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

Paul E. McKenney
2021-03-04T20:07:48Z

On Thu, Mar 04, 2021 at 12:58:54PM +0100, Rafael J. Wysocki wrote:
> On 3/4/2021 1:23 AM, paulmck@kernel.org wrote:
> > From: Frederic Weisbecker <frederic@kernel.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@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar<mingo@kernel.org>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.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);
>
>