[RFC,03/14] drm/sched: Implement RR via FIFO

Commit Message

Tvrtko Ursulin Dec. 30, 2024, 4:52 p.m. UTC

From: Tvrtko Ursulin <[e-mail]>

Round-robin being the non-default policy and unclear how much it is used,
we can notice that it can be implemented using the FIFO data structures if
we only invent a fake submit timestamp which is monotonically increasing
inside drm_sched_rq instances.

So instead of remembering which was the last entity the scheduler worker
picked, we can bump the picked one to the bottom of the tree, achieving
the same round-robin behaviour.

Advantage is that we can consolidate to a single code path and remove a
bunch of code. Downside is round-robin mode now needs to lock on the job
pop path but that should not be visible.

Signed-off-by: Tvrtko Ursulin <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: Matthew Brost <[e-mail]>
Cc: Philipp Stanner <[e-mail]>
---
 drivers/gpu/drm/scheduler/sched_entity.c | 53 ++++++++-----
 drivers/gpu/drm/scheduler/sched_main.c   | 99 +++---------------------
 include/drm/gpu_scheduler.h              |  5 +-
 3 files changed, 48 insertions(+), 109 deletions(-)

Patch

diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
index 8e910586979e..cb5f596b48b7 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -473,9 +473,20 @@  drm_sched_job_dependency(struct drm_sched_job *job,
 	return NULL;
 }
 
+static ktime_t
+drm_sched_rq_get_rr_deadline(struct drm_sched_rq *rq)
+{
+	lockdep_assert_held(&rq->lock);
+
+	rq->rr_deadline = ktime_add_ns(rq->rr_deadline, 1);
+
+	return rq->rr_deadline;
+}
+
 struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
 {
-	struct drm_sched_job *sched_job;
+	struct drm_sched_job *sched_job, *next_job;
+	struct drm_sched_rq *rq;
 
 	sched_job = to_drm_sched_job(spsc_queue_peek(&entity->job_queue));
 	if (!sched_job)
@@ -510,24 +521,28 @@  struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
 	 * Update the entity's location in the min heap according to
 	 * the timestamp of the next job, if any.
 	 */
-	if (drm_sched_policy == DRM_SCHED_POLICY_FIFO) {
-		struct drm_sched_job *next;
-		struct drm_sched_rq *rq;
+	next_job = to_drm_sched_job(spsc_queue_peek(&entity->job_queue));
 
-		spin_lock(&entity->lock);
-		rq = entity->rq;
-		spin_lock(&rq->lock);
-		next = to_drm_sched_job(spsc_queue_peek(&entity->job_queue));
-		if (next) {
-			drm_sched_rq_update_fifo_locked(entity, rq,
-							next->submit_ts);
-		} else {
-			drm_sched_rq_remove_fifo_locked(entity, rq);
-		}
-		spin_unlock(&rq->lock);
-		spin_unlock(&entity->lock);
+	spin_lock(&entity->lock);
+	rq = entity->rq;
+	spin_lock(&rq->lock);
+
+	if (next_job) {
+		ktime_t ts;
+
+		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
+			ts = next_job->submit_ts;
+		else
+			ts = drm_sched_rq_get_rr_deadline(rq);
+
+		drm_sched_rq_update_fifo_locked(entity, rq, ts);
+	} else {
+		drm_sched_rq_remove_fifo_locked(entity, rq);
 	}
 
+	spin_unlock(&rq->lock);
+	spin_unlock(&entity->lock);
+
 	/* Jobs and entities might have different lifecycles. Since we're
 	 * removing the job from the entities queue, set the jobs entity pointer
 	 * to NULL to prevent any future access of the entity through this job.
@@ -624,9 +639,9 @@  void drm_sched_entity_push_job(struct drm_sched_job *sched_job)
 
 		spin_lock(&rq->lock);
 		drm_sched_rq_add_entity(rq, entity);
-
-		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
-			drm_sched_rq_update_fifo_locked(entity, rq, submit_ts);
+		if (drm_sched_policy == DRM_SCHED_POLICY_RR)
+			submit_ts = drm_sched_rq_get_rr_deadline(rq);
+		drm_sched_rq_update_fifo_locked(entity, rq, submit_ts);
 
 		spin_unlock(&rq->lock);
 		spin_unlock(&entity->lock);
diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index 9beb4c611988..eb22b1b7de36 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -189,7 +189,6 @@  static void drm_sched_rq_init(struct drm_gpu_scheduler *sched,
 	spin_lock_init(&rq->lock);
 	INIT_LIST_HEAD(&rq->entities);
 	rq->rb_tree_root = RB_ROOT_CACHED;
-	rq->current_entity = NULL;
 	rq->sched = sched;
 }
 
@@ -235,82 +234,13 @@  void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
 	atomic_dec(rq->sched->score);
 	list_del_init(&entity->list);
 
-	if (rq->current_entity == entity)
-		rq->current_entity = NULL;
-
-	if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
-		drm_sched_rq_remove_fifo_locked(entity, rq);
+	drm_sched_rq_remove_fifo_locked(entity, rq);
 
 	spin_unlock(&rq->lock);
 }
 
 /**
- * drm_sched_rq_select_entity_rr - Select an entity which could provide a job to run
- *
- * @sched: the gpu scheduler
- * @rq: scheduler run queue to check.
- *
- * Try to find the next ready entity.
- *
- * Return an entity if one is found; return an error-pointer (!NULL) if an
- * entity was ready, but the scheduler had insufficient credits to accommodate
- * its job; return NULL, if no ready entity was found.
- */
-static struct drm_sched_entity *
-drm_sched_rq_select_entity_rr(struct drm_gpu_scheduler *sched,
-			      struct drm_sched_rq *rq)
-{
-	struct drm_sched_entity *entity;
-
-	spin_lock(&rq->lock);
-
-	entity = rq->current_entity;
-	if (entity) {
-		list_for_each_entry_continue(entity, &rq->entities, list) {
-			if (drm_sched_entity_is_ready(entity)) {
-				/* If we can't queue yet, preserve the current
-				 * entity in terms of fairness.
-				 */
-				if (!drm_sched_can_queue(sched, entity)) {
-					spin_unlock(&rq->lock);
-					return ERR_PTR(-ENOSPC);
-				}
-
-				rq->current_entity = entity;
-				reinit_completion(&entity->entity_idle);
-				spin_unlock(&rq->lock);
-				return entity;
-			}
-		}
-	}
-
-	list_for_each_entry(entity, &rq->entities, list) {
-		if (drm_sched_entity_is_ready(entity)) {
-			/* If we can't queue yet, preserve the current entity in
-			 * terms of fairness.
-			 */
-			if (!drm_sched_can_queue(sched, entity)) {
-				spin_unlock(&rq->lock);
-				return ERR_PTR(-ENOSPC);
-			}
-
-			rq->current_entity = entity;
-			reinit_completion(&entity->entity_idle);
-			spin_unlock(&rq->lock);
-			return entity;
-		}
-
-		if (entity == rq->current_entity)
-			break;
-	}
-
-	spin_unlock(&rq->lock);
-
-	return NULL;
-}
-
-/**
- * drm_sched_rq_select_entity_fifo - Select an entity which provides a job to run
+ * drm_sched_rq_select_entity - Select an entity which provides a job to run
  *
  * @sched: the gpu scheduler
  * @rq: scheduler run queue to check.
@@ -322,32 +252,29 @@  drm_sched_rq_select_entity_rr(struct drm_gpu_scheduler *sched,
  * its job; return NULL, if no ready entity was found.
  */
 static struct drm_sched_entity *
-drm_sched_rq_select_entity_fifo(struct drm_gpu_scheduler *sched,
-				struct drm_sched_rq *rq)
+drm_sched_rq_select_entity(struct drm_gpu_scheduler *sched,
+			   struct drm_sched_rq *rq)
 {
+	struct drm_sched_entity *entity = NULL;
 	struct rb_node *rb;
 
 	spin_lock(&rq->lock);
 	for (rb = rb_first_cached(&rq->rb_tree_root); rb; rb = rb_next(rb)) {
-		struct drm_sched_entity *entity;
-
 		entity = rb_entry(rb, struct drm_sched_entity, rb_tree_node);
 		if (drm_sched_entity_is_ready(entity)) {
-			/* If we can't queue yet, preserve the current entity in
-			 * terms of fairness.
-			 */
 			if (!drm_sched_can_queue(sched, entity)) {
-				spin_unlock(&rq->lock);
-				return ERR_PTR(-ENOSPC);
+				entity = ERR_PTR(-ENOSPC);
+				break;
 			}
 
 			reinit_completion(&entity->entity_idle);
 			break;
 		}
+		entity = NULL;
 	}
 	spin_unlock(&rq->lock);
 
-	return rb ? rb_entry(rb, struct drm_sched_entity, rb_tree_node) : NULL;
+	return entity;
 }
 
 /**
@@ -1045,20 +972,18 @@  void drm_sched_wakeup(struct drm_gpu_scheduler *sched)
 static struct drm_sched_entity *
 drm_sched_select_entity(struct drm_gpu_scheduler *sched)
 {
-	struct drm_sched_entity *entity;
+	struct drm_sched_entity *entity = NULL;
 	int i;
 
 	/* Start with the highest priority.
 	 */
 	for (i = DRM_SCHED_PRIORITY_KERNEL; i < sched->num_rqs; i++) {
-		entity = drm_sched_policy == DRM_SCHED_POLICY_FIFO ?
-			drm_sched_rq_select_entity_fifo(sched, sched->sched_rq[i]) :
-			drm_sched_rq_select_entity_rr(sched, sched->sched_rq[i]);
+		entity = drm_sched_rq_select_entity(sched, sched->sched_rq[i]);
 		if (entity)
 			break;
 	}
 
-	return IS_ERR(entity) ? NULL : entity;
+	return IS_ERR(entity) ? NULL : entity;;
 }
 
 /**
diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
index 978ca621cc13..db65600732b9 100644
--- a/include/drm/gpu_scheduler.h
+++ b/include/drm/gpu_scheduler.h
@@ -245,8 +245,7 @@  struct drm_sched_entity {
  * struct drm_sched_rq - queue of entities to be scheduled.
  *
  * @sched: the scheduler to which this rq belongs to.
- * @lock: protects @entities, @rb_tree_root and @current_entity.
- * @current_entity: the entity which is to be scheduled.
+ * @lock: protects @entities, @rb_tree_root and @rr_deadline.
  * @entities: list of the entities to be scheduled.
  * @rb_tree_root: root of time based priority queue of entities for FIFO scheduling
  *
@@ -259,7 +258,7 @@  struct drm_sched_rq {
 
 	spinlock_t			lock;
 	/* Following members are protected by the @lock: */
-	struct drm_sched_entity		*current_entity;
+	ktime_t				rr_deadline;
 	struct list_head		entities;
 	struct rb_root_cached		rb_tree_root;
 };