linux-kernel - Re: [RESEND PATCH v7 7/7] cpuidle/poll_state: Poll via smp_cond_load_relaxed

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Message-ID: <CAJZ5v0hSvzHfsE4nrEW-Ey0dnJ+m=dSU-f1RywGNU0Xyi3jXtQ@mail.gmail.com>
Date: Tue, 28 Oct 2025 13:30:28 +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: [RESEND PATCH v7 7/7] cpuidle/poll_state: Poll via smp_cond_load_relaxed_timeout()

On Tue, Oct 28, 2025 at 6:32 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 done only intermittently (once every
> POLL_IDLE_RELAX_COUNT iterations). 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.
>
> This is close to what smp_cond_load_relaxed_timeout() provides. So,
> restructure the loop and fold the loop condition and the timeout check
> in smp_cond_load_relaxed_timeout().

Well, it is close, but is it close enough?

> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  drivers/cpuidle/poll_state.c | 29 ++++++++---------------------
>  1 file changed, 8 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..dc7f4b424fec 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -8,35 +8,22 @@
>  #include <linux/sched/clock.h>
>  #include <linux/sched/idle.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();
> +       u64 time_end;
> +       u32 flags = 0;
>
>         dev->poll_time_limit = false;
>
> +       time_end = local_clock_noinstr() + cpuidle_poll_time(drv, dev);

Is there any particular reason for doing this unconditionally?  If
not, then it looks like an arbitrary unrelated change to me.

> +
>         raw_local_irq_enable();
>         if (!current_set_polling_and_test()) {
> -               unsigned int loop_count = 0;
> -               u64 limit;
> -
> -               limit = cpuidle_poll_time(drv, dev);
> -
> -               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;
> -                       }
> -               }
> +               flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
> +                                                     (VAL & _TIF_NEED_RESCHED),
> +                                                     (local_clock_noinstr() >= time_end));

So my understanding of this is that it reduces duplication with some
other places doing similar things.  Fair enough.

However, since there is "timeout" in the name, I'd expect it to take
the timeout as an argument.

> +               dev->poll_time_limit = !(flags & _TIF_NEED_RESCHED);
>         }
>         raw_local_irq_disable();
>
> --