Re: [PATCH] drm/i915/pmu: Use local64_try_cmpxchg in i915_pmu_event_read

[Jani Nikula]

On Mon, 03 Jul 2023, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
> in i915_pmu_event_read.  x86 CMPXCHG instruction returns success in ZF flag,
> so this change saves a compare after cmpxchg (and related move instruction
> in front of cmpxchg).
>
> Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> fails. There is no need to re-read the value in the loop.
>
> No functional change intended.
>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/i915_pmu.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> index d35973b41186..108b675088ba 100644
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -696,12 +696,11 @@ static void i915_pmu_event_read(struct perf_event *event)
>  		event->hw.state = PERF_HES_STOPPED;
>  		return;
>  	}
> -again:
> -	prev = local64_read(&hwc->prev_count);
> -	new = __i915_pmu_event_read(event);
>  
> -	if (local64_cmpxchg(&hwc->prev_count, prev, new) != prev)
> -		goto again;
> +	prev = local64_read(&hwc->prev_count);
> +	do {
> +		new = __i915_pmu_event_read(event);
> +	} while (!local64_try_cmpxchg(&hwc->prev_count, &prev, new));

Chased through the documentation again, and pushed to drm-intel-next.

Thanks for the patch.

BR,
Jani.

>  
>  	local64_add(new - prev, &event->count);
>  }

-- 
Jani Nikula, Intel