linux-kernel - RE: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

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Date: Mon, 26 Feb 2024 08:36:34 +0000
From: "Tomohiro Misono (Fujitsu)" <misono.tomohiro@...itsu.com>
To: 'Mihai Carabas' <mihai.carabas@...cle.com>,
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Subject: RE: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed

Hi,
> Subject: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed
> 
> 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 | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..1e45be906e72 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -13,6 +13,7 @@
>  static int __cpuidle poll_idle(struct cpuidle_device *dev,
>  			       struct cpuidle_driver *drv, int index)
>  {
> +	unsigned long ret;
>  	u64 time_start;
> 
>  	time_start = local_clock_noinstr();
> @@ -26,12 +27,16 @@ 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;
> +
> +			ret = smp_cond_load_relaxed(&current_thread_info()->flags,
> +						    VAL & _TIF_NEED_RESCHED ||
> +						    loop_count++ >= POLL_IDLE_RELAX_COUNT);
> +
> +			if (!(ret & _TIF_NEED_RESCHED))
> +				break;

Should this be "if (ret & _TIF_NEED_RESCHED) since we want to break here
if the flag is set, or am I misunderstood?

Regards,
Tomohiro

> +
>  			if (local_clock_noinstr() - time_start > limit) {
>  				dev->poll_time_limit = true;
>  				break;
> --
> 1.8.3.1
> 
> 
> _______________________________________________
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> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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