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

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Message-ID: <874irkun26.fsf@oracle.com>
Date: Mon, 27 Oct 2025 13:06:25 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: rafael@...nel.org, daniel.lezcano@...aro.org
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.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, 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 v7 7/7] cpuidle/poll_state: Poll via
 smp_cond_load_relaxed_timeout()


Hi Rafael, Daniel,

Could you take a look at the proposed smp_cond_load_relaxed_timeout()
interface and how it's used here and see if that looks fine to you?


Thanks
Ankur

Ankur Arora <ankur.a.arora@...cle.com> writes:

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


--
ankur