Date:   Fri, 26 Nov 2021 17:18:17 +0000
From:   Vincent Donnefort <vincent.donnefort@arm.com>
To:     Valentin Schneider <Valentin.Schneider@arm.com>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>, peterz@infradead.org,
        mingo@redhat.com, linux-kernel@vger.kernel.org,
        mgorman@techsingularity.net, dietmar.eggemann@arm.com
Subject: Re: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a
 task
Message-ID: <20211126171817.GA3798214@ubiquitous>
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On Fri, Nov 26, 2021 at 04:49:12PM +0000, Valentin Schneider wrote:
> On 26/11/21 15:40, Vincent Guittot wrote:
> > On Fri, 26 Nov 2021 at 14:32, Valentin Schneider
> > <Valentin.Schneider@arm.com> wrote:
> >>         /*
> >> -        * Allow a per-cpu kthread to stack with the wakee if the
> >> -        * kworker thread and the tasks previous CPUs are the same.
> >> -        * The assumption is that the wakee queued work for the
> >> -        * per-cpu kthread that is now complete and the wakeup is
> >> -        * essentially a sync wakeup. An obvious example of this
> >> +        * Allow a per-cpu kthread to stack with the wakee if the kworker thread
> >> +        * and the tasks previous CPUs are the same.  The assumption is that the
> >> +        * wakee queued work for the per-cpu kthread that is now complete and
> >> +        * the wakeup is essentially a sync wakeup. An obvious example of this
> >>          * pattern is IO completions.
> >> +        *
> >> +        * Ensure the wakeup is issued by the kthread itself, and don't match
> >> +        * against the idle task because that could override the
> >> +        * available_idle_cpu(target) check done higher up.
> >>          */
> >> -       if (is_per_cpu_kthread(current) &&
> >> +       if (is_per_cpu_kthread(current) && !is_idle_task(current) &&
> >
> > still i don't see the need of !is_idle_task(current)
> >
> 
> Admittedly, belts and braces. The existing condition checks rq->nr_running <= 1
> which can lead to coscheduling when the wakeup is issued by the idle task
> (or even if rq->nr_running == 0, you can have rq->ttwu_pending without
> having sent an IPI due to polling). Essentially this overrides the first
> check in sis() that uses idle_cpu(target) (prev == smp_processor_id() ==
> target).
> 
> I couldn't prove such wakeups can happen right now, but if/when they do
> (AIUI it would just take someone to add a wake_up_process() down some
> smp_call_function() callback) then we'll need the above. If you're still
> not convinced by now, I won't push it further.

From a quick experiment, even with the asym_fits_capacity(), I can trigger
the following:

[    0.118855] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
[    0.128214] select_idle_sibling: wakee=rcu_gp:3 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
[    0.137327] select_idle_sibling: wakee=rcu_par_gp:4 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
[    0.147221] select_idle_sibling: wakee=kworker/u16:0:7 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
[    0.156994] select_idle_sibling: wakee=mm_percpu_wq:8 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
[    0.171943] select_idle_sibling: wakee=rcu_sched:10 nr_cpus_allowed=8 current=swapper/0:1 in_task=1

So the in_task() condition doesn't appear to be enough to filter wakeups
while we have the swapper as a current.

> 
> >
> >> +           in_task() &&
> >>             prev == smp_processor_id() &&
> >>             this_rq()->nr_running <= 1) {
> >>                 return prev;
> >>