[PATCH] drm/atomic: Perform blocking commits on workqueue

[Ville Syrjälä]

On Tue, Sep 26, 2023 at 01:05:49PM -0400, Ray Strode wrote:
> From: Ray Strode <rstrode at redhat.com>
> 
> A drm atomic commit can be quite slow on some hardware. It can lead
> to a lengthy queue of commands that need to get processed and waited
> on before control can go back to user space.
> 
> If user space is a real-time thread, that delay can have severe
> consequences, leading to the process getting killed for exceeding
> rlimits.
> 
> This commit addresses the problem by always running the slow part of
> a commit on a workqueue, separated from the task initiating the
> commit.
> 
> This change makes the nonblocking and blocking paths work in the same way,
> and as a result allows the task to sleep and not use up its
> RLIMIT_RTTIME allocation.
> 
> Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2861
> Signed-off-by: Ray Strode <rstrode at redhat.com>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 292e38eb6218..1a1e68d98d38 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -2028,64 +2028,63 @@ int drm_atomic_helper_commit(struct drm_device *dev,
>  	 * This is the point of no return - everything below never fails except
>  	 * when the hw goes bonghits. Which means we can commit the new state on
>  	 * the software side now.
>  	 */
>  
>  	ret = drm_atomic_helper_swap_state(state, true);
>  	if (ret)
>  		goto err;
>  
>  	/*
>  	 * Everything below can be run asynchronously without the need to grab
>  	 * any modeset locks at all under one condition: It must be guaranteed
>  	 * that the asynchronous work has either been cancelled (if the driver
>  	 * supports it, which at least requires that the framebuffers get
>  	 * cleaned up with drm_atomic_helper_cleanup_planes()) or completed
>  	 * before the new state gets committed on the software side with
>  	 * drm_atomic_helper_swap_state().
>  	 *
>  	 * This scheme allows new atomic state updates to be prepared and
>  	 * checked in parallel to the asynchronous completion of the previous
>  	 * update. Which is important since compositors need to figure out the
>  	 * composition of the next frame right after having submitted the
>  	 * current layout.
>  	 *
>  	 * NOTE: Commit work has multiple phases, first hardware commit, then
>  	 * cleanup. We want them to overlap, hence need system_unbound_wq to
>  	 * make sure work items don't artificially stall on each another.
>  	 */
>  
>  	drm_atomic_state_get(state);
> -	if (nonblock)
> -		queue_work(system_unbound_wq, &state->commit_work);
> -	else
> -		commit_tail(state);
> +	queue_work(system_unbound_wq, &state->commit_work);
> +	if (!nonblock)
> +		flush_work(&state->commit_work);

Here's my earlier take on this: https://patchwork.freedesktop.org/series/108668/
execpt I went further and moved the flush past the unlock in the end.

-- 
Ville Syrjälä
Intel