linux-kernel - Re: [PATCH] sched: Warn on long periods of pending need

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Date:   Wed, 17 Mar 2021 17:06:31 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched: Warn on long periods of pending need_resched

On Wed, Mar 17, 2021 at 1:25 AM Ingo Molnar <mingo@...nel.org> wrote:
>
> * Josh Don <joshdon@...gle.com> wrote:
>
> > If resched_latency_warn_ms is set to the default value, only one warning
> > will be produced per boot.
>
> Looks like a value hack, should probably be a separate flag,
> defaulting to warn-once.

Agreed, done.

> > This warning only exists under CONFIG_SCHED_DEBUG. If it goes off, it is
> > likely that there is a missing cond_resched() somewhere.
>
> CONFIG_SCHED_DEBUG is default-y, so most distros have it enabled.

To avoid log spam for people who don't care, I was considering having
the feature default disabled. Perhaps a better alternative is to only
show a single line warning and not print the full backtrace by
default. Does the latter sound good to you?

> > +#ifdef CONFIG_KASAN
> > +#define RESCHED_DEFAULT_WARN_LATENCY_MS 101
> > +#define RESCHED_BOOT_QUIET_SEC 600
> > +#else
> > +#define RESCHED_DEFAULT_WARN_LATENCY_MS 51
> > +#define RESCHED_BOOT_QUIET_SEC 300
> >  #endif
> > +int sysctl_resched_latency_warn_ms = RESCHED_DEFAULT_WARN_LATENCY_MS;
> > +#endif /* CONFIG_SCHED_DEBUG */
>
> I'd really just make this a single value - say 100 or 200 msecs.

Replacing these both with a single value (the more conservative
default of 100ms and 600s).

> > +static inline void resched_latency_warn(int cpu, u64 latency)
> > +{
> > +     static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
> > +
> > +     WARN(__ratelimit(&latency_check_ratelimit),
> > +          "CPU %d: need_resched set for > %llu ns (%d ticks) "
> > +          "without schedule\n",
> > +          cpu, latency, cpu_rq(cpu)->ticks_without_resched);
> > +}
>
> Could you please put the 'sched:' prefix into scheduler warnings.
> Let's have a bit of a namespace structure in new warnings.

Sounds good, done.