[RFC v7 03/12] drm/sched: Implement RR via FIFO
Tvrtko Ursulin
tvrtko.ursulin at igalia.com
Thu Jul 24 14:19:12 UTC 2025
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 simply bump the picked one to the bottom of the tree, which
ensures round-robin behaviour between all active queued jobs.
If the picked job was the last from a given entity, we remember the
assigned fake timestamp and use it to re-insert the job once it re-joins
the queue. This ensures job neither overtakes all already queued jobs,
neither it goes last. Instead it keeps the position after the currently
queued jobs and before the ones which haven't yet been queued at the point
the entity left the queue.
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 <tvrtko.ursulin at igalia.com>
Cc: Christian König <christian.koenig at amd.com>
Cc: Danilo Krummrich <dakr at kernel.org>
Cc: Matthew Brost <matthew.brost at intel.com>
Cc: Philipp Stanner <phasta at kernel.org>
---
drivers/gpu/drm/scheduler/sched_entity.c | 51 ++++++++++------
drivers/gpu/drm/scheduler/sched_main.c | 76 ++----------------------
include/drm/gpu_scheduler.h | 16 +++--
3 files changed, 51 insertions(+), 92 deletions(-)
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
index 8867b95ab089..a7ba6dcede23 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -451,9 +451,24 @@ drm_sched_job_dependency(struct drm_sched_job *job,
return NULL;
}
+static ktime_t
+drm_sched_rq_get_rr_ts(struct drm_sched_rq *rq, struct drm_sched_entity *entity)
+{
+ ktime_t ts;
+
+ lockdep_assert_held(&entity->lock);
+ lockdep_assert_held(&rq->lock);
+
+ ts = ktime_add_ns(rq->rr_ts, 1);
+ entity->rr_ts = ts;
+ rq->rr_ts = ts;
+
+ return ts;
+}
+
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;
sched_job = drm_sched_entity_queue_peek(entity);
if (!sched_job)
@@ -488,21 +503,21 @@ 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;
+ next_job = drm_sched_entity_queue_peek(entity);
+ if (next_job) {
+ struct drm_sched_rq *rq;
+ ktime_t ts;
- next = drm_sched_entity_queue_peek(entity);
- if (next) {
- struct drm_sched_rq *rq;
-
- spin_lock(&entity->lock);
- rq = entity->rq;
- spin_lock(&rq->lock);
- drm_sched_rq_update_fifo_locked(entity, rq,
- next->submit_ts);
- spin_unlock(&rq->lock);
- spin_unlock(&entity->lock);
- }
+ spin_lock(&entity->lock);
+ rq = entity->rq;
+ spin_lock(&rq->lock);
+ if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
+ ts = next_job->submit_ts;
+ else
+ ts = drm_sched_rq_get_rr_ts(rq, entity);
+ drm_sched_rq_update_fifo_locked(entity, rq, ts);
+ spin_unlock(&rq->lock);
+ spin_unlock(&entity->lock);
}
/* Jobs and entities might have different lifecycles. Since we're
@@ -609,9 +624,9 @@ 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 = entity->rr_ts;
+ 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 5a550fd76bf0..a0fca48ede9f 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -185,7 +185,6 @@ 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;
}
@@ -231,74 +230,13 @@ 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))
- goto found;
- }
- }
-
- list_for_each_entry(entity, &rq->entities, list) {
- if (drm_sched_entity_is_ready(entity))
- goto found;
-
- if (entity == rq->current_entity)
- break;
- }
-
- spin_unlock(&rq->lock);
-
- return NULL;
-
-found:
- if (!drm_sched_can_queue(sched, entity)) {
- /*
- * If scheduler cannot take more jobs signal the caller to not
- * consider lower priority queues.
- */
- entity = ERR_PTR(-ENOSPC);
- } else {
- rq->current_entity = entity;
- reinit_completion(&entity->entity_idle);
- }
-
- spin_unlock(&rq->lock);
-
- return entity;
-}
-
-/**
- * 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.
@@ -310,8 +248,8 @@ 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 rb_node *rb;
@@ -1093,15 +1031,13 @@ 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;
}
diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
index 323a505e6e6a..802a0060db55 100644
--- a/include/drm/gpu_scheduler.h
+++ b/include/drm/gpu_scheduler.h
@@ -94,7 +94,8 @@ struct drm_sched_entity {
* @lock:
*
* Lock protecting the run-queue (@rq) to which this entity belongs,
- * @priority and the list of schedulers (@sched_list, @num_sched_list).
+ * @priority, the list of schedulers (@sched_list, @num_sched_list) and
+ * the @rr_ts field.
*/
spinlock_t lock;
@@ -142,6 +143,13 @@ struct drm_sched_entity {
*/
enum drm_sched_priority priority;
+ /**
+ * @rr_ts:
+ *
+ * Fake timestamp of the last popped job from the entity.
+ */
+ ktime_t rr_ts;
+
/**
* @job_queue: the list of jobs of this entity.
*/
@@ -239,8 +247,8 @@ 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_ts.
+ * @rr_ts: monotonically incrementing fake timestamp for RR mode
* @entities: list of the entities to be scheduled.
* @rb_tree_root: root of time based priority queue of entities for FIFO scheduling
*
@@ -253,7 +261,7 @@ struct drm_sched_rq {
spinlock_t lock;
/* Following members are protected by the @lock: */
- struct drm_sched_entity *current_entity;
+ ktime_t rr_ts;
struct list_head entities;
struct rb_root_cached rb_tree_root;
};
--
2.48.0
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