linux-kernel - Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Message-ID: <20231211114642.GB24899@willie-the-truck>
Date:   Mon, 11 Dec 2023 11:46:42 +0000
From:   Will Deacon <will@...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,
        catalin.marinas@....com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, pbonzini@...hat.com,
        wanpengli@...cent.com, vkuznets@...hat.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, akpm@...ux-foundation.org,
        pmladek@...e.com, peterz@...radead.org, dianders@...omium.org,
        npiggin@...il.com, rick.p.edgecombe@...el.com,
        joao.m.martins@...cle.com, juerg.haefliger@...onical.com,
        mic@...ikod.net, arnd@...db.de, ankur.a.arora@...cle.com
Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed

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@...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(&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