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

Frederic Weisbecker
2021-01-11T12:57:54Z

On Mon, Jan 11, 2021 at 01:25:59PM +0100, Peter Zijlstra wrote:
> 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.

Right.

>
> - 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.

Ok then, I'll wait for that instead.

>
> > +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(), "...");

Right! But I guess I'll drop that patch now.

Thanks.