Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

[Will Deacon]

On Mon, Nov 20, 2023 at 04:01:38PM +0200, Mihai Carabas 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@xxxxxxxxxxxxx>
> Signed-off-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
> ---
>  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(&current_thread_info()->flags,
> +					      (VAL & _TIF_NEED_RESCHED) ||
> +					      (loop_count++ >= POLL_IDLE_RELAX_COUNT));
> +
> +			if (loop_count < POLL_IDLE_RELAX_COUNT)
> +				break;
> +
>  			if (local_clock_noinstr() - time_start > limit) {
>  				dev->poll_time_limit = true;
>  				break;

Doesn't this make ARCH_HAS_CPU_RELAX a complete misnomer?

Will