Re: [PATCH v4] drm/sched: Add FIFO sched policy to run queue v3

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Inlined:

On 2022-09-22 12:15, Andrey Grodzovsky wrote:
> 
> On 2022-09-22 11:03, Luben Tuikov wrote:
>> The title of this patch has "v3", but "v4" in the title prefix.
>> If you're using "-v" to git-format-patch, please remove the "v3" from the title.
>>
>> Inlined:
>>
>> On 2022-09-21 14:28, Andrey Grodzovsky wrote:
>>> When many entities competing for same run queue on
>>> the same scheduler When many entities have  unacceptably long wait
>>> time for some jobs waiting stuck in the run queue before being picked
>>> up are observed (seen using  GPUVis).
>> Use this as your opening:
>>
>> "When many entities are competing for the same run queue on the same scheduler,
>> we observe an unusually long wait times and some jobs get starved. This has
>> been observed on GPUVis."
>>
>>> The issue is due to the Round Robin policy used by schedulers
>>> to pick up the next entity's job queue for execution. Under stress
>>> of many entities and long job queues within entity some
>>> jobs could be stack for very long time in it's entity's
>> "stuck", not "stack".
>>
>>> queue before being popped from the queue and executed
>>> while for other entities with smaller job queues a job
>>> might execute earlier even though that job arrived later
>>> then the job in the long queue.
>>>     
>>> Fix:
>>> Add FIFO selection policy to entities in run queue, chose next entity
>>> on run queue in such order that if job on one entity arrived
>>> earlier then job on another entity the first job will start
>>> executing earlier regardless of the length of the entity's job
>>> queue.
>>>     
>>> v2:
>>> Switch to rb tree structure for entities based on TS of
>>> oldest job waiting in the job queue of an entity. Improves next
>>> entity extraction to O(1). Entity TS update
>>> O(log N) where N is the number of entities in the run-queue
>>>     
>>> Drop default option in module control parameter.
>>>
>>> v3:
>>> Various cosmetical fixes and minor refactoring of fifo update function. (Luben)
>>>
>>> v4:
>>> Switch drm_sched_rq_select_entity_fifo to in order search (Luben)
>>>     
>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
>>> Tested-by: Li Yunxiang (Teddy) <Yunxiang.Li@xxxxxxx>
>>> ---
>>>   drivers/gpu/drm/scheduler/sched_entity.c |  26 +++++-
>>>   drivers/gpu/drm/scheduler/sched_main.c   | 107 ++++++++++++++++++++++-
>>>   include/drm/gpu_scheduler.h              |  32 +++++++
>>>   3 files changed, 159 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
>>> index 6b25b2f4f5a3..f3ffce3c9304 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>>> @@ -73,6 +73,7 @@ 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;
>>> +	RB_CLEAR_NODE(&entity->rb_tree_node);
>>>   
>>>   	if(num_sched_list)
>>>   		entity->rq = &sched_list[0]->sched_rq[entity->priority];
>>> @@ -417,14 +418,16 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
>>>   
>>>   	sched_job = to_drm_sched_job(spsc_queue_peek(&entity->job_queue));
>>>   	if (!sched_job)
>>> -		return NULL;
>>> +		goto skip;
>>>   
>>>   	while ((entity->dependency =
>>>   			drm_sched_job_dependency(sched_job, entity))) {
>>>   		trace_drm_sched_job_wait_dep(sched_job, entity->dependency);
>>>   
>>> -		if (drm_sched_entity_add_dependency_cb(entity))
>>> -			return NULL;
>>> +		if (drm_sched_entity_add_dependency_cb(entity)) {
>>> +			sched_job = NULL;
>>> +			goto skip;
>>> +		}
>>>   	}
>>>   
>>>   	/* skip jobs from entity that marked guilty */
>>> @@ -443,6 +446,16 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
>>>   	smp_wmb();
>>>   
>>>   	spsc_queue_pop(&entity->job_queue);
>>> +
>>> +	/*
>>> +	 * It's when head job is extracted we can access the next job (or empty)
>>> +	 * queue and update the entity location in the min heap accordingly.
>>> +	 */
>>> +skip:
>>> +	if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
>>> +		drm_sched_rq_update_fifo(entity,
>>> +					 (sched_job ? sched_job->submit_ts : ktime_get()));
>>> +
>>>   	return sched_job;
>>>   }
>>>   
>>> @@ -502,11 +515,13 @@ void drm_sched_entity_push_job(struct drm_sched_job *sched_job)
>>>   {
>>>   	struct drm_sched_entity *entity = sched_job->entity;
>>>   	bool first;
>>> +	ktime_t ts =  ktime_get();
>>>   
>>>   	trace_drm_sched_job(sched_job, entity);
>>>   	atomic_inc(entity->rq->sched->score);
>>>   	WRITE_ONCE(entity->last_user, current->group_leader);
>>>   	first = spsc_queue_push(&entity->job_queue, &sched_job->queue_node);
>>> +	sched_job->submit_ts = ts;
>>>   
>>>   	/* first job wakes up scheduler */
>>>   	if (first) {
>>> @@ -518,8 +533,13 @@ void drm_sched_entity_push_job(struct drm_sched_job *sched_job)
>>>   			DRM_ERROR("Trying to push to a killed entity\n");
>>>   			return;
>>>   		}
>>> +
>>>   		drm_sched_rq_add_entity(entity->rq, entity);
>>>   		spin_unlock(&entity->rq_lock);
>>> +
>>> +		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
>>> +			drm_sched_rq_update_fifo(entity, ts);
>>> +
>>>   		drm_sched_wakeup(entity->rq->sched);
>>>   	}
>>>   }
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 4f2395d1a791..565707a1c5c7 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -62,6 +62,64 @@
>>>   #define to_drm_sched_job(sched_job)		\
>>>   		container_of((sched_job), struct drm_sched_job, queue_node)
>>>   
>>> +int drm_sched_policy = DRM_SCHED_POLICY_RR;
>>> +
>>> +/**
>>> + * DOC: sched_policy (int)
>>> + * Used to override default entities scheduling policy in a run queue.
>>> + */
>>> +MODULE_PARM_DESC(sched_policy,
>>> +		 "specify schedule policy for entities on a runqueue (DRM_SCHED_POLICY_RR = Round Robin (default), DRM_SCHED_POLICY_FIFO  = use FIFO");
>>> +module_param_named(sched_policy, drm_sched_policy, int, 0444);
>>> +
>>> +static __always_inline bool drm_sched_entity_compare_before(struct rb_node *a,
>>> +							    const struct rb_node *b)
>>> +{
>>> +	struct drm_sched_entity *ent_a =  rb_entry((a), struct drm_sched_entity, rb_tree_node);
>>> +	struct drm_sched_entity *ent_b =  rb_entry((b), struct drm_sched_entity, rb_tree_node);
>>> +
>>> +	return ktime_before(ent_a->oldest_job_waiting, ent_b->oldest_job_waiting);
>>> +}
>>> +
>>> +static inline void drm_sched_rq_remove_fifo_locked(struct drm_sched_entity *entity)
>>> +{
>>> +	struct drm_sched_rq *rq = entity->rq;
>>> +
>>> +	if (!RB_EMPTY_NODE(&entity->rb_tree_node)) {
>>> +		rb_erase_cached(&entity->rb_tree_node, &rq->rb_tree_root);
>>> +		RB_CLEAR_NODE(&entity->rb_tree_node);
>>> +	}
>>> +}
>>> +
>>> +static inline void drm_sched_rq_update_fifo_locked(struct drm_sched_entity *entity,
>>> +						   ktime_t ts)
>>> +{
>>> +	struct drm_sched_rq *rq = entity->rq;
>>> +
>>> +	drm_sched_rq_remove_fifo_locked(entity);
>>> +
>>> +	entity->oldest_job_waiting = ts;
>>> +
>>> +	rb_add_cached(&entity->rb_tree_node, &rq->rb_tree_root,
>>> +		      drm_sched_entity_compare_before);
>>> +}
>>> +
>>> +void drm_sched_rq_update_fifo(struct drm_sched_entity *entity, ktime_t ts)
>>> +{
>>> +	/*
>>> +	 * Both locks need to be grabbed, one to protect from entity->rq change
>>> +	 * for entity from within concurrent drm_sched_entity_select_rq and the
>>> +	 * other to update the rb tree structure.
>>> +	 */
>>> +	spin_lock(&entity->rq_lock);
>>> +	spin_lock(&entity->rq->lock);
>>> +
>>> +	drm_sched_rq_update_fifo_locked(entity, ts);
>>> +
>>> +	spin_unlock(&entity->rq->lock);
>>> +	spin_unlock(&entity->rq_lock);
>>> +}
>> I thought you were going to drop one of the locks here?
>> Address this by either updating the comment to explain to future programmers
>> what is going on here and why the locking is not consistent (2 vs 1 lock),
>> or drop one of the locks, as per my previous review.
> 
> 
> Note that after last review drm_sched_rq_update_fifo_locked is not 
> called anywhere
> besides drm_sched_rq_update_fifo and so becomes obsolete and I will 
> remove it now.
> In this case the double locking above is consistent and the reason is 
> explained in the
> comment above.

WHen you say "it's consistent" you mean it's being done from other places, I suppose.


> 
> 
>>
>>> +
>>>   /**
>>>    * drm_sched_rq_init - initialize a given run queue struct
>>>    *
>>> @@ -75,6 +133,7 @@ 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;
>>>   }
>>> @@ -92,9 +151,12 @@ void drm_sched_rq_add_entity(struct drm_sched_rq *rq,
>>>   {
>>>   	if (!list_empty(&entity->list))
>>>   		return;
>>> +
>>>   	spin_lock(&rq->lock);
>>> +
>>>   	atomic_inc(rq->sched->score);
>>>   	list_add_tail(&entity->list, &rq->entities);
>>> +
>>>   	spin_unlock(&rq->lock);
>>>   }
>>>   
>>> @@ -111,23 +173,30 @@ void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
>>>   {
>>>   	if (list_empty(&entity->list))
>>>   		return;
>>> +
>>>   	spin_lock(&rq->lock);
>>> +
>>>   	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);
>>> +
>>>   	spin_unlock(&rq->lock);
>>>   }
>>>   
>>>   /**
>>> - * drm_sched_rq_select_entity - Select an entity which could provide a job to run
>>> + * drm_sched_rq_select_entity_rr - Select an entity which could provide a job to run
>>>    *
>>>    * @rq: scheduler run queue to check.
>>>    *
>>>    * Try to find a ready entity, returns NULL if none found.
>>>    */
>>>   static struct drm_sched_entity *
>>> -drm_sched_rq_select_entity(struct drm_sched_rq *rq)
>>> +drm_sched_rq_select_entity_rr(struct drm_sched_rq *rq)
>>>   {
>>>   	struct drm_sched_entity *entity;
>>>   
>>> @@ -163,6 +232,36 @@ drm_sched_rq_select_entity(struct drm_sched_rq *rq)
>>>   	return NULL;
>>>   }
>>>   
>>> +/**
>>> + * drm_sched_rq_select_entity_fifo - Select an entity which provides a job to run
>>> + *
>>> + * @rq: scheduler run queue to check.
>>> + *
>>> + * Find oldest waiting ready entity, returns NULL if none found.
>>> + */
>>> +static struct drm_sched_entity *
>>> +drm_sched_rq_select_entity_fifo(struct drm_sched_rq *rq)
>>> +{
>>> +	struct rb_node *rb;
>>> +	bool found = false;
>>> +	struct drm_sched_entity *entity;
>>> +
>>> +	spin_lock(&rq->lock);
>>> +	for (rb = rb_first_cached(&rq->rb_tree_root); rb; rb = rb_next(rb)) {
>>> +		entity = rb_entry(rb, struct drm_sched_entity, rb_tree_node);
>>> +
>>> +		if (drm_sched_entity_is_ready(entity)) {
>>> +			rq->current_entity = entity;
>>> +			reinit_completion(&entity->entity_idle);
>>> +			found = true;
>>> +			break;
>>> +		}
>>> +	}
>>> +	spin_unlock(&rq->lock);
>>> +
>>> +	return found ? entity : NULL;
>>> +}
>> You really don't need "found", and you don't need "entity" to be outside the loop.
>>
>> As per my last review, use this (which I've compiled and run):
>>
>> static struct drm_sched_entity *
>> drm_sched_rq_select_entity_fifo(struct drm_sched_rq *rq)
>> {
>> 	struct rb_node *rb;
>>
>> 	spin_lock(&rq->lock);
>> 	for (rb = rb_first_cached(&rq->rb_tree_root); rb; rb = rb_next(rb)) {
>> 		struct drm_sched_entity *entity;
>>
>> 		entity = rb_entry(rb, struct drm_sched_entity, rb_tree_node);
>> 		if (drm_sched_entity_is_ready(entity)) {
>> 			rq->current_entity = entity;
>> 			reinit_completion(&entity->entity_idle);
>> 			break;
>> 		}
>> 	}
>> 	spin_unlock(&rq->lock);
>>
>> 	return rb ? rb_entry(rb, struct drm_sched_entity, rb_tree_node) : NULL;
>> }
>>
>> The only way we can exit the loop is,
>> 1. The loop invariant is false, i.e. rb == NULL, or
>> 2. The "break;" jump from inside the if () inside the loop.
>>
>> Also note that "rb" does NOT need to be initialized, since, the for() statement
>> is always executed, and the assignment "rb = rb_first_cached(&rq->rb_tree_root);"
>> initializes it--this is why GCC doesn't complain :-)
>>
>> Also note that you don't need to export "entity" as it makes the for() loop relocatable
>> to another function with only having a dependency on "rb" being defined--the loop
>> is self-contained.
>>
>> At the "return" statement, we know that we've exited the loop, by either the loop
>> invariant being false, i.e. rb == NULL, or by the "break;" jump, in which case
>> we know that rb != NULL. This is why the "return;" statement as presented above works
>> correctly.
>>
>> Please use that function in the way it is above, which is minimal and mature.
> 
> 
> Makes sense, missed the point that at the end rb will be set to NULL

Yeah, use the loop as I've written it above.

Okay, send out v5.

Regards,
Luben

> 
> Andrey
> 
> 
>>
>> Regards,
>> Luben
>>
>>> +
>>>   /**
>>>    * drm_sched_job_done - complete a job
>>>    * @s_job: pointer to the job which is done
>>> @@ -803,7 +902,9 @@ drm_sched_select_entity(struct drm_gpu_scheduler *sched)
>>>   
>>>   	/* Kernel run queue has higher priority than normal run queue*/
>>>   	for (i = DRM_SCHED_PRIORITY_COUNT - 1; i >= DRM_SCHED_PRIORITY_MIN; i--) {
>>> -		entity = drm_sched_rq_select_entity(&sched->sched_rq[i]);
>>> +		entity = drm_sched_policy != DRM_SCHED_POLICY_FIFO ?
>>> +				drm_sched_rq_select_entity_rr(&sched->sched_rq[i]) :
>>> +				drm_sched_rq_select_entity_fifo(&sched->sched_rq[i]);
>>>   		if (entity)
>>>   			break;
>>>   	}
>>> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
>>> index 599855c6a672..1f7d9dd1a444 100644
>>> --- a/include/drm/gpu_scheduler.h
>>> +++ b/include/drm/gpu_scheduler.h
>>> @@ -50,6 +50,12 @@ enum drm_sched_priority {
>>>   	DRM_SCHED_PRIORITY_UNSET = -2
>>>   };
>>>   
>>> +/* Used to chose between FIFO and RR jobs scheduling */
>>> +extern int drm_sched_policy;
>>> +
>>> +#define DRM_SCHED_POLICY_RR    0
>>> +#define DRM_SCHED_POLICY_FIFO  1
>>> +
>>>   /**
>>>    * struct drm_sched_entity - A wrapper around a job queue (typically
>>>    * attached to the DRM file_priv).
>>> @@ -196,6 +202,21 @@ struct drm_sched_entity {
>>>   	 * drm_sched_entity_fini().
>>>   	 */
>>>   	struct completion		entity_idle;
>>> +
>>> +	/**
>>> +	 * @oldest_job_waiting:
>>> +	 *
>>> +	 * Marks earliest job waiting in SW queue
>>> +	 */
>>> +	ktime_t				oldest_job_waiting;
>>> +
>>> +	/**
>>> +	 * @rb_tree_node:
>>> +	 *
>>> +	 * The node used to insert this entity into time based priority queue
>>> +	 */
>>> +	struct rb_node			rb_tree_node;
>>> +
>>>   };
>>>   
>>>   /**
>>> @@ -205,6 +226,7 @@ struct drm_sched_entity {
>>>    * @sched: the scheduler to which this rq belongs to.
>>>    * @entities: list of the entities to be scheduled.
>>>    * @current_entity: the entity which is to be scheduled.
>>> + * @rb_tree_root: root of time based priory queue of entities for FIFO scheduling
>>>    *
>>>    * Run queue is a set of entities scheduling command submissions for
>>>    * one specific ring. It implements the scheduling policy that selects
>>> @@ -215,6 +237,7 @@ struct drm_sched_rq {
>>>   	struct drm_gpu_scheduler	*sched;
>>>   	struct list_head		entities;
>>>   	struct drm_sched_entity		*current_entity;
>>> +	struct rb_root_cached		rb_tree_root;
>>>   };
>>>   
>>>   /**
>>> @@ -314,6 +337,13 @@ struct drm_sched_job {
>>>   
>>>   	/** @last_dependency: tracks @dependencies as they signal */
>>>   	unsigned long			last_dependency;
>>> +
>>> +	/**
>>> +	 * @submit_ts:
>>> +	 *
>>> +	 * When the job was pushed into the entity queue.
>>> +	 */
>>> +	ktime_t                         submit_ts;
>>>   };
>>>   
>>>   static inline bool drm_sched_invalidate_job(struct drm_sched_job *s_job,
>>> @@ -503,6 +533,8 @@ void drm_sched_rq_add_entity(struct drm_sched_rq *rq,
>>>   void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
>>>   				struct drm_sched_entity *entity);
>>>   
>>> +void drm_sched_rq_update_fifo(struct drm_sched_entity *entity, ktime_t ts);
>>> +
>>>   int drm_sched_entity_init(struct drm_sched_entity *entity,
>>>   			  enum drm_sched_priority priority,
>>>   			  struct drm_gpu_scheduler **sched_list,



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