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

Rafael J. Wysocki
2021-03-04T12:00:33Z

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>

> ---
> 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);