[Linaro-mm-sig] Re: [PATCH 1/5] drm/sched: Protect entity->last_scheduled with spinlock

On 01/07/2026 09:59, Philipp Stanner wrote:

The entity->last_scheduled field has always been set and read with special RCU functions in addition to memory barriers.

This was added in

commit 70102d77ff22 ("drm/scheduler: add drm_sched_entity_error and use rcu for last_scheduled")

however, no proper justification for that mechanism was provided. There seems to be no obvious reason, since the entity lock is available and taken at all places that evaluate the last_scheduled field. The only exception is drm_sched_entity_error(), which is not performance critical in any way.

Improve robustness, readability and maintainability by replacing RCU and barriers with the lock.

First thing, and regardless of other strands of discussion, I think it should be squashed with 3/5 instead of that one undoing the introduction of lock-unlock-lock-unlock.

For what the main topic is concerned, I really like the removal of all the rcu_dereference_check(, true) lines and the memory barriers.

But I also think the commit message should explain better what code paths are now taking an extra lock - under which circumstances is the lock now taken for all scheduler users, and which amdgpu paths use drm_sched_entity_error() a lot so could be affected. I doubt it creates a measurable performance impact but it needs to be explained.

I am also happy to give it a spin on the Steam Deck to see if I can observe anything.

Signed-off-by: Philipp Stanner phasta@kernel.org

drivers/gpu/drm/scheduler/sched_entity.c | 50 ++++++++++-------------- include/drm/gpu_scheduler.h | 9 ++--- 2 files changed, 25 insertions(+), 34 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c index c51101ec70c1..91aec20611ad 100644 --- a/drivers/gpu/drm/scheduler/sched_entity.c +++ b/drivers/gpu/drm/scheduler/sched_entity.c @@ -135,7 +135,6 @@ int drm_sched_entity_init(struct drm_sched_entity *entity, entity->num_sched_list = num_sched_list; entity->sched_list = num_sched_list > 1 ? sched_list : NULL; entity->rq = &sched_list[0]->rq;

• RCU_INIT_POINTER(entity->last_scheduled, NULL); RB_CLEAR_NODE(&entity->rb_tree_node); init_completion(&entity->entity_idle);

@@ -201,10 +200,10 @@ int drm_sched_entity_error(struct drm_sched_entity *entity) struct dma_fence *fence; int r;

• rcu_read_lock();
• fence = rcu_dereference(entity->last_scheduled);

• spin_lock(&entity->lock);
• fence = entity->last_scheduled; r = fence ? fence->error : 0;

• rcu_read_unlock();

• spin_unlock(&entity->lock);

return r; } @@ -287,9 +286,10 @@ void drm_sched_entity_kill(struct drm_sched_entity *entity) /* Make sure this entity is not used by the scheduler at the moment */ wait_for_completion(&entity->entity_idle);

• /* The entity is guaranteed to not be used by the scheduler */
• prev = rcu_dereference_check(entity->last_scheduled, true);

• spin_lock(&entity->lock);
• prev = entity->last_scheduled; dma_fence_get(prev);
• spin_unlock(&entity->lock); while ((job = drm_sched_entity_queue_pop(entity))) { struct drm_sched_fence *s_fence = job->s_fence;

@@ -381,8 +381,7 @@ void drm_sched_entity_fini(struct drm_sched_entity *entity) entity->dependency = NULL; }

• dma_fence_put(rcu_dereference_check(entity->last_scheduled, true));
• RCU_INIT_POINTER(entity->last_scheduled, NULL);

• dma_fence_put(entity->last_scheduled); drm_sched_entity_stats_put(entity->stats); } EXPORT_SYMBOL(drm_sched_entity_fini);

@@ -507,6 +506,10 @@ drm_sched_job_dependency(struct drm_sched_job *job, struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity) {

• /* Helper to avoid dropping the reference while the entity lock is held,

• * just to have some more robustness.
  

• */
  

I don't get this comment. Neither the placement or the content.

Regards,

Tvrtko

• struct dma_fence *prev_last_scheduled; struct drm_sched_job *sched_job;

sched_job = drm_sched_entity_queue_peek(entity); @@ -523,19 +526,14 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity) if (entity->guilty && atomic_read(entity->guilty)) dma_fence_set_error(&sched_job->s_fence->finished, -ECANCELED);

• dma_fence_put(rcu_dereference_check(entity->last_scheduled, true));
• rcu_assign_pointer(entity->last_scheduled,

• 	   dma_fence_get(&sched_job->s_fence->finished));
  

• /*

• * If the queue is empty we allow drm_sched_entity_select_rq() to
  

• * locklessly access ->last_scheduled. This only works if we set the
  

• * pointer before we dequeue and if we a write barrier here.
  

• */
  

• smp_wmb();

• spin_lock(&entity->lock);
• prev_last_scheduled = entity->last_scheduled;
• entity->last_scheduled = dma_fence_get(&sched_job->s_fence->finished);
• spin_unlock(&entity->lock);

spsc_queue_pop(&entity->job_queue);

• dma_fence_put(prev_last_scheduled); drm_sched_rq_pop_entity(entity);

/* Jobs and entities might have different lifecycles. Since we're @@ -561,21 +559,15 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity) if (spsc_queue_count(&entity->job_queue)) return;

• /*

• * Only when the queue is empty are we guaranteed that
  

• * drm_sched_run_job_work() cannot change entity->last_scheduled. To
  

• * enforce ordering we need a read barrier here. See
  

• * drm_sched_entity_pop_job() for the other side.
  

• */
  

• smp_rmb();
• fence = rcu_dereference_check(entity->last_scheduled, true);

• spin_lock(&entity->lock);
• fence = entity->last_scheduled;

/* stay on the same engine if the previous job hasn't finished */

• if (fence && !dma_fence_is_signaled(fence))

• if (fence && !dma_fence_is_signaled(fence)) {

• spin_unlock(&entity->lock);
  

  return;
• }

• spin_lock(&entity->lock); sched = drm_sched_pick_best(entity->sched_list, entity->num_sched_list); rq = sched ? &sched->rq : NULL; if (rq != entity->rq) {

diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h index d61c19e78182..176ff1f936cd 100644 --- a/include/drm/gpu_scheduler.h +++ b/include/drm/gpu_scheduler.h @@ -100,7 +100,8 @@ 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, @last_scheduled and the list of schedulers (@sched_list,
  

• * @num_sched_list).
  

  */ spinlock_t lock;

@@ -202,11 +203,9 @@ struct drm_sched_entity { /** * @last_scheduled: *

• * Points to the finished fence of the last scheduled job. Only written
  

• * by drm_sched_entity_pop_job(). Can be accessed locklessly from
  

• * drm_sched_job_arm() if the queue is empty.
  

• * Points to the finished fence of the last scheduled job.
  

  */

• struct dma_fence __rcu *last_scheduled;

• struct dma_fence *last_scheduled;

/** * @last_user: last group leader pushing a job into the entity.