linux-kernel - Re: [PATCH v2 1/1] sched: watchdog: Touch kernel watchdog with sched count

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Message-ID: <CAOBoifg5wGYb_XvZiRysf0iH=binaiVZE2ds=cU=6Y+KhbQJAA@mail.gmail.com>
Date:   Wed, 21 Oct 2020 16:48:38 -0700
From:   Xi Wang <xii@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Don <joshdon@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2 1/1] sched: watchdog: Touch kernel watchdog with sched count

On Wed, Oct 21, 2020 at 3:13 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Oct 20, 2020 at 01:57:04PM -0700, Xi Wang wrote:
>
> > +     if (watchdog_touch_with_sched) {
> > +             /* Trigger reschedule for the next round */
> > +             set_tsk_need_resched(current);
> > +             set_preempt_need_resched();
>
> Blergh.. that's gross. This relies on this being in IRQ context and
> either: PREEMPT=y *OR* this always being from userspace. Otherwise
> there's no guarantee the return-from-interrupt will actually schedule.
>

Maybe I missed something but I think immediate rescheduling is not
required? E.g. software watchdog should fire if there is a kernel busy
loop and kernel preemption is not enabled. The current method ends up
with a thread wakeup so there is no guaranteed reschedule either?

-Xi