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

[Alex Deucher]

Applied.  Thanks!

On Wed, Aug 5, 2026 at 4:53 AM Christian König <christian.koenig@xxxxxxx> wrote:
>
> On 7/29/26 12:26, 2564278112@xxxxxx wrote:
> > From: Wang Jiang <jiangwang@xxxxxxxxxx>
> >
> > 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 <jiangwang@xxxxxxxxxx>
>
> 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 <christian.koenig@xxxxxxx>
>
> 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
> >
>