Re: [PATCH v2] tick/nohz: Reevaluate the tick for kicks flushed outside hardirq

[Frederic Weisbecker]

Le Fri, Oct 23, 2026 at 01:54:57AM +0800, Peng Wang a écrit :
> The nohz full kick relies on IRQ exit to reevaluate the tick. However,
> a remote kick can also be flushed from task context without an IRQ
> exit guaranteed to follow. If a tick dependency is set or an earlier
> timer is queued after the idle-exit reevaluation, the required tick
> update can be delayed until another interrupt.
> 
> Reevaluate the tick directly from the callback outside hardirq context.
> Keep the existing IRQ-exit handling for hardirq callbacks. The offline
> flush remains harmless because tick_sched_timer_dying() has already
> cleared TS_FLAG_NOHZ.
> 
> Fixes: 4b44a21dd640 ("irq_work, smp: Allow irq_work on call_single_queue")
> Fixes: a5183862e76f ("tick/nohz: Conditionally restart tick on idle exit")
> Suggested-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Co-developed-by: He Rongguang <herongguang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: He Rongguang <herongguang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Peng Wang <peng_wang@xxxxxxxxxxxxxxxxx>
> ---
> Changes in v2:
> - Reevaluate the tick directly outside hardirq instead of requeueing,
>   as suggested by Frederic.
> - Use the full update for both dependencies and timer deadlines. Keep
>   IRQ-exit handling for hardirq callbacks and the existing NOHZ guard
>   for CPU teardown.
> 
> v1: https://lore.kernel.org/all/43668cf88d976084e2865ec07dea82b64ad402ba.1792240863.git.peng_wang@xxxxxxxxxxxxxxxxx/
> 
>  kernel/time/tick-sched.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 6c3fea386713..ece2c21cec84 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -400,9 +400,13 @@ static bool can_stop_full_tick(int cpu, struct tick_sched *ts)
>  	return true;
>  }
>  
> +static void tick_nohz_full_update_tick(struct tick_sched *ts);
> +
>  static void nohz_full_kick_func(struct irq_work *work)
>  {
> -	/* Empty, the tick restart happens on tick_nohz_irq_exit() */
> +	/* Task-context flushes may not be followed by an IRQ exit. */
> +	if (!in_hardirq())
> +		tick_nohz_full_update_tick(this_cpu_ptr(&tick_cpu_sched));

Just in case, even though it shouldn't happen, just have the following sanity
checks to return early:

if (WARN_ON_ONCE(!tick_nohz_full_cpu(smp_processor_id())))
    return;

if (!in_hardirq()) {
    if (WARN_ON_ONCE(tick_sched_flag_test(ts, TS_FLAG_INIDLE)))
       return;
    tick_nohz_full_update_tick(this_cpu_ptr(&tick_cpu_sched));
}

Thanks!

>  }
>  
>  static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) =
> 
> base-commit: 70dd4ee72eb2d5793c62e71c14e3aba78e5a4e56
> -- 
> 2.47.3
> 

-- 
Frederic Weisbecker
SUSE Labs