Re: [PATCH] drm/radeon: restore hardware polling in fence_is_signaled to fix performance regression

On 7/29/26 12:26, [email protected] wrote:
> From: Wang Jiang <[email protected]>
> 
> Commit 9eb00b5f5697b ("drm/radeon: delete radeon_fence_process in
> is_signaled, no deadlock") removed the hardware polling from
> radeon_fence_is_signaled() to fix a self-deadlock caused by
> wake_up_all(&rdev->fence_queue) being called with the fence queue
> lock held.
> 
> However, removing the polling entirely causes significant performance
> regression (e.g. glxgears FPS drop) because the fence signaled check
> becomes purely passive — it only reads the cached last_seq without
> probing the GPU, so completed GPU work is not detected in time,
> causing unnecessary CPU stalls in sync-heavy workloads.
> 
> Fix this by calling radeon_fence_activity() directly instead of
> radeon_fence_process(). radeon_fence_activity() reads the hardware
> fence counter and updates last_seq via atomic ops without calling
> wake_up_all(), thus avoiding the deadlock while restoring timely
> fence detection.

Yeah I already feared that removing this in commit 9eb00b5f5697b could cause issues.

Adding Philip and Danilo since we recently had a discussion about the necessity of this.
> Fixes: 9eb00b5f5697b ("drm/radeon: delete radeon_fence_process in is_signaled, no deadlock")
> Signed-off-by: Wang Jiang <[email protected]>

As far as I remember the radeon code the solution should work, but I don't have time to double check.

Acked-by: Christian König <[email protected]>

Thanks,
Christian.

> ---
>  drivers/gpu/drm/radeon/radeon_fence.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/radeon/radeon_fence.c b/drivers/gpu/drm/radeon/radeon_fence.c
> index 02a40e4750c7..299865569252 100644
> --- a/drivers/gpu/drm/radeon/radeon_fence.c
> +++ b/drivers/gpu/drm/radeon/radeon_fence.c
> @@ -360,6 +360,13 @@ static bool radeon_fence_is_signaled(struct dma_fence *f)
>         if (atomic64_read(&rdev->fence_drv[ring].last_seq) >= seq)
>                 return true;
> 
> +       if (down_read_trylock(&rdev->exclusive_lock)) {
> +               radeon_fence_activity(rdev, ring);
> +               up_read(&rdev->exclusive_lock);
> +
> +               if (atomic64_read(&rdev->fence_drv[ring].last_seq) >= seq)
> +                       return true;
> +       }
>         return false;
>  }
> 
> --
> 2.25.1
>