[PATCH] drm/scheduler: fix race condition in load balancer

[Christian König]

Am 14.01.20 um 16:43 schrieb Nirmoy Das:
> Jobs submitted in an entity should execute in the order those jobs
> are submitted. We make sure that by checking entity->job_queue in
> drm_sched_entity_select_rq() so that we don't loadbalance jobs within
> an entity.
>
> But because we update entity->job_queue later in drm_sched_entity_push_job(),
> there remains a open window when it is possibe that entity->rq might get
> updated by drm_sched_entity_select_rq() which should not be allowed.

NAK, concurrent calls to 
drm_sched_job_init()/drm_sched_entity_push_job() are not allowed in the 
first place or otherwise we mess up the fence sequence order and risk 
memory corruption.

>
> Changes in this part also improves job distribution.
> Below are test results after running amdgpu_test from mesa drm
>
> Before this patch:
>
> sched_name     num of many times it got scheduled
> =========      ==================================
> sdma0          314
> sdma1          32
> comp_1.0.0     56
> comp_1.1.0     0
> comp_1.1.1     0
> comp_1.2.0     0
> comp_1.2.1     0
> comp_1.3.0     0
> comp_1.3.1     0
>
> After this patch:
>
> sched_name     num of many times it got scheduled
> =========      ==================================
>   sdma1          243
>   sdma0          164
>   comp_1.0.1     14
>   comp_1.1.0     11
>   comp_1.1.1     10
>   comp_1.2.0     15
>   comp_1.2.1     14
>   comp_1.3.0     10
>   comp_1.3.1     10

Well that is still rather nice to have, why does that happen?

Christian.

>
> Fixes: 35e160e781a048 (drm/scheduler: change entities rq even earlier)
>
> Signed-off-by: Nirmoy Das <nirmoy.das at amd.com>
> Reported-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer at amd.com>
> ---
>   drivers/gpu/drm/scheduler/sched_entity.c | 9 +++++++--
>   drivers/gpu/drm/scheduler/sched_main.c   | 1 +
>   include/drm/gpu_scheduler.h              | 1 +
>   3 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> index 2e3a058fc239..8414e084b6ac 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -67,6 +67,7 @@ int drm_sched_entity_init(struct drm_sched_entity *entity,
>   	entity->priority = priority;
>   	entity->sched_list = num_sched_list > 1 ? sched_list : NULL;
>   	entity->last_scheduled = NULL;
> +	entity->loadbalance_on = true;
>   
>   	if(num_sched_list)
>   		entity->rq = &sched_list[0]->sched_rq[entity->priority];
> @@ -447,6 +448,9 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
>   	entity->last_scheduled = dma_fence_get(&sched_job->s_fence->finished);
>   
>   	spsc_queue_pop(&entity->job_queue);
> +	if (!spsc_queue_count(&entity->job_queue))
> +		entity->loadbalance_on = true;
> +
>   	return sched_job;
>   }
>   
> @@ -463,7 +467,8 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
>   	struct dma_fence *fence;
>   	struct drm_sched_rq *rq;
>   
> -	if (spsc_queue_count(&entity->job_queue) || entity->num_sched_list <= 1)
> +	atomic_inc(&entity->rq->sched->num_jobs);
> +	if ((entity->num_sched_list <= 1) || !entity->loadbalance_on)
>   		return;
>   
>   	fence = READ_ONCE(entity->last_scheduled);
> @@ -477,6 +482,7 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
>   		entity->rq = rq;
>   	}
>   
> +	entity->loadbalance_on = false;
>   	spin_unlock(&entity->rq_lock);
>   }
>   
> @@ -498,7 +504,6 @@ void drm_sched_entity_push_job(struct drm_sched_job *sched_job,
>   	bool first;
>   
>   	trace_drm_sched_job(sched_job, entity);
> -	atomic_inc(&entity->rq->sched->num_jobs);
>   	WRITE_ONCE(entity->last_user, current->group_leader);
>   	first = spsc_queue_push(&entity->job_queue, &sched_job->queue_node);
>   
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 3fad5876a13f..00fdc350134e 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -562,6 +562,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>   		return -ENOENT;
>   
>   	sched = entity->rq->sched;
> +	atomic_inc(&entity->rq->sched->num_jobs);
>   
>   	job->sched = sched;
>   	job->entity = entity;
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index 96a1a1b7526e..a5190869d323 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -97,6 +97,7 @@ struct drm_sched_entity {
>   	struct dma_fence                *last_scheduled;
>   	struct task_struct		*last_user;
>   	bool 				stopped;
> +	bool				loadbalance_on;
>   	struct completion		entity_idle;
>   };
>