linux-kernel - RE: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load

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Message-ID:
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Date: Fri, 9 Aug 2024 05:54:26 +0000
From: "Tomohiro Misono (Fujitsu)" <misono.tomohiro@...itsu.com>
To: 'Ankur Arora' <ankur.a.arora@...cle.com>, "linux-pm@...r.kernel.org"
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Subject: RE: [PATCH v6 01/10] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()

> Subject: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load_relaxed()
> 
> From: Mihai Carabas <mihai.carabas@...cle.com>
> 
> The inner loop in poll_idle() polls up to POLL_IDLE_RELAX_COUNT times,
> checking to see if the thread has the TIF_NEED_RESCHED bit set. The
> loop exits once the condition is met, or if the poll time limit has
> been exceeded.
> 
> To minimize the number of instructions executed each iteration, the
> time check is done only infrequently (once every POLL_IDLE_RELAX_COUNT
> iterations). In addition, each loop iteration executes cpu_relax()
> which on certain platforms provides a hint to the pipeline that the
> loop is busy-waiting, thus allowing the processor to reduce power
> consumption.
> 
> However, cpu_relax() is defined optimally only on x86. On arm64, for
> instance, it is implemented as a YIELD which only serves a hint to the
> CPU that it prioritize a different hardware thread if one is available.
> arm64, however, does expose a more optimal polling mechanism via
> smp_cond_load_relaxed() which uses LDXR, WFE to wait until a store
> to a specified region.
> 
> So restructure the loop, folding both checks in smp_cond_load_relaxed().
> Also, move the time check to the head of the loop allowing it to exit
> straight-away once TIF_NEED_RESCHED is set.
> 
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  drivers/cpuidle/poll_state.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..532e4ed19e0f 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -21,21 +21,21 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
> 
>  	raw_local_irq_enable();
>  	if (!current_set_polling_and_test()) {
> -		unsigned int loop_count = 0;
> +		unsigned int loop_count;
>  		u64 limit;
> 
>  		limit = cpuidle_poll_time(drv, dev);
> 
>  		while (!need_resched()) {
> -			cpu_relax();
> -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> -				continue;
> -
>  			loop_count = 0;
>  			if (local_clock_noinstr() - time_start > limit) {
>  				dev->poll_time_limit = true;
>  				break;
>  			}
> +
> +			smp_cond_load_relaxed(&current_thread_info()->flags,
> +					      VAL & _TIF_NEED_RESCHED ||
> +					      loop_count++ >= POLL_IDLE_RELAX_COUNT);
>  		}
>  	}
>  	raw_local_irq_disable();
> --
> 2.43.5

Reviewed-by: Misono Tomohiro <misono.tomohiro@...itsu.com>

Regards,
Tomohiro