linux-kernel - Re: [PATCH v8 12/12] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed

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Message-ID: <CAJZ5v0imk5kdqunNGvK+6_BPh2_k89RPPC8B4MDDF1GLZrUhLQ@mail.gmail.com>
Date: Mon, 15 Dec 2025 12:11:59 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org, 
	bpf@...r.kernel.org, arnd@...db.de, catalin.marinas@....com, will@...nel.org, 
	peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com, 
	harisokn@...zon.com, cl@...two.org, ast@...nel.org, rafael@...nel.org, 
	daniel.lezcano@...aro.org, memxor@...il.com, zhenglifeng1@...wei.com, 
	xueshuai@...ux.alibaba.com, joao.m.martins@...cle.com, 
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v8 12/12] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

On Mon, Dec 15, 2025 at 5:55 AM Ankur Arora <ankur.a.arora@...cle.com> wrote:
>
> The inner loop in poll_idle() polls over the thread_info flags,
> waiting to see if the thread has TIF_NEED_RESCHED set. The loop
> exits once the condition is met, or if the poll time limit has
> been exceeded.
>
> To minimize the number of instructions executed in each iteration,
> the time check is rate-limited. In addition, each loop iteration
> executes cpu_relax() which on certain platforms provides a hint to
> the pipeline that the loop busy-waits, allowing the processor to
> reduce power consumption.
>
> Switch over to tif_need_resched_relaxed_wait() instead, since that
> provides exactly that.
>
> However, given that when running in idle we want to minimize our power
> consumption, continue to depend on CONFIG_ARCH_HAS_CPU_RELAX as that
> serves as an indicator that the platform supports an optimized version
> of tif_need_resched_relaxed_wait() (via
> smp_cond_load_acquire_timeout()).
>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: linux-pm@...r.kernel.org
> Suggested-by: "Rafael J. Wysocki" <rafael@...nel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>
> Notes:
>   - use tif_need_resched_relaxed_wait() instead of
>     smp_cond_load_relaxed_timeout()
>
>  drivers/cpuidle/poll_state.c | 27 +++++----------------------
>  1 file changed, 5 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index c7524e4c522a..20136b3a08c2 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -6,41 +6,24 @@
>  #include <linux/cpuidle.h>
>  #include <linux/export.h>
>  #include <linux/irqflags.h>
> -#include <linux/sched.h>
> -#include <linux/sched/clock.h>
>  #include <linux/sched/idle.h>
>  #include <linux/sprintf.h>
>  #include <linux/types.h>
>
> -#define POLL_IDLE_RELAX_COUNT  200
> -
>  static int __cpuidle poll_idle(struct cpuidle_device *dev,
>                                struct cpuidle_driver *drv, int index)
>  {
> -       u64 time_start;
> -
> -       time_start = local_clock_noinstr();
> -
>         dev->poll_time_limit = false;
>
>         raw_local_irq_enable();
>         if (!current_set_polling_and_test()) {
> -               unsigned int loop_count = 0;
> -               u64 limit;
> +               s64 limit;
> +               bool nr_set;

It doesn't look like the nr_set variable is really needed.

>
> -               limit = cpuidle_poll_time(drv, dev);
> +               limit = (s64)cpuidle_poll_time(drv, dev);

Is the explicit cast needed to suppress a warning?  If not, I'd drop it.

>
> -               while (!need_resched()) {
> -                       cpu_relax();
> -                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> -                               continue;
> -
> -                       loop_count = 0;
> -                       if (local_clock_noinstr() - time_start > limit) {
> -                               dev->poll_time_limit = true;
> -                               break;
> -                       }
> -               }
> +               nr_set = tif_need_resched_relaxed_wait(limit);
> +               dev->poll_time_limit = !nr_set;

This can be

dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);

>         }
>         raw_local_irq_disable();
>
> --