linux-kernel - Re: [PATCH v2] sched/idle: disable tick in idle=poll idle entry

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Message-ID: <aQONGWu1lM27erA3@localhost.localdomain>
Date: Thu, 30 Oct 2025 17:06:49 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Marcelo Tosatti <mtosatti@...hat.com>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2] sched/idle: disable tick in idle=poll idle entry

(Adding more people in Cc)

Le Wed, Oct 29, 2025 at 03:00:56PM -0300, Marcelo Tosatti a écrit :
> 
> Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5 
> ("tick/nohz: Conditionally restart tick on idle exit") allows
> a nohz_full CPU to enter idle and return from it with the 
> scheduler tick disabled (since the tick might be undesired noise).
> 
> The idle=poll case still unconditionally restarts the tick when entering
> idle.
> 
> To reduce the noise for that case as well, stop the tick when entering
> idle, for the idle=poll case.
> 
> Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the
> case where a new timer is added from an interrupt. This breaks out of
> cpu_idle_poll and rearms the timer if necessary.
> 
> ---
> 
> v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)
> 
>  include/linux/sched.h    |    2 ++
>  kernel/sched/core.c      |   10 ++++++++++
>  kernel/sched/idle.c      |    2 +-
>  kernel/time/tick-sched.c |    1 +
>  4 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index cbb7340c5866..1f6938dc20cd 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
>  
>  DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
>  
> +void set_tif_resched_if_polling(int cpu);
> +
>  #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f1ebf67b48e2..f0b84600084b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
>  	return true;
>  }
>  
> +void set_tif_resched_if_polling(int cpu)
> +{
> +	set_nr_if_polling(cpu_rq(cpu)->idle);
> +}
> +
>  #else
>  static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
>  {
> @@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
>  {
>  	return false;
>  }
> +
> +void set_tif_resched_if_polling(int cpu)
> +{
> +	set_tsk_need_resched(cpu_rq(cpu)->idle);
> +}
>  #endif
>  
>  static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index c39b089d4f09..428c2d1cbd1b 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -324,7 +324,7 @@ 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();
> +			tick_nohz_idle_stop_tick();

Shouldn't we simply remove the tick_nohz_idle_restart_tick() line? The nohz_full
CPU should have entered here with the tick disabled already.

Also non-nohz_full systems shouldn't care.

>  			cpu_idle_poll();
>  		} else {
>  			cpuidle_idle_call();
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index c527b421c865..efc3653999dc 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -408,6 +408,7 @@ void tick_nohz_full_kick_cpu(int cpu)
>  	if (!tick_nohz_full_cpu(cpu))
>  		return;
>  
> +	set_tif_resched_if_polling(cpu);

Perhaps stuff that within wake_up_full_nohz_cpu() and call
set_nr_if_polling() directly. Also this needs a big comment.

Thanks.

>  	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
>  }
>  
> 

-- 
Frederic Weisbecker
SUSE Labs