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From:   Josh Don <joshdon@google.com>
Date:   Wed, 17 Mar 2021 17:17:37 -0700
Message-ID: <CABk29Nu8iYDzY+GHa+z7oJyGF_0JKdF9+-zBbiL7C2hgSfHqMg@mail.gmail.com>
Subject: Re: [PATCH] sched: Warn on long periods of pending need_resched
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        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>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org,
        David Rientjes <rientjes@google.com>,
        Oleg Rombakh <olegrom@google.com>, Paul Turner <pjt@google.com>
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On Wed, Mar 17, 2021 at 1:31 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Josh Don <joshdon@google.com> wrote:
>
> > +static inline u64 resched_latency_check(struct rq *rq)
> > +{
> > +     int latency_warn_ms = READ_ONCE(sysctl_resched_latency_warn_ms);
> > +     bool warn_only_once = (latency_warn_ms == RESCHED_DEFAULT_WARN_LATENCY_MS);
> > +     u64 need_resched_latency, now = rq_clock(rq);
> > +     static bool warned_once;
> > +
> > +     if (warn_only_once && warned_once)
> > +             return 0;
> > +
> > +     if (!need_resched() || latency_warn_ms < 2)
> > +             return 0;
> > +
> > +     /* Disable this warning for the first few mins after boot */
> > +     if (now < RESCHED_BOOT_QUIET_SEC * NSEC_PER_SEC)
> > +             return 0;
> > +
> > +     if (!rq->last_seen_need_resched_ns) {
> > +             rq->last_seen_need_resched_ns = now;
> > +             rq->ticks_without_resched = 0;
> > +             return 0;
> > +     }
> > +
> > +     rq->ticks_without_resched++;
>
> So AFAICS this will only really do something useful on full-nohz
> kernels with sufficiently long scheduler ticks, right?

Not quite sure what you mean; it is actually the inverse? Since we
rely on the tick to detect the resched latency, on nohz-full we won't
have detection on cpus running a single thread. The ideal scenario is
!nohz-full and tick interval << warn_ms.

> On other kernels the scheduler tick interrupt, when it returns to
> user-space, will trigger a reschedule if it sees a need_resched.

True for the case where we return to userspace, but we could instead
be executing in a non-preemptible region of the kernel. This is where
we've seen/fixed kernel bugs.

Best,
Josh