linux-kernel - Re: [PATCH tip/core/rcu 02/12] timer: Report ignored local enqueue in nohz mode

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Date:   Thu, 4 Mar 2021 12:06:24 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH tip/core/rcu 02/12] timer: Report ignored local enqueue
 in nohz mode

On Thu, Mar 04, 2021 at 12:58:54PM +0100, Rafael J. Wysocki wrote:
> On 3/4/2021 1:23 AM, paulmck@...nel.org wrote:
> > From: Frederic Weisbecker <frederic@...nel.org>
> > 
> > Enqueuing a local timer after the tick has been stopped will result in
> > the timer being ignored until the next random interrupt.
> > 
> > Perform sanity checks to report these situations.
> > 
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar<mingo@...nel.org>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Applied, thank you!

							Thanx, Paul

> > ---
> >   kernel/sched/core.c | 24 +++++++++++++++++++++++-
> >   1 file changed, 23 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index ca2bb62..4822371 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -674,6 +674,26 @@ int get_nohz_timer_target(void)
> >   	return cpu;
> >   }
> > +static void wake_idle_assert_possible(void)
> > +{
> > +#ifdef CONFIG_SCHED_DEBUG
> > +	/* Timers are re-evaluated after idle IRQs */
> > +	if (in_hardirq())
> > +		return;
> > +	/*
> > +	 * Same as hardirqs, assuming they are executing
> > +	 * on IRQ tail. Ksoftirqd shouldn't reach here
> > +	 * as the timer base wouldn't be idle. And inline
> > +	 * softirq processing after a call to local_bh_enable()
> > +	 * within idle loop sound too fun to be considered here.
> > +	 */
> > +	if (in_serving_softirq())
> > +		return;
> > +
> > +	WARN_ON_ONCE("Late timer enqueue may be ignored\n");
> > +#endif
> > +}
> > +
> >   /*
> >    * When add_timer_on() enqueues a timer into the timer wheel of an
> >    * idle CPU then this timer might expire before the next timer event
> > @@ -688,8 +708,10 @@ static void wake_up_idle_cpu(int cpu)
> >   {
> >   	struct rq *rq = cpu_rq(cpu);
> > -	if (cpu == smp_processor_id())
> > +	if (cpu == smp_processor_id()) {
> > +		wake_idle_assert_possible();
> >   		return;
> > +	}
> >   	if (set_nr_and_not_polling(rq->idle))
> >   		smp_send_reschedule(cpu);
> 
>