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

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Nov 06, 2023 at 11:10:21PM -0500, 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 <ltuikov89@gmail.com>
> Fixes: f7fe64ad0f22ff ("drm/sched: Split free_job into own work item")

Reviewed-by: Matthew Brost <matthew.brost@intel.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
> -- 
> 2.42.1
>