Re: [PATCH] drm/amdgpu: Use local64_try_cmpxchg in amdgpu_perf_read

[Alex Deucher <alexdeucher@gmail.com>]

Applied.  Thanks!

Alex

On Mon, Jul 3, 2023 at 7:16 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
> in amdgpu_perf_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: Alex Deucher <alexander.deucher@amd.com>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c
> index 71ee361d0972..6e91ea1de5aa 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c
> @@ -276,9 +276,8 @@ static void amdgpu_perf_read(struct perf_event *event)
>             (!pe->adev->df.funcs->pmc_get_count))
>                 return;
>
> +       prev = local64_read(&hwc->prev_count);
>         do {
> -               prev = local64_read(&hwc->prev_count);
> -
>                 switch (hwc->config_base) {
>                 case AMDGPU_PMU_EVENT_CONFIG_TYPE_DF:
>                 case AMDGPU_PMU_EVENT_CONFIG_TYPE_XGMI:
> @@ -289,7 +288,7 @@ static void amdgpu_perf_read(struct perf_event *event)
>                         count = 0;
>                         break;
>                 }
> -       } while (local64_cmpxchg(&hwc->prev_count, prev, count) != prev);
> +       } while (!local64_try_cmpxchg(&hwc->prev_count, &prev, count));
>
>         local64_add(count - prev, &event->count);
>  }
> --
> 2.41.0
>