[Intel-xe] [PATCH] drm/sched: Don't disturb the entity when in RR-mode scheduling

[Luben Tuikov]

Hi,

On 2023-11-07 19:41, Danilo Krummrich wrote:
> On 11/7/23 05:10, Luben Tuikov wrote:
>> Don't call drm_sched_select_entity() in drm_sched_run_job_queue().  In fact,
>> rename __drm_sched_run_job_queue() to just drm_sched_run_job_queue(), and let
>> it do just that, schedule the work item for execution.
>>
>> The problem is that drm_sched_run_job_queue() calls drm_sched_select_entity()
>> to determine if the scheduler has an entity ready in one of its run-queues,
>> and in the case of the Round-Robin (RR) scheduling, the function
>> drm_sched_rq_select_entity_rr() does just that, selects the _next_ entity
>> which is ready, sets up the run-queue and completion and returns that
>> entity. The FIFO scheduling algorithm is unaffected.
>>
>> Now, since drm_sched_run_job_work() also calls drm_sched_select_entity(), then
>> in the case of RR scheduling, that would result in drm_sched_select_entity()
>> having been called twice, which may result in skipping a ready entity if more
>> than one entity is ready. This commit fixes this by eliminating the call to
>> drm_sched_select_entity() from drm_sched_run_job_queue(), and leaves it only
>> in drm_sched_run_job_work().
>>
>> v2: Rebased on top of Tvrtko's renames series of patches. (Luben)
>>      Add fixes-tag. (Tvrtko)
>>
>> Signed-off-by: Luben Tuikov <ltuikov89 at gmail.com>
>> Fixes: f7fe64ad0f22ff ("drm/sched: Split free_job into own work item")
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 16 +++-------------
>>   1 file changed, 3 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 27843e37d9b769..cd0dc3f81d05f0 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -256,10 +256,10 @@ drm_sched_rq_select_entity_fifo(struct drm_sched_rq *rq)
>>   }
>>   
>>   /**
>> - * __drm_sched_run_job_queue - enqueue run-job work
>> + * drm_sched_run_job_queue - enqueue run-job work
>>    * @sched: scheduler instance
>>    */
>> -static void __drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>> +static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>>   {
>>   	if (!READ_ONCE(sched->pause_submit))
>>   		queue_work(sched->submit_wq, &sched->work_run_job);
>> @@ -928,7 +928,7 @@ static bool drm_sched_can_queue(struct drm_gpu_scheduler *sched)
>>   void drm_sched_wakeup(struct drm_gpu_scheduler *sched)
>>   {
>>   	if (drm_sched_can_queue(sched))
>> -		__drm_sched_run_job_queue(sched);
>> +		drm_sched_run_job_queue(sched);
>>   }
>>   
>>   /**
>> @@ -1040,16 +1040,6 @@ drm_sched_pick_best(struct drm_gpu_scheduler **sched_list,
>>   }
>>   EXPORT_SYMBOL(drm_sched_pick_best);
>>   
>> -/**
>> - * drm_sched_run_job_queue - enqueue run-job work if there are ready entities
>> - * @sched: scheduler instance
>> - */
>> -static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>> -{
>> -	if (drm_sched_select_entity(sched))
> 
> Hm, now that I rebase my patch to implement dynamic job-flow control I recognize that
> we probably need the peek semantics here. If we do not select an entity here, we also
> do not check whether the corresponding job fits on the ring.
> 
> Alternatively, we simply can't do this check in drm_sched_wakeup(). The consequence would
> be that we don't detect that we need to wait for credits to free up before the run work is
> already executing and the run work selects an entity.

So I rebased v5 on top of the latest drm-misc-next, and looked around and found out that
drm_sched_wakeup() is missing drm_sched_entity_is_ready(). It should look like the following,

void drm_sched_wakeup(struct drm_gpu_scheduler *sched,
		      struct drm_sched_entity *entity)
{
	if (drm_sched_entity_is_ready(entity))
		if (drm_sched_can_queue(sched, entity))
			drm_sched_run_job_queue(sched);
}

See the attached patch. (Currently running with base-commit and the attached patch.)
-- 
Regards,
Luben
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