linux-kernel - [PATCH] drm/sched: Discourage usage of separate workqueues

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Message-ID: <20250604081657.124453-2-phasta@kernel.org>
Date: Wed,  4 Jun 2025 10:16:58 +0200
From: Philipp Stanner <phasta@...nel.org>
To: Matthew Brost <matthew.brost@...el.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Philipp Stanner <phasta@...nel.org>,
	Christian König <ckoenig.leichtzumerken@...il.com>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] drm/sched: Discourage usage of separate workqueues

struct drm_sched_init_args provides the possibility of letting the
scheduler use user-controlled workqueues, instead of the scheduler
creating its own workqueues. It's currently not documented who would
want to use that.

Not sharing the submit_wq between driver and scheduler has the advantage
of no negative intereference between them being able to occur (e.g.,
MMU notifier callbacks waiting for fences to get signaled). A separate
timeout_wq should rarely be necessary, since using the system_wq could,
in the worst case, delay a timeout.

Discourage the usage of own workqueues in the documentation.

Suggested-by: Danilo Krummrich <dakr@...nel.org>
Signed-off-by: Philipp Stanner <phasta@...nel.org>
---
 include/drm/gpu_scheduler.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
index 81dcbfc8c223..11740d745223 100644
--- a/include/drm/gpu_scheduler.h
+++ b/include/drm/gpu_scheduler.h
@@ -590,14 +590,17 @@ struct drm_gpu_scheduler {
  *
  * @ops: backend operations provided by the driver
  * @submit_wq: workqueue to use for submission. If NULL, an ordered wq is
- *	       allocated and used.
+ *	       allocated and used. It is recommended to pass NULL unless there
+ *	       is a good reason not to.
  * @num_rqs: Number of run-queues. This may be at most DRM_SCHED_PRIORITY_COUNT,
  *	     as there's usually one run-queue per priority, but may be less.
  * @credit_limit: the number of credits this scheduler can hold from all jobs
  * @hang_limit: number of times to allow a job to hang before dropping it.
  *		This mechanism is DEPRECATED. Set it to 0.
  * @timeout: timeout value in jiffies for submitted jobs.
- * @timeout_wq: workqueue to use for timeout work. If NULL, the system_wq is used.
+ * @timeout_wq: workqueue to use for timeout work. If NULL, the system_wq is
+ *		used. It is recommended to pass NULL unless there is a good
+ *		reason not to.
  * @score: score atomic shared with other schedulers. May be NULL.
  * @name: name (typically the driver's name). Used for debugging
  * @dev: associated device. Used for debugging
-- 
2.49.0