Re: [RFC PATCH 6/8] sched: Report local wake up on resched blind zone within idle loop

Peter Zijlstra
2021-01-11T12:27:02Z

On Sat, Jan 09, 2021 at 03:05:34AM +0100, Frederic Weisbecker wrote:
> The idle loop has several need_resched() checks that make sure we don't
> miss a rescheduling request. This means that any wake up performed on
> the local runqueue after the last generic need_resched() check is going
> to have its rescheduling silently ignored. This has happened in the
> past with rcu kthreads awaken from rcu_idle_enter() for example.
>
> Perform sanity checks to report these situations.

I really don't like this..

- it's too specific to the actual reschedule condition, any wakeup this
late is dodgy, not only those that happen to cause a local
reschedule.

- we can already test this with unwind and checking against __cpuidle

- moving all of __cpuidle into noinstr would also cover this. And we're
going to have to do that anyway.

> +void noinstr sched_resched_local_assert_allowed(void)
> +{
> + if (this_rq()->resched_local_allow)
> + return;
> +

> + /*
> + * Idle interrupts break the CPU from its pause and
> + * rescheduling happens on idle loop exit.
> + */
> + if (in_hardirq())
> + return;
> +
> + /*
> + * What applies to hardirq also applies to softirq as
> + * we assume they execute on hardirq tail. Ksoftirqd
> + * shouldn't have resched_local_allow == 0.
> + * We also assume that no local_bh_enable() call may
> + * execute softirqs inline on fragile idle/entry
> + * path...
> + */
> + if (in_serving_softirq())
> + return;
> +
> + WARN_ONCE(1, "Late current task rescheduling may be lost\n");

That seems like it wants to be:

WARN_ONCE(in_task(), "...");

> +}