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 <20190729152957.GA21958@cmpxchg.org>
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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Mon, 29 Jul 2019 09:27:20 -0700
Message-ID: <CAJuCfpFsPn9MeS-pWcrAj1ZcVZXJNn94Vina6se2MgApgEFxHQ@mail.gmail.com>
Subject: Re: [PATCH] psi: get poll_work to run when calling poll syscall next time
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Jason Xing <kerneljasonxing@linux.alibaba.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dennis Zhou <dennis@kernel.org>, axboe@kernel.dk,
        lizefan@huawei.com, Tejun Heo <tj@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jul 29, 2019 at 8:30 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> Hi Jason,
>
> On Tue, Jul 23, 2019 at 02:45:39PM +0800, Jason Xing wrote:
> > Only when calling the poll syscall the first time can user
> > receive POLLPRI correctly. After that, user always fails to
> > acquire the event signal.
> >
> > Reproduce case:
> > 1. Get the monitor code in Documentation/accounting/psi.txt
> > 2. Run it, and wait for the event triggered.
> > 3. Kill and restart the process.
> >
> > If the user doesn't kill the monitor process, it seems the
> > poll_work works fine. After killing and restarting the monitor,
> > the poll_work in kernel will never run again due to the wrong
> > value of poll_scheduled. Therefore, we should reset the value
> > as group_init() does after the last trigger is destroyed.
> >
> > Signed-off-by: Jason Xing <kerneljasonxing@linux.alibaba.com>
>
> Good catch, and the fix makes sense to me. However, it was a bit hard
> to understand how the problem occurs:
>
> > ---
> >  kernel/sched/psi.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > index 7acc632..66f4385 100644
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -1133,6 +1133,12 @@ static void psi_trigger_destroy(struct kref *ref)
> >       if (kworker_to_destroy) {
> >               kthread_cancel_delayed_work_sync(&group->poll_work);
> >               kthread_destroy_worker(kworker_to_destroy);
> > +             /*
> > +              * The poll_work should have the chance to be put into the
> > +              * kthread queue when calling poll syscall next time. So
> > +              * reset poll_scheduled to zero as group_init() does
> > +              */
> > +             atomic_set(&group->poll_scheduled, 0);
>
> The question is why we can end up with poll_scheduled = 1 but the work
> not running (which would reset it to 0). And the answer is because the
> scheduling side sees group->poll_kworker under RCU protection and then
> schedules it, but here we cancel the work and destroy the worker. The
> cancel needs to pair with resetting the poll_scheduled flag:
>
>         if (kworker_to_destroy) {
>                 /*
>                  * After the RCU grace period has expired, the worker
>                  * can no longer be found through group->poll_kworker.
>                  * But it might have been already scheduled before
>                  * that - deschedule it cleanly before destroying it.
>                  */
>                 kthread_cancel_delayed_work_sync(&group->poll_work);
>                 atomic_set(&group->poll_scheduled, 0);
>
>                 kthread_destroy_worker(kworker_to_destroy);
>         }
>
> With that change, please add:
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Thanks!

The changes makes sense to me as well. Thanks!

Reviewed-by: Suren Baghdasaryan <surenb@google.com>