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Date:   Thu, 8 Jun 2023 15:25:04 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/idle: disable tick in idle=poll idle entry
Message-ID: <ZIHWsAC+n6mrDyFT@lothringen>
References: <ZIEqlkIASx2F2DRF@tpad>
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On Wed, Jun 07, 2023 at 10:10:46PM -0300, Marcelo Tosatti wrote:
> 
> 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.
> 
> tick_nohz_idle_exit (called if the NEED_RESCHED bit is set) 
> is responsible for re-enabling the tick when necessary. 
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> ---
> 
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 342f58a329f5..fdc240988ea9 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -276,7 +276,7 @@ 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();
>  			cpu_idle_poll();

cpu_idle_poll() re-enables interrupts and only checks need_resched(). If an
interrupt happens and queues a new timer without waking up a task, the timer
gets ignored.

>  		} else {
>  			cpuidle_idle_call();
>