun 2021 17:45:26 +0200 [thread overview]
Message-ID: <CAKfTPtDj6E00o4ZFDJ+kJKqy8J3oLm-mVSajUnHpufFCRiX_8g@mail.gmail.com> (raw)
In-Reply-To: <87mtrrb2jw.mognet@arm.com>
On Tue, 15 Jun 2021 at 16:55, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
>
> Hi,
>
> On 15/06/21 20:15, Yafang Shao wrote:
>
> > - Prev version
> > https://lore.kernel.org/lkml/CAKfTPtBd349eyDhA5ThCAHFd83cGMQKb_LDxD4QvyP-cJOBjqA@mail.gmail.com/
> >
> > - Similar discussion
> > https://lore.kernel.org/lkml/CAKfTPtBygNcVewbb0GQOP5xxO96am3YeTZNP5dK9BxKHJJAL-g@mail.gmail.com/
>
> I knew that sounded familiar :-)
>
> > ---
> > 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.
> > + */
>
> This can be summarized as in the below, no?
>
> /*
> * Don't cause a higher-than-CFS task to be preempted by
> * the CPU stopper.
> */
IMO, it's worth keeping the explanation that we are there because:
- a CFS task that was running during the 1st step : if
(busiest->nr_running > 1) { ...
so we didn't pull the task
- but it has been preempted while lb was deciding if it needs an
active load balance
so maybe something like:
/*
* Don't kick the active_load_balance_cpu_stop,
* if the CFS task has been preempted by higher
* priority task in the meantime.
*/
>
> > + if (busiest->curr->sched_class != &fair_sched_class) {
> > + raw_spin_unlock_irqrestore(&busiest->lock,
> > + flags);
> > + goto out;
>
> Since you goto out this could be moved before the
>
> env.flags &= ~LBF_ALL_PINNED;
>
> above (it only has an impact if you'd goto out_balanced).
Good point. My comment to move this test after env.flags &=
~LBF_ALL_PINNED; was valid only with goto out_one_pinned
>
> > + }
> > +
>
> Other than the above, this looks OK to me.
>
> Back then I had argued that having a >CFS task and holding the remote rq
> lock could let us invoke detach_one_task() locally (rather than on the
> stopper thread), but realistically if we got to this !ld_moved condition
> then the chances of us actually pulling something here are very slim (we'd
> depend on enqueues happening between ~detach_tasks() and here).
>
> > /*
> > * ->active_balance synchronizes accesses to
> > * ->active_balance_work. Once set, it's cleared
> > --
> > 2.17.1
next prev parent reply other threads:[~2021-06-15 15:45 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-15 12:15 [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task Yafang Shao
2021-06-15 14:55 ` Valentin Schneider
2021-06-15 15:45 ` Vincent Guittot [this message]
2021-06-15 16:39 ` Valentin Schneider
2021-06-15 20:35 ` Peter Zijlstra
2021-06-16 1:44 ` Yafang Shao
2021-06-16 7:15 ` Peter Zijlstra
2021-06-16 7:29 ` Vincent Guittot
2021-06-16 8:29 ` Peter Zijlstra
2021-06-16 9:49 ` Yafang Shao
2021-06-16 9:45 ` Yafang Shao
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