linux-kernel - Re: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Date: Mon, 29 Jan 2024 15:52:19 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Mihai Carabas <mihai.carabas@...cle.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	catalin.marinas@....com, will@...nel.org, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com, 
	pbonzini@...hat.com, wanpengli@...cent.com, vkuznets@...hat.com, 
	rafael@...nel.org, daniel.lezcano@...aro.org, akpm@...ux-foundation.org, 
	pmladek@...e.com, peterz@...radead.org, dianders@...omium.org, 
	npiggin@...il.com, rick.p.edgecombe@...el.com, joao.m.martins@...cle.com, 
	juerg.haefliger@...onical.com, mic@...ikod.net, arnd@...db.de, 
	ankur.a.arora@...cle.com
Subject: Re: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

On Mon, Jan 29, 2024 at 12:56 PM Mihai Carabas <mihai.carabas@...cle.com> wrote:
>
> cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> smp_cond_load_relaxed which basically does a "wfe".
>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
> ---
>  drivers/cpuidle/poll_state.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..440cd713e39a 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>
>                 limit = cpuidle_poll_time(drv, dev);
>
> -               while (!need_resched()) {
> -                       cpu_relax();
> -                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> -                               continue;
> -
> +               for (;;) {
>                         loop_count = 0;
> +
> +                       smp_cond_load_relaxed(&current_thread_info()->flags,
> +                                             (VAL & _TIF_NEED_RESCHED) ||
> +                                             (loop_count++ >= POLL_IDLE_RELAX_COUNT));

The inner parens are not necessary AFAICS.

Also, doesn't this return a value which can be used for checking if
_TIF_NEED_RESCHED is set instead of the condition below?

> +
> +                       if (loop_count < POLL_IDLE_RELAX_COUNT)
> +                               break;
> +
>                         if (local_clock_noinstr() - time_start > limit) {
>                                 dev->poll_time_limit = true;
>                                 break;
> --