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

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Date:   Mon, 11 Jan 2021 13:56:55 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: Re: [RFC PATCH 6/8] sched: Report local wake up on resched blind
 zone within idle loop

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.