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Message-ID: <3eb74ad0-408b-46de-b3b3-d4955508d143@redhat.com>
Date:   Tue, 31 Oct 2023 13:19:44 +0100
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Subject: Re: [PATCH drm-misc-next v4] drm/sched: implement dynamic job-flow
 control
To:     =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        airlied@gmail.com, daniel@ffwll.ch, matthew.brost@intel.com,
        boris.brezillon@collabora.com, faith@gfxstrand.net,
        luben.tuikov@amd.com
Cc:     dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <20231031002655.38707-1-dakr@redhat.com>
 <a3ab8a98-d2c3-4974-978e-782811ff0548@amd.com>
Content-Language: en-US
From:   Danilo Krummrich <dakr@redhat.com>
Organization: RedHat
In-Reply-To: <a3ab8a98-d2c3-4974-978e-782811ff0548@amd.com>
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On 10/31/23 12:13, Christian König wrote:
> 
> 
> Am 31.10.23 um 01:26 schrieb Danilo Krummrich:
>> 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
>>
>> Changes in V4:
>> ==============
>>    - address Lubens comments regarding documentation
>>    - switch to drm_WARN() variants
>>    - WARN_ON() drivers passing in zero job credits for both drm_sched_job_init()
>>      and the update_job_credits() callback
>>    - don't retry with another runq if job doesn't fit on the ring to prevent
>>      priority inversion
>>    - rebase onto drm-misc-next (will probably land before Matt's series)
>>
>> Patch also available in [1].
>>
>> [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        | 148 ++++++++++++++----
>>   drivers/gpu/drm/v3d/v3d_gem.c                 |   2 +-
>>   include/drm/gpu_scheduler.h                   |  31 +++-
>>   12 files changed, 156 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 295f0353a02e..0febe8ceb43b 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 e62b04109b2f..2a845f9c34ec 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 fd755e953487..5dfde149441a 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 may register a callback (update_job_credits) provided by
>> + * struct drm_sched_backend_ops to update the job's credits dynamically. The
>> + * scheduler executes this callback every time the scheduler considers a job for
>> + * execution and subsequently checks whether the job fits the scheduler's credit
>> + * limit.
>> + */
>> +
>>   #include <linux/kthread.h>
>>   #include <linux/wait.h>
>>   #include <linux/sched.h>
>> @@ -76,6 +100,46 @@ 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;
>> +
>> +    drm_WARN_ON(sched, 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;
>> +
> 
>> +    if (sched->ops->update_job_credits) {
>> +        s_job->credits = sched->ops->update_job_credits(s_job);
>> +
>> +        drm_WARN(sched, !s_job->credits,
>> +             "Jobs with zero credits bypass job-flow control\n");
>> +    }
> 
> I'm still absolutely not keen about that.
> 
> Adding another callback adds more complexity into scheduler driver interface, so at the end of the day we have more which could be implemented incorrectly.

I already commented on this concern in [1]. If this isn't satisfying yet,
please send an answer to [1], such that I don't need to re-write everything.

> 
> So this needs to be justified somehow and to be honest I don't see the value in filling in the ring buffers more than necessary.

I think both Boris and me gave a justification, it's just that it seems you're
not accepting it. Again, if there are concerns left, please continue the discussion
in [1], such that we don't need to start over.

- Danilo

[1] https://lore.kernel.org/all/ZT%2FuPhLK7gan61Ns@pollux/

> 
> Regards,
> Christian.
> 
> 
>> +
>> +    drm_WARN_ON(sched, 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)
>>   {
>> @@ -187,12 +251,14 @@ 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;
>> @@ -202,6 +268,14 @@ 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)) {
>> +                    spin_unlock(&rq->lock);
>> +                    return ERR_PTR(-ENOSPC);
>> +                }
>> +
>>                   rq->current_entity = entity;
>>                   reinit_completion(&entity->entity_idle);
>>                   spin_unlock(&rq->lock);
>> @@ -211,8 +285,15 @@ 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)) {
>> +                spin_unlock(&rq->lock);
>> +                return ERR_PTR(-ENOSPC);
>> +            }
>> +
>>               rq->current_entity = entity;
>>               reinit_completion(&entity->entity_idle);
>>               spin_unlock(&rq->lock);
>> @@ -231,12 +312,14 @@ drm_sched_rq_select_entity_rr(struct drm_sched_rq *rq)
>>   /**
>>    * 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;
>> @@ -246,6 +329,14 @@ 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, preserve the current entity in
>> +             * terms of fairness.
>> +             */
>> +            if (!drm_sched_can_queue(sched, entity)) {
>> +                spin_unlock(&rq->lock);
>> +                return ERR_PTR(-ENOSPC);
>> +            }
>> +
>>               rq->current_entity = entity;
>>               reinit_completion(&entity->entity_idle);
>>               break;
>> @@ -267,7 +358,7 @@ 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);
>> @@ -468,7 +559,7 @@ 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.
>> @@ -529,7 +620,7 @@ 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;
>> @@ -613,6 +704,8 @@ 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
>> @@ -630,7 +723,7 @@ 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
>> @@ -647,6 +740,10 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>           return -ENOMEM;
>>       INIT_LIST_HEAD(&job->list);
>> +    job->credits = credits;
>> +
>> +    drm_WARN(job->sched, !credits,
>> +         "Jobs with zero credits bypass job-flow control\n");
>>       xa_init_flags(&job->dependencies, XA_FLAGS_ALLOC);
>> @@ -853,27 +950,17 @@ void drm_sched_job_cleanup(struct drm_sched_job *job)
>>   }
>>   EXPORT_SYMBOL(drm_sched_job_cleanup);
>> -/**
>> - * 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_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))
>>           wake_up_interruptible(&sched->wake_up_worker);
>>   }
>> @@ -890,19 +977,16 @@ 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;
>>       }
>> -    return entity;
>> +    return IS_ERR(entity) ? NULL : entity;
>>   }
>>   /**
>> @@ -1043,7 +1127,7 @@ static int drm_sched_main(void *param)
>>           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);
>> @@ -1078,7 +1162,7 @@ static int drm_sched_main(void *param)
>>    * @sched: scheduler instance
>>    * @ops: backend operations for this scheduler
>>    * @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
>> @@ -1091,14 +1175,14 @@ static int drm_sched_main(void *param)
>>    */
>>   int drm_sched_init(struct drm_gpu_scheduler *sched,
>>              const struct drm_sched_backend_ops *ops,
>> -           u32 num_rqs, uint32_t hw_submission, unsigned int hang_limit,
>> +           u32 num_rqs, u32 credit_limit, unsigned int 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;
>>       sched->timeout = timeout;
>>       sched->timeout_wq = timeout_wq ? : system_wq;
>> @@ -1140,7 +1224,7 @@ 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);
>>       atomic_set(&sched->_score, 0);
>>       atomic64_set(&sched->job_id_count, 0);
>> diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
>> index afa7d170d1ff..8dc5a7988488 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 653e0eec9743..8bafa81300b2 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 +466,27 @@ 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 callback returns the number of credits this job would take if
>> +     * pushed to the hardware. Drivers may use this to dynamically update
>> +     * the job's credit count. For instance, deduct the number of credits
>> +     * for already signalled native fences.
>> +     *
>> +     * This callback is optional.
>> +     */
>> +    u32 (*update_job_credits)(struct drm_sched_job *sched_job);
>>   };
>>   /**
>>    * 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,
>> @@ -480,7 +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.
>>    * @timeout_wq: workqueue used to queue @work_tdr
>>    * @work_tdr: schedules a delayed call to @drm_sched_job_timedout after the
>> @@ -500,14 +516,14 @@ 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        wake_up_worker;
>>       wait_queue_head_t        job_scheduled;
>> -    atomic_t            hw_rq_count;
>>       atomic64_t            job_id_count;
>>       struct workqueue_struct        *timeout_wq;
>>       struct delayed_work        work_tdr;
>> @@ -524,14 +540,14 @@ struct drm_gpu_scheduler {
>>   int drm_sched_init(struct drm_gpu_scheduler *sched,
>>              const struct drm_sched_backend_ops *ops,
>> -           u32 num_rqs, uint32_t hw_submission, unsigned int hang_limit,
>> +           u32 num_rqs, u32 credit_limit, unsigned int 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);
>> @@ -552,7 +568,8 @@ void drm_sched_entity_modify_sched(struct drm_sched_entity *entity,
>>                                      unsigned int num_sched_list);
>>   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);
>>   void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad);
>>   void drm_sched_start(struct drm_gpu_scheduler *sched, bool full_recovery);
>>   void drm_sched_resubmit_jobs(struct drm_gpu_scheduler *sched);
>>
>> base-commit: b2139fb5051554a7f297e4ad584ef1bc26c76d5d
>