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Date:   Thu, 26 Oct 2023 21:03:38 -0400
From:   Luben Tuikov <ltuikov89@...il.com>
To:     Danilo Krummrich <dakr@...hat.com>, airlied@...il.com,
        daniel@...ll.ch, matthew.brost@...el.com,
        boris.brezillon@...labora.com, christian.koenig@....com,
        faith@...strand.net, luben.tuikov@....com
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH drm-misc-next v3] drm/sched: implement dynamic job-flow
 control

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@...hat.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.

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.

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);
> 

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