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Message-ID: <429be5db-a783-4dcf-a1f2-c3842fe6e114@redhat.com>
Date:   Fri, 27 Oct 2023 16:47:41 +0200
MIME-Version: 1.0
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>
 <8c0abef0-e24b-419a-b3b5-78f112c6618b@gmail.com>
From:   Danilo Krummrich <dakr@redhat.com>
Organization: RedHat
In-Reply-To: <8c0abef0-e24b-419a-b3b5-78f112c6618b@gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 10/27/23 03:03, Luben Tuikov wrote:
> On 2023-10-26 17:13, 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.
> 
> I see it now--never mind.
> 
> (I would've probably added a comment--something like
> /* Incoming queue empty--nothing to schedule, return false. */
> as it circles all the way back to where we pick the next job to schedule.)
> 
> So, I ported this patch to drm-misc-next and then to 6.6-rc7 and ran
> with it. All is good.

That's great!

> 
> I think this patch is ready to go in after addressing the cosmetic fixes
> to comments and clarity--if you also feel the same way.

Yes, as mentioned in my previous reply, gonna address them soon in a V4.

- Danilo

> 
> Thanks! Great job!
> 
> Regards,
> Luben
> 
> 
>>
>>> +
>>> +	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.
>>
>>> +	 * 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.
>>
>> 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);
>>