Re: [PATCH 06/10] timer: Report ignored local enqueue in nohz mode

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 11, 2021 at 01:37:04PM +0100, Frederic Weisbecker wrote:
> 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.
> 
> Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/sched/core.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ca2bb629595f..24552911f92b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -674,6 +674,22 @@ int get_nohz_timer_target(void)
>  	return cpu;
>  }
>  
> +/* Make sure the timer won't be ignored in dynticks-idle case */
> +static void wake_idle_assert_possible(void)
> +{
> +#ifdef CONFIG_SCHED_DEBUG
> +	/*
> +	 * Timers are re-evaluated after idle IRQs. In case of softirq,
> +	 * we assume 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.
> +	 */
> +	WARN_ONCE(in_task(),
> +		  "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 +704,10 @@ 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);

I'm not entirely sure I understand this one. What's the callchain that
leads to this?