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Message-ID: <CAJZ5v0gPoLan0M2A-x5V=ZNCxbYdv5e5DvNZTyo6Bd3e9HThYQ@mail.gmail.com>
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,
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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(¤t_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;
> --
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