Re: [PATCH] sched: Warn on long periods of pending need_resched

[Josh Don]

On Wed, Mar 17, 2021 at 1:25 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Josh Don <joshdon@xxxxxxxxxx> 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.