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.
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. + */ + 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.