Re: [PATCH] sched/idle: disable tick in idle=poll idle entry

Frederic Weisbecker
2023-06-08T13:25:12Z

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();
>