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Message-ID: <3d7beeb5-1f10-4837-9c14-6a0f40d83b3c@redhat.com>
Date:   Fri, 27 Oct 2023 16:45:37 +0200
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User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH drm-misc-next v3] drm/sched: implement dynamic job-flow
 control
Content-Language: en-US
To:     Luben Tuikov <ltuikov89@gmail.com>, airlied@gmail.com,
        daniel@ffwll.ch, matthew.brost@intel.com,
        boris.brezillon@collabora.com, christian.koenig@amd.com,
        faith@gfxstrand.net, luben.tuikov@amd.com
Cc:     linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
References: <20231026161431.5934-1-dakr@redhat.com>
 <c95c4dbf-3d97-477a-9074-a4e6a4394189@gmail.com>
From:   Danilo Krummrich <dakr@redhat.com>
Organization: RedHat
In-Reply-To: <c95c4dbf-3d97-477a-9074-a4e6a4394189@gmail.com>
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Precedence: bulk

Hi Luben,

On 10/26/23 23:13, Luben Tuikov wrote:
> On 2023-10-26 12:13, Danilo Krummrich wrote:
>> Currently, job flow control is implemented simply by limiting the number
>> of jobs in flight. Therefore, a scheduler is initialized with a credit
>> limit that corresponds to the number of jobs which can be sent to the
>> hardware.
>>
>> This implies that for each job, drivers need to account for the maximum
>> job size possible in order to not overflow the ring buffer.
>>
>> However, there are drivers, such as Nouveau, where the job size has a
>> rather large range. For such drivers it can easily happen that job
>> submissions not even filling the ring by 1% can block subsequent
>> submissions, which, in the worst case, can lead to the ring run dry.
>>
>> In order to overcome this issue, allow for tracking the actual job size
>> instead of the number of jobs. Therefore, add a field to track a job's
>> credit count, which represents the number of credits a job contributes
>> to the scheduler's credit limit.
>>
>> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
>> ---
>> Changes in V2:
>> ==============
>>    - fixed up influence on scheduling fairness due to consideration of a job's
>>      size
>>      - If we reach a ready entity in drm_sched_select_entity() but can't actually
>>        queue a job from it due to size limitations, just give up and go to sleep
>>        until woken up due to a pending job finishing, rather than continue to try
>>        other entities.
>>    - added a callback to dynamically update a job's credits (Boris)
>>    - renamed 'units' to 'credits'
>>    - fixed commit message and comments as requested by Luben
>>
>> Changes in V3:
>> ==============
>>    - rebased onto V7 of the "DRM scheduler changes for Xe" series by Matt
>>    - move up drm_sched_can_queue() instead of adding a forward declaration
>>      (Boris)
>>    - add a drm_sched_available_credits() helper (Boris)
>>    - adjust control flow in drm_sched_rq_select_entity_fifo() to Luben's proposal
>>    - re-phrase a few comments and fix a typo (Luben)
>>    - change naming of all structures credit fields and function parameters to the
>>      following scheme
>>      - drm_sched_job::credits
>>      - drm_gpu_scheduler::credit_limit
>>      - drm_gpu_scheduler::credit_count
>>      - drm_sched_init(..., u32 credit_limit, ...)
>>      - drm_sched_job_init(..., u32 credits, ...)
>>    - add proper documentation for the scheduler's job-flow control mechanism
>>
>> This patch is based on V7 of the "DRM scheduler changes for Xe" series. [1]
>> provides a branch based on drm-misc-next, with the named series and this patch
>> on top of it.
>>
>> [1] https://gitlab.freedesktop.org/nouvelles/kernel/-/commits/sched-credits/
>> ---
>>   Documentation/gpu/drm-mm.rst                  |   6 +
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_job.c       |   2 +-
>>   drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c  |   2 +-
>>   drivers/gpu/drm/lima/lima_sched.c             |   2 +-
>>   drivers/gpu/drm/msm/msm_gem_submit.c          |   2 +-
>>   drivers/gpu/drm/nouveau/nouveau_sched.c       |   2 +-
>>   drivers/gpu/drm/panfrost/panfrost_drv.c       |   2 +-
>>   .../gpu/drm/scheduler/gpu_scheduler_trace.h   |   2 +-
>>   drivers/gpu/drm/scheduler/sched_entity.c      |   4 +-
>>   drivers/gpu/drm/scheduler/sched_main.c        | 142 ++++++++++++++----
>>   drivers/gpu/drm/v3d/v3d_gem.c                 |   2 +-
>>   include/drm/gpu_scheduler.h                   |  33 +++-
>>   12 files changed, 152 insertions(+), 49 deletions(-)
>>
>> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
>> index 602010cb6894..acc5901ac840 100644
>> --- a/Documentation/gpu/drm-mm.rst
>> +++ b/Documentation/gpu/drm-mm.rst
>> @@ -552,6 +552,12 @@ Overview
>>   .. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
>>      :doc: Overview
>>   
>> +Flow Control
>> +------------
>> +
>> +.. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
>> +   :doc: Flow Control
>> +
>>   Scheduler Function References
>>   -----------------------------
>>   
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
>> index 1f357198533f..62bb7fc7448a 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
>> @@ -115,7 +115,7 @@ int amdgpu_job_alloc(struct amdgpu_device *adev, struct amdgpu_vm *vm,
>>   	if (!entity)
>>   		return 0;
>>   
>> -	return drm_sched_job_init(&(*job)->base, entity, owner);
>> +	return drm_sched_job_init(&(*job)->base, entity, 1, owner);
>>   }
>>   
>>   int amdgpu_job_alloc_with_ib(struct amdgpu_device *adev,
>> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c b/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
>> index 2416c526f9b0..3d0f8d182506 100644
>> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
>> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
>> @@ -535,7 +535,7 @@ int etnaviv_ioctl_gem_submit(struct drm_device *dev, void *data,
>>   
>>   	ret = drm_sched_job_init(&submit->sched_job,
>>   				 &ctx->sched_entity[args->pipe],
>> -				 submit->ctx);
>> +				 1, submit->ctx);
>>   	if (ret)
>>   		goto err_submit_put;
>>   
>> diff --git a/drivers/gpu/drm/lima/lima_sched.c b/drivers/gpu/drm/lima/lima_sched.c
>> index 23a6276f1332..cdcb37ff62c3 100644
>> --- a/drivers/gpu/drm/lima/lima_sched.c
>> +++ b/drivers/gpu/drm/lima/lima_sched.c
>> @@ -123,7 +123,7 @@ int lima_sched_task_init(struct lima_sched_task *task,
>>   	for (i = 0; i < num_bos; i++)
>>   		drm_gem_object_get(&bos[i]->base.base);
>>   
>> -	err = drm_sched_job_init(&task->base, &context->base, vm);
>> +	err = drm_sched_job_init(&task->base, &context->base, 1, vm);
>>   	if (err) {
>>   		kfree(task->bos);
>>   		return err;
>> diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
>> index 99744de6c05a..c002cabe7b9c 100644
>> --- a/drivers/gpu/drm/msm/msm_gem_submit.c
>> +++ b/drivers/gpu/drm/msm/msm_gem_submit.c
>> @@ -48,7 +48,7 @@ static struct msm_gem_submit *submit_create(struct drm_device *dev,
>>   		return ERR_PTR(ret);
>>   	}
>>   
>> -	ret = drm_sched_job_init(&submit->base, queue->entity, queue);
>> +	ret = drm_sched_job_init(&submit->base, queue->entity, 1, queue);
>>   	if (ret) {
>>   		kfree(submit->hw_fence);
>>   		kfree(submit);
>> diff --git a/drivers/gpu/drm/nouveau/nouveau_sched.c b/drivers/gpu/drm/nouveau/nouveau_sched.c
>> index 7e64b5ef90fb..1b2cc3f2e1c7 100644
>> --- a/drivers/gpu/drm/nouveau/nouveau_sched.c
>> +++ b/drivers/gpu/drm/nouveau/nouveau_sched.c
>> @@ -89,7 +89,7 @@ nouveau_job_init(struct nouveau_job *job,
>>   
>>   	}
>>   
>> -	ret = drm_sched_job_init(&job->base, &entity->base, NULL);
>> +	ret = drm_sched_job_init(&job->base, &entity->base, 1, NULL);
>>   	if (ret)
>>   		goto err_free_chains;
>>   
>> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
>> index b834777b409b..54d1c19bea84 100644
>> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
>> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
>> @@ -274,7 +274,7 @@ static int panfrost_ioctl_submit(struct drm_device *dev, void *data,
>>   
>>   	ret = drm_sched_job_init(&job->base,
>>   				 &file_priv->sched_entity[slot],
>> -				 NULL);
>> +				 1, NULL);
>>   	if (ret)
>>   		goto out_put_job;
>>   
>> diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler_trace.h b/drivers/gpu/drm/scheduler/gpu_scheduler_trace.h
>> index 3143ecaaff86..f8ed093b7356 100644
>> --- a/drivers/gpu/drm/scheduler/gpu_scheduler_trace.h
>> +++ b/drivers/gpu/drm/scheduler/gpu_scheduler_trace.h
>> @@ -51,7 +51,7 @@ DECLARE_EVENT_CLASS(drm_sched_job,
>>   			   __assign_str(name, sched_job->sched->name);
>>   			   __entry->job_count = spsc_queue_count(&entity->job_queue);
>>   			   __entry->hw_job_count = atomic_read(
>> -				   &sched_job->sched->hw_rq_count);
>> +				   &sched_job->sched->credit_count);
>>   			   ),
>>   	    TP_printk("entity=%p, id=%llu, fence=%p, ring=%s, job count:%u, hw job count:%d",
>>   		      __entry->entity, __entry->id,
>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
>> index 409e4256f6e7..b79e0672b94f 100644
>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>> @@ -370,7 +370,7 @@ static void drm_sched_entity_wakeup(struct dma_fence *f,
>>   		container_of(cb, struct drm_sched_entity, cb);
>>   
>>   	drm_sched_entity_clear_dep(f, cb);
>> -	drm_sched_wakeup_if_can_queue(entity->rq->sched);
>> +	drm_sched_wakeup_if_can_queue(entity->rq->sched, entity);
>>   }
>>   
>>   /**
>> @@ -602,7 +602,7 @@ void drm_sched_entity_push_job(struct drm_sched_job *sched_job)
>>   		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
>>   			drm_sched_rq_update_fifo(entity, submit_ts);
>>   
>> -		drm_sched_wakeup_if_can_queue(entity->rq->sched);
>> +		drm_sched_wakeup_if_can_queue(entity->rq->sched, entity);
>>   	}
>>   }
>>   EXPORT_SYMBOL(drm_sched_entity_push_job);
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 246213963928..3cc3dc0091fc 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -48,6 +48,30 @@
>>    * through the jobs entity pointer.
>>    */
>>   
>> +/**
>> + * DOC: Flow Control
>> + *
>> + * The DRM GPU scheduler provides a flow control mechanism to regulate the rate
>> + * in which the jobs fetched from scheduler entities are executed.
>> + *
>> + * In this context the &drm_gpu_scheduler keeps track of a driver specified
>> + * credit limit representing the capacity of this scheduler and a credit count;
>> + * every &drm_sched_job carries a driver specified number of credits.
>> + *
>> + * Once a job is executed (but not yet finished), the job's credits contribute
>> + * to the scheduler's credit count until the job is finished. If by executing
>> + * one more job the scheduler's credit count would exceed the scheduler's
>> + * credit limit, the job won't be executed. Instead, the scheduler will wait
>> + * until the credit count has decreased enough to not overflow its credit limit.
>> + * This implies waiting for previously executed jobs.
>> + *
>> + * Optionally, drivers can register a callback (update_job_credits) provided by
> 
> "can" --> "may".
> 
>> + * struct drm_sched_backend_ops to update the job's credits dynamically. The
>> + * scheduler will execute this callback every time the scheduler considers a job
> 
> No future tense, "will execute" --> "executes", because when this patch lands,
> it all becomes present reality. See below for a reworded paragraph.
> 
>> + * for execution and subsequently checks whether the job fits the scheduler's
>> + * credit limit.
> 
> This here is a good explanation of what update_job_credits() does, but the one
> where this callback is defined in the scheduler ops is not very clear (see further down
> into the patch).
> 
> I think we shouldn't use the word "update" as we don't really have any "update" code,
> a la,
> 	if old value != new value, then
> 		old value = new value # update it
> in the code using this new sched op.
> 
> Perhaps it should be a "get_job_credits()" as this is how this function is used
> in drm_sched_can_queue().
> 
> Perhaps this adds some clarity:
> 
> * Optionally, drivers may register a sched ops callback, get_job_credits(), which
> * returns the number of credits the job passed as an argument would take when
> * submitted to driver/hardware. The scheduler executes this callback every time it
> * considers a job for execution in order to check if the job's credits fit
> * into the scheduler's credit limit.
> 
> (FWIW, it may be doing some "update" into the driver/firmware/hardware, but DRM
> has no visibility or care for that, since DRM is using it simply as "tell me
> the number or credits this job would take.")
> 
>> + */
>> +
>>   #include <linux/wait.h>
>>   #include <linux/sched.h>
>>   #include <linux/completion.h>
>> @@ -75,6 +99,46 @@ int drm_sched_policy = DRM_SCHED_POLICY_FIFO;
>>   MODULE_PARM_DESC(sched_policy, "Specify the scheduling policy for entities on a run-queue, " __stringify(DRM_SCHED_POLICY_RR) " = Round Robin, " __stringify(DRM_SCHED_POLICY_FIFO) " = FIFO (default).");
>>   module_param_named(sched_policy, drm_sched_policy, int, 0444);
>>   
>> +static u32 drm_sched_available_credits(struct drm_gpu_scheduler *sched)
>> +{
>> +	u32 credits;
>> +
>> +	WARN_ON(check_sub_overflow(sched->credit_limit,
>> +				   atomic_read(&sched->credit_count),
>> +				   &credits));
>> +
>> +	return credits;
>> +}
>> +
>> +/**
>> + * drm_sched_can_queue -- Can we queue more to the hardware?
>> + * @sched: scheduler instance
>> + * @entity: the scheduler entity
>> + *
>> + * Return true if we can push at least one more job from @entity, false
>> + * otherwise.
>> + */
>> +static bool drm_sched_can_queue(struct drm_gpu_scheduler *sched,
>> +				struct drm_sched_entity *entity)
>> +{
>> +	struct drm_sched_job *s_job;
>> +
>> +	s_job = to_drm_sched_job(spsc_queue_peek(&entity->job_queue));
>> +	if (!s_job)
>> +		return false;
> 
> We don't have this gating logic at the moment, and I don't think
> we need it.
> 
> At the current state of the code, we don't care if there's jobs
> in the incoming queue or not.
> 
> The only thing we should check here for is the credit availability
> for _this job_, as you do in the code below.
> 
>> +
>> +	if (sched->ops->update_job_credits) {
>> +		u32 credits = sched->ops->update_job_credits(s_job);
>> +
>> +		if (credits)
>> +			s_job->credits = credits;
>> +	}
>> +
>> +	WARN_ON(s_job->credits > sched->credit_limit);
>> +
>> +	return drm_sched_available_credits(sched) >= s_job->credits;
>> +}
>> +
>>   static __always_inline bool drm_sched_entity_compare_before(struct rb_node *a,
>>   							    const struct rb_node *b)
>>   {
>> @@ -186,12 +250,14 @@ void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
>>   /**
>>    * drm_sched_rq_select_entity_rr - Select an entity which could provide a job to run
>>    *
>> + * @sched: the gpu scheduler
>>    * @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_rr(struct drm_sched_rq *rq)
>> +drm_sched_rq_select_entity_rr(struct drm_gpu_scheduler *sched,
>> +			      struct drm_sched_rq *rq)
>>   {
>>   	struct drm_sched_entity *entity;
>>   
>> @@ -201,6 +267,12 @@ drm_sched_rq_select_entity_rr(struct drm_sched_rq *rq)
>>   	if (entity) {
>>   		list_for_each_entry_continue(entity, &rq->entities, list) {
>>   			if (drm_sched_entity_is_ready(entity)) {
>> +				/* If we can't queue yet, preserve the current
>> +				 * entity in terms of fairness.
>> +				 */
>> +				if (!drm_sched_can_queue(sched, entity))
>> +					goto out;
>> +
>>   				rq->current_entity = entity;
>>   				reinit_completion(&entity->entity_idle);
>>   				spin_unlock(&rq->lock);
>> @@ -210,8 +282,13 @@ drm_sched_rq_select_entity_rr(struct drm_sched_rq *rq)
>>   	}
>>   
>>   	list_for_each_entry(entity, &rq->entities, list) {
>> -
>>   		if (drm_sched_entity_is_ready(entity)) {
>> +			/* If we can't queue yet, preserve the current entity in
>> +			 * terms of fairness.
>> +			 */
>> +			if (!drm_sched_can_queue(sched, entity))
>> +				goto out;
>> +
>>   			rq->current_entity = entity;
>>   			reinit_completion(&entity->entity_idle);
>>   			spin_unlock(&rq->lock);
>> @@ -222,20 +299,22 @@ drm_sched_rq_select_entity_rr(struct drm_sched_rq *rq)
>>   			break;
>>   	}
>>   
>> +out:
>>   	spin_unlock(&rq->lock);
>> -
>>   	return NULL;
>>   }
>>   
>>   /**
>>    * drm_sched_rq_select_entity_fifo - Select an entity which provides a job to run
>>    *
>> + * @sched: the gpu scheduler
>>    * @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)
>> +drm_sched_rq_select_entity_fifo(struct drm_gpu_scheduler *sched,
>> +				struct drm_sched_rq *rq)
>>   {
>>   	struct rb_node *rb;
>>   
>> @@ -245,6 +324,15 @@ drm_sched_rq_select_entity_fifo(struct drm_sched_rq *rq)
>>   
>>   		entity = rb_entry(rb, struct drm_sched_entity, rb_tree_node);
>>   		if (drm_sched_entity_is_ready(entity)) {
>> +			/* If we can't queue yet, don't pick another entity
>> +			 * which's job might fit and wait until we got space for
> 
> 			 * whose job might not fit and wait until we get space for
> Three fixes:                 ^              ^                         ^
> 
>> +			 * this one in terms of fairness.
> 
>                           * this one in terms of credit availability.
> 
> It's not the "fairness", it's the number of credits we check for in drm_sched_can_queue() below.
> 
>> +			 */
>> +			if (!drm_sched_can_queue(sched, entity)) {
>> +				rb = NULL;
>> +				break;
>> +			}
>> +
> 
> That's good.
> 
>>   			rq->current_entity = entity;
>>   			reinit_completion(&entity->entity_idle);
>>   			break;
>> @@ -266,18 +354,6 @@ static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>>   		queue_work(sched->submit_wq, &sched->work_run_job);
>>   }
>>   
>> -/**
>> - * drm_sched_can_queue -- Can we queue more to the hardware?
>> - * @sched: scheduler instance
>> - *
>> - * Return true if we can push more jobs to the hw, otherwise false.
>> - */
>> -static bool drm_sched_can_queue(struct drm_gpu_scheduler *sched)
>> -{
>> -	return atomic_read(&sched->hw_rq_count) <
>> -		sched->hw_submission_limit;
>> -}
>> -
>>   /**
>>    * drm_sched_select_entity - Select next entity to process
>>    *
>> @@ -291,14 +367,11 @@ drm_sched_select_entity(struct drm_gpu_scheduler *sched)
>>   	struct drm_sched_entity *entity;
>>   	int i;
>>   
>> -	if (!drm_sched_can_queue(sched))
>> -		return NULL;
>> -
>>   	/* Kernel run queue has higher priority than normal run queue*/
>>   	for (i = sched->num_rqs - 1; i >= DRM_SCHED_PRIORITY_MIN; i--) {
>>   		entity = drm_sched_policy == DRM_SCHED_POLICY_FIFO ?
>> -			drm_sched_rq_select_entity_fifo(sched->sched_rq[i]) :
>> -			drm_sched_rq_select_entity_rr(sched->sched_rq[i]);
>> +			drm_sched_rq_select_entity_fifo(sched, sched->sched_rq[i]) :
>> +			drm_sched_rq_select_entity_rr(sched, sched->sched_rq[i]);
>>   		if (entity)
>>   			break;
>>   	}
>> @@ -353,7 +426,7 @@ static void drm_sched_job_done(struct drm_sched_job *s_job, int result)
>>   	struct drm_sched_fence *s_fence = s_job->s_fence;
>>   	struct drm_gpu_scheduler *sched = s_fence->sched;
>>   
>> -	atomic_dec(&sched->hw_rq_count);
>> +	atomic_sub(s_job->credits, &sched->credit_count);
>>   	atomic_dec(sched->score);
>>   
>>   	trace_drm_sched_process_job(s_fence);
>> @@ -576,7 +649,7 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
>>   					      &s_job->cb)) {
>>   			dma_fence_put(s_job->s_fence->parent);
>>   			s_job->s_fence->parent = NULL;
>> -			atomic_dec(&sched->hw_rq_count);
>> +			atomic_sub(s_job->credits, &sched->credit_count);
>>   		} else {
>>   			/*
>>   			 * remove job from pending_list.
>> @@ -637,7 +710,7 @@ void drm_sched_start(struct drm_gpu_scheduler *sched, bool full_recovery)
>>   	list_for_each_entry_safe(s_job, tmp, &sched->pending_list, list) {
>>   		struct dma_fence *fence = s_job->s_fence->parent;
>>   
>> -		atomic_inc(&sched->hw_rq_count);
>> +		atomic_add(s_job->credits, &sched->credit_count);
>>   
>>   		if (!full_recovery)
>>   			continue;
>> @@ -718,6 +791,8 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>    * drm_sched_job_init - init a scheduler job
>>    * @job: scheduler job to init
>>    * @entity: scheduler entity to use
>> + * @credits: the number of credits this job contributes to the schedulers
>> + * credit limit
>>    * @owner: job owner for debugging
>>    *
>>    * Refer to drm_sched_entity_push_job() documentation
>> @@ -735,7 +810,7 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>    */
>>   int drm_sched_job_init(struct drm_sched_job *job,
>>   		       struct drm_sched_entity *entity,
>> -		       void *owner)
>> +		       u32 credits, void *owner)
>>   {
>>   	if (!entity->rq) {
>>   		/* This will most likely be followed by missing frames
>> @@ -752,6 +827,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>   		return -ENOMEM;
>>   
>>   	INIT_LIST_HEAD(&job->list);
>> +	job->credits = credits ? credits : 1;
>>   
>>   	xa_init_flags(&job->dependencies, XA_FLAGS_ALLOC);
>>   
>> @@ -961,12 +1037,14 @@ EXPORT_SYMBOL(drm_sched_job_cleanup);
>>   /**
>>    * drm_sched_wakeup_if_can_queue - Wake up the scheduler
>>    * @sched: scheduler instance
>> + * @entity: the scheduler entity
>>    *
>>    * Wake up the scheduler if we can queue jobs.
>>    */
>> -void drm_sched_wakeup_if_can_queue(struct drm_gpu_scheduler *sched)
>> +void drm_sched_wakeup_if_can_queue(struct drm_gpu_scheduler *sched,
>> +				   struct drm_sched_entity *entity)
>>   {
>> -	if (drm_sched_can_queue(sched))
>> +	if (drm_sched_can_queue(sched, entity))
>>   		drm_sched_run_job_queue(sched);
>>   }
>>   
>> @@ -1104,7 +1182,7 @@ static void drm_sched_run_job_work(struct work_struct *w)
>>   
>>   	s_fence = sched_job->s_fence;
>>   
>> -	atomic_inc(&sched->hw_rq_count);
>> +	atomic_add(sched_job->credits, &sched->credit_count);
>>   	drm_sched_job_begin(sched_job);
>>   
>>   	trace_drm_run_job(sched_job, entity);
>> @@ -1139,7 +1217,7 @@ static void drm_sched_run_job_work(struct work_struct *w)
>>    * @submit_wq: workqueue to use for submission. If NULL, an ordered wq is
>>    *	       allocated and used
>>    * @num_rqs: number of runqueues, one for each priority, up to DRM_SCHED_PRIORITY_COUNT
>> - * @hw_submission: number of hw submissions that can be in flight
>> + * @credit_limit: the number of credits this scheduler can hold from all jobs
>>    * @hang_limit: number of times to allow a job to hang before dropping it
>>    * @timeout: timeout value in jiffies for the scheduler
>>    * @timeout_wq: workqueue to use for timeout work. If NULL, the system_wq is
>> @@ -1153,14 +1231,14 @@ static void drm_sched_run_job_work(struct work_struct *w)
>>   int drm_sched_init(struct drm_gpu_scheduler *sched,
>>   		   const struct drm_sched_backend_ops *ops,
>>   		   struct workqueue_struct *submit_wq,
>> -		   u32 num_rqs, unsigned hw_submission, unsigned hang_limit,
>> +		   u32 num_rqs, u32 credit_limit, unsigned hang_limit,
>>   		   long timeout, struct workqueue_struct *timeout_wq,
>>   		   atomic_t *score, const char *name, struct device *dev)
>>   {
>>   	int i, ret;
>>   
>>   	sched->ops = ops;
>> -	sched->hw_submission_limit = hw_submission;
>> +	sched->credit_limit = credit_limit;
>>   	sched->name = name;
>>   	if (submit_wq) {
>>   		sched->submit_wq = submit_wq;
>> @@ -1211,7 +1289,7 @@ int drm_sched_init(struct drm_gpu_scheduler *sched,
>>   	init_waitqueue_head(&sched->job_scheduled);
>>   	INIT_LIST_HEAD(&sched->pending_list);
>>   	spin_lock_init(&sched->job_list_lock);
>> -	atomic_set(&sched->hw_rq_count, 0);
>> +	atomic_set(&sched->credit_count, 0);
>>   	INIT_DELAYED_WORK(&sched->work_tdr, drm_sched_job_timedout);
>>   	INIT_WORK(&sched->work_run_job, drm_sched_run_job_work);
>>   	INIT_WORK(&sched->work_free_job, drm_sched_free_job_work);
>> diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
>> index 2e94ce788c71..8479e5302f7b 100644
>> --- a/drivers/gpu/drm/v3d/v3d_gem.c
>> +++ b/drivers/gpu/drm/v3d/v3d_gem.c
>> @@ -417,7 +417,7 @@ v3d_job_init(struct v3d_dev *v3d, struct drm_file *file_priv,
>>   	job->free = free;
>>   
>>   	ret = drm_sched_job_init(&job->base, &v3d_priv->sched_entity[queue],
>> -				 v3d_priv);
>> +				 1, v3d_priv);
>>   	if (ret)
>>   		goto fail;
>>   
>> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
>> index e5a6166eb152..a911b3f5917e 100644
>> --- a/include/drm/gpu_scheduler.h
>> +++ b/include/drm/gpu_scheduler.h
>> @@ -321,6 +321,7 @@ struct drm_sched_fence *to_drm_sched_fence(struct dma_fence *f);
>>    * @sched: the scheduler instance on which this job is scheduled.
>>    * @s_fence: contains the fences for the scheduling of job.
>>    * @finish_cb: the callback for the finished fence.
>> + * @credits: the number of credits this job contributes to the scheduler
>>    * @work: Helper to reschdeule job kill to different context.
>>    * @id: a unique id assigned to each job scheduled on the scheduler.
>>    * @karma: increment on every hang caused by this job. If this exceeds the hang
>> @@ -340,6 +341,8 @@ struct drm_sched_job {
>>   	struct drm_gpu_scheduler	*sched;
>>   	struct drm_sched_fence		*s_fence;
>>   
>> +	u32				credits;
>> +
>>   	/*
>>   	 * work is used only after finish_cb has been used and will not be
>>   	 * accessed anymore.
>> @@ -463,13 +466,29 @@ struct drm_sched_backend_ops {
>>            * and it's time to clean it up.
>>   	 */
>>   	void (*free_job)(struct drm_sched_job *sched_job);
>> +
>> +	/**
>> +	 * @update_job_credits: Called once the scheduler is considering this
>> +	 * job for execution.
>> +	 *
> 
> This whole paragraph isn't very clear of what update_job_credits does.
> Perhaps a simple and straightforward name would be more clear:
> 	get_job_credits: Returns the number of credits this job would
> 		take.

I still very much prefer 'update' over 'get' for the following reasons.

1. 'Update' describes very well what the driver should do when receiving this
    callback. It should update the number of credits the job requires at the time
    it receives the callback.

2. 'get', as well as 'put', is typically used in a different context in the kernel
    and hence this might create confusion.

> 
>> +	 * Drivers may use this to update the job's submission credits, which is
>> +	 * useful to e.g. deduct the number of native fences which have been
>> +	 * signaled meanwhile.
>> +	 *
>> +	 * The callback must either return the new number of submission credits
>> +	 * for the given job, or zero if no update is required.
> 
> The word "update" is confusing here and it implies that DRM should keep track
> of the previous value this function had returned.
> 
> Perhaps we can just say:
> 	* This callback returns the number of credits this job would take if pushed
> 	* to the driver/hardware. It returns 0, if this job would take no credits.

For this version of the patch this would be wrong, 0 really means that we keep
job->credits as it is. However, I'd probably change this in V4 as requested by Boris,
such that we accept 0 credits, but WARN() on it. Unless, we consider it to be valid
use-case to have jobs that are not flow-controlled at all.

All other comments look good, gonna address them in a V4.

- Danilo

> 
> Else, driver writers would have to deduce what to return here by looking at
> drm_sched_can_queue() which effectively does:
> 	s_job->credits = sched->ops->update_job_credits(s_job).
> 
>> +	 *
>> +	 * This callback is optional.
>> +	 */
>> +	u32 (*update_job_credits)(struct drm_sched_job *sched_job);
>>   };
>>   
> 
> The rest is good--thanks for the changes!
> 
>>   /**
>>    * struct drm_gpu_scheduler - scheduler instance-specific data
>>    *
>>    * @ops: backend operations provided by the driver.
>> - * @hw_submission_limit: the max size of the hardware queue.
>> + * @credit_limit: the credit limit of this scheduler
>> + * @credit_count: the current credit count of this scheduler
>>    * @timeout: the time after which a job is removed from the scheduler.
>>    * @name: name of the ring for which this scheduler is being used.
>>    * @num_rqs: Number of run-queues. This is at most DRM_SCHED_PRIORITY_COUNT,
>> @@ -478,7 +497,6 @@ struct drm_sched_backend_ops {
>>    * @job_scheduled: once @drm_sched_entity_do_release is called the scheduler
>>    *                 waits on this wait queue until all the scheduled jobs are
>>    *                 finished.
>> - * @hw_rq_count: the number of jobs currently in the hardware queue.
>>    * @job_id_count: used to assign unique id to the each job.
>>    * @submit_wq: workqueue used to queue @work_run_job and @work_free_job
>>    * @timeout_wq: workqueue used to queue @work_tdr
>> @@ -502,13 +520,13 @@ struct drm_sched_backend_ops {
>>    */
>>   struct drm_gpu_scheduler {
>>   	const struct drm_sched_backend_ops	*ops;
>> -	uint32_t			hw_submission_limit;
>> +	u32				credit_limit;
>> +	atomic_t			credit_count;
>>   	long				timeout;
>>   	const char			*name;
>>   	u32                             num_rqs;
>>   	struct drm_sched_rq             **sched_rq;
>>   	wait_queue_head_t		job_scheduled;
>> -	atomic_t			hw_rq_count;
>>   	atomic64_t			job_id_count;
>>   	struct workqueue_struct		*submit_wq;
>>   	struct workqueue_struct		*timeout_wq;
>> @@ -530,14 +548,14 @@ struct drm_gpu_scheduler {
>>   int drm_sched_init(struct drm_gpu_scheduler *sched,
>>   		   const struct drm_sched_backend_ops *ops,
>>   		   struct workqueue_struct *submit_wq,
>> -		   u32 num_rqs, uint32_t hw_submission, unsigned hang_limit,
>> +		   u32 num_rqs, u32 credit_limit, unsigned hang_limit,
>>   		   long timeout, struct workqueue_struct *timeout_wq,
>>   		   atomic_t *score, const char *name, struct device *dev);
>>   
>>   void drm_sched_fini(struct drm_gpu_scheduler *sched);
>>   int drm_sched_job_init(struct drm_sched_job *job,
>>   		       struct drm_sched_entity *entity,
>> -		       void *owner);
>> +		       u32 credits, void *owner);
>>   void drm_sched_job_arm(struct drm_sched_job *job);
>>   int drm_sched_job_add_dependency(struct drm_sched_job *job,
>>   				 struct dma_fence *fence);
>> @@ -559,7 +577,8 @@ void drm_sched_entity_modify_sched(struct drm_sched_entity *entity,
>>   
>>   void drm_sched_tdr_queue_imm(struct drm_gpu_scheduler *sched);
>>   void drm_sched_job_cleanup(struct drm_sched_job *job);
>> -void drm_sched_wakeup_if_can_queue(struct drm_gpu_scheduler *sched);
>> +void drm_sched_wakeup_if_can_queue(struct drm_gpu_scheduler *sched,
>> +				   struct drm_sched_entity *entity);
>>   bool drm_sched_wqueue_ready(struct drm_gpu_scheduler *sched);
>>   void drm_sched_wqueue_stop(struct drm_gpu_scheduler *sched);
>>   void drm_sched_wqueue_start(struct drm_gpu_scheduler *sched);
>