Re: [PATCH 2/3] sched/idle: Stop the tick on polling idle on nohz_full CPUs

[Frederic Weisbecker <frederic@kernel.org>]

Le Fri, Nov 06, 2026 at 05:06:14PM +0100, Marco Crivellari a écrit :
> With idle=poll, the tick fires while in user space, interrupting the task
> running there. This is a problem when the CPU is nohz_full, as it
> introduces noise into the workload.
> 
> To fix this, stop the tick when entering polling idle on nohz_full CPUs,
> while on other CPUs keep restarting it.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Link: https://lore.kernel.org/all/aQiWfnnSzxsnwa2o@tpad/
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> ---
>  include/linux/tick.h     |  2 ++
>  kernel/sched/idle.c      |  5 ++++-
>  kernel/time/tick-sched.c | 18 ++++++++++++++++++
>  3 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 8bfbb053f4fd..be4940f76777 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -129,6 +129,7 @@ extern bool tick_nohz_is_active(void);
>  extern bool tick_nohz_tick_stopped(void);
>  extern bool tick_nohz_tick_stopped_cpu(int cpu);
>  extern void tick_nohz_idle_stop_tick(void);
> +extern void tick_nohz_full_idle_stop_tick(void);
>  extern void tick_nohz_idle_retain_tick(void);
>  extern void tick_nohz_idle_restart_tick(void);
>  extern void tick_nohz_idle_enter(void);
> @@ -144,6 +145,7 @@ static inline bool tick_nohz_is_active(void) { return false; }
>  static inline int tick_nohz_tick_stopped(void) { return 0; }
>  static inline int tick_nohz_tick_stopped_cpu(int cpu) { return 0; }
>  static inline void tick_nohz_idle_stop_tick(void) { }
> +static inline void tick_nohz_full_idle_stop_tick(void) { }
>  static inline void tick_nohz_idle_retain_tick(void) { }
>  static inline void tick_nohz_idle_restart_tick(void) { }
>  static inline void tick_nohz_idle_enter(void) { }
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index e0c10a5dbda2..226d02d24c73 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -349,7 +349,10 @@ static void do_idle(void)
>  		 * idle as we know that the IPI is going to arrive right away.
>  		 */
>  		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
> -			tick_nohz_idle_restart_tick();
> +			if (cpu_idle_force_poll)
> +				tick_nohz_full_idle_stop_tick();
> +			else
> +				tick_nohz_idle_restart_tick();
>  			cpu_idle_poll();
>  		} else {
>  			cpuidle_idle_call(got_tick);
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index a9f622fdeb12..3625ad9d2268 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1179,6 +1179,24 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
>  	return true;
>  }
>  
> +/**
> + * tick_nohz_full_idle_stop_tick - stop the tick on polling idle on NOHZ
> + *
> + * On nohz_full CPUs the tick restarted for polling idle would keep
> + * running after idle exit and interrupt userspace once more; so stop it
> + * instaed. On other CPUs, keep restarting it as before. Must be called

*instead

Also it should mention that we want to stop the tick before polling
and not after because we don't want the tick stop induced latency after
the task wake-up.

> + * with interrupts disabled.
> + */
> +void tick_nohz_full_idle_stop_tick(void)

I would rather call that tick_nohz_idle_poll_enter() because it is
very specific and it's not just about stopping the nohz_full tick
in idle, it's also restarting the tick if in dynticks-idle.

Thanks.

> +{
> +	lockdep_assert_irqs_disabled();
> +
> +	if (tick_nohz_full_cpu(smp_processor_id()))
> +		tick_nohz_idle_stop_tick();
> +	else
> +		tick_nohz_idle_restart_tick();
> +}
> +
>  /**
>   * tick_nohz_idle_stop_tick - stop the idle tick from the idle task
>   *
> -- 
> 2.56.0
> 

-- 
Frederic Weisbecker
SUSE Labs