Re: [PATCH] drm/sched: Don't disturb the entity when in RR-mode scheduling

[Danilo Krummrich]

On 11/7/23 05:10, Luben Tuikov wrote:

Don't call drm_sched_select_entity() in drm_sched_run_job_queue().  In fact,
rename __drm_sched_run_job_queue() to just drm_sched_run_job_queue(), and let
it do just that, schedule the work item for execution.

The problem is that drm_sched_run_job_queue() calls drm_sched_select_entity()
to determine if the scheduler has an entity ready in one of its run-queues,
and in the case of the Round-Robin (RR) scheduling, the function
drm_sched_rq_select_entity_rr() does just that, selects the _next_ entity
which is ready, sets up the run-queue and completion and returns that
entity. The FIFO scheduling algorithm is unaffected.

Now, since drm_sched_run_job_work() also calls drm_sched_select_entity(), then
in the case of RR scheduling, that would result in drm_sched_select_entity()
having been called twice, which may result in skipping a ready entity if more
than one entity is ready. This commit fixes this by eliminating the call to
drm_sched_select_entity() from drm_sched_run_job_queue(), and leaves it only
in drm_sched_run_job_work().

v2: Rebased on top of Tvrtko's renames series of patches. (Luben)
     Add fixes-tag. (Tvrtko)

Signed-off-by: Luben Tuikov <ltuiko...@gmail.com>
Fixes: f7fe64ad0f22ff ("drm/sched: Split free_job into own work item")

Reviewed-by: Danilo Krummrich <d...@redhat.com>

---
  drivers/gpu/drm/scheduler/sched_main.c | 16 +++-------------
  1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
b/drivers/gpu/drm/scheduler/sched_main.c
index 27843e37d9b769..cd0dc3f81d05f0 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -256,10 +256,10 @@ drm_sched_rq_select_entity_fifo(struct drm_sched_rq *rq)
  }

/**

- * __drm_sched_run_job_queue - enqueue run-job work
+ * drm_sched_run_job_queue - enqueue run-job work
   * @sched: scheduler instance
   */
-static void __drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
+static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
  {
        if (!READ_ONCE(sched->pause_submit))
                queue_work(sched->submit_wq, &sched->work_run_job);
@@ -928,7 +928,7 @@ static bool drm_sched_can_queue(struct drm_gpu_scheduler 
*sched)
  void drm_sched_wakeup(struct drm_gpu_scheduler *sched)
  {
        if (drm_sched_can_queue(sched))
-               __drm_sched_run_job_queue(sched);
+               drm_sched_run_job_queue(sched);
  }

/**

@@ -1040,16 +1040,6 @@ drm_sched_pick_best(struct drm_gpu_scheduler 
**sched_list,
  }
  EXPORT_SYMBOL(drm_sched_pick_best);

-/**

- * drm_sched_run_job_queue - enqueue run-job work if there are ready entities
- * @sched: scheduler instance
- */
-static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
-{
-       if (drm_sched_select_entity(sched))
-               __drm_sched_run_job_queue(sched);
-}
-
  /**
   * drm_sched_free_job_work - worker to call free_job
   *

base-commit: 27d9620e9a9a6bc27a646b464b85860d91e21af3