+ Christian's e-mail which he reads :)
On 10/09/2026 08:59, 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.
FWIW the entry paths that I found from the amdgpu side are from the job
submission side where it is called a few times for each job via
amdgpu_ctx_get_entity(), amdgpu_job_prepare_job() and
amdgpu_vm_generation(). Last one being both in amdgpu_job_alloc() and
amdgpu_job_run(). If I did not miss any, the change effectively adds
four lock-unlock cycles to the job submission path. Plus one more for
schedulers which use load balancing in some cases.
I don't know if avoiding that was the reason RCU was chosen, perhaps
Christian remembers.
Also, to repeat what I said before, I benchmarked this a bit under
extreme circumstances (4x non vsynced vkgears) and did not find a
regression in fps.
Improve robustness, readability and maintainability by replacing RCU and
barriers with the lock.
Signed-off-by: Philipp Stanner <[email protected]>
---
drivers/gpu/drm/scheduler/sched_entity.c | 50 ++++++++++--------------
include/drm/gpu_scheduler.h | 10 ++---
2 files changed, 24 insertions(+), 36 deletions(-)
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
b/drivers/gpu/drm/scheduler/sched_entity.c
index e168f445f2ab..fa4a9388e5c9 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -136,7 +136,6 @@ int drm_sched_entity_init(struct drm_sched_entity *entity,
DRM_SCHED_PRIORITY_KERNEL : priority;
entity->num_sched_list = num_sched_list;
entity->sched_list = num_sched_list > 1 ? sched_list : NULL;
- RCU_INIT_POINTER(entity->last_scheduled, NULL);
RB_CLEAR_NODE(&entity->rb_tree_node);
if (!sched_list[0]->sched_rq) {
@@ -233,10 +232,10 @@ 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;
}
@@ -319,9 +318,10 @@ 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);
Here you can compact with the idiomatic:
prev = dma_fence_get(entity->last_scheduled);
Otherwise looks plausible to me. Lets see what sashiko will have to say
this time round.
Regards,
Tvrtko
+ spin_unlock(&entity->lock);
while ((job = drm_sched_entity_queue_pop(entity))) {
struct drm_sched_fence *s_fence = job->s_fence;
@@ -413,8 +413,7 @@ 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);
@@ -536,6 +535,10 @@ 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.
+ */
+ struct dma_fence *prev_last_scheduled;
struct drm_sched_job *sched_job;
sched_job = drm_sched_entity_queue_peek(entity);
@@ -552,22 +555,15 @@ 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);
spsc_queue_pop(&entity->job_queue);
drm_sched_rq_pop_entity(entity);
spin_unlock(&entity->lock);
+ dma_fence_put(prev_last_scheduled);
+
/* 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.
@@ -591,21 +587,15 @@ 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->sched_rq[entity->rq_priority] : NULL;
if (rq != entity->rq) {
diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
index 7a64cc11de08..aeeea6efc623 100644
--- a/include/drm/gpu_scheduler.h
+++ b/include/drm/gpu_scheduler.h
@@ -100,8 +100,8 @@ struct drm_sched_entity {
* @lock:
*
* Lock protecting the run-queue (@rq) to which this entity belongs,
- * @priority, the list of schedulers (@sched_list, @num_sched_list) and
- * the @rr_ts field.
+ * @priority, @last_scheduled and the list of schedulers (@sched_list,
+ * @num_sched_list).
*/
spinlock_t lock;
@@ -215,11 +215,9 @@ 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.
