[PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

[Okanovic, Haris]

On Thu, 2024-02-15 at 09:41 +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 at infradead.org>
> Signed-off-by: Mihai Carabas <mihai.carabas at oracle.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);

Is it necessary to repeat this 200 times with a wfe poll? Does kvm not
implement a timeout period?

Could you make it configurable? This patch improves certain workloads
on AWS Graviton instances as well, but blocks up to 6ms in 200 * 30us
increments before going to wfi, which is a bit excessive.

> +
> +			if (!(ret & _TIF_NEED_RESCHED))
> +				break;
> +
>  			if (local_clock_noinstr() - time_start > limit) {
>  				dev->poll_time_limit = true;
>  				break;