[RFC v4 04/16] drm/sched: Avoid double re-lock on the job free path

[Tvrtko Ursulin]

On 12/05/2025 13:49, Philipp Stanner wrote:
> On Fri, 2025-04-25 at 11:20 +0100, Tvrtko Ursulin wrote:
>> Currently the job free work item will lock sched->job_list_lock first
>> time
>> to see if there are any jobs, free a single job, and then lock again
>> to
>> decide whether to re-queue itself if there are more finished jobs.
>>
>> Since drm_sched_get_finished_job() already looks at the second job in
>> the
>> queue we can simply add the signaled check and have it return the
>> presence
>> of more jobs to free to the caller. That way the work item does not
>> have
>> to lock the list again and repeat the signaled check.
> 
> Are you convinced that this is worth it?

I cannot see a reason for the lazy code which re-locks only to get the 
same boolean state it already peeked at so yes, I am. Maybe CPU vendors 
don't mind burning extra cycles to sell us faster chips, I don't know. :D

More interesting angle is that the patch removes the potential 
opportunistic signaling from the fence worker (the bad old
evil dma_fence_is_signaled).

> I'm torn. It's rare that one returns a status through a boolean by
> reference. >
> Independently from that, this is a candidate which certainly can be
> branched out from this series, to make the series completely about the
> new scheduling policy, not general other improvements.

If I get an r-b I can easily send it standalone. Until then I let it simmer.

Regards,

Tvrtko

>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>> Cc: Christian König <christian.koenig at amd.com>
>> Cc: Danilo Krummrich <dakr at kernel.org>
>> Cc: Matthew Brost <matthew.brost at intel.com>
>> Cc: Philipp Stanner <phasta at kernel.org>
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 39 +++++++++++-------------
>> --
>>   1 file changed, 16 insertions(+), 23 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c
>> b/drivers/gpu/drm/scheduler/sched_main.c
>> index 86e40157b09b..a45b02fd2af3 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -365,22 +365,6 @@ static void __drm_sched_run_free_queue(struct
>> drm_gpu_scheduler *sched)
>>   		queue_work(sched->submit_wq, &sched->work_free_job);
>>   }
>>   
>> -/**
>> - * drm_sched_run_free_queue - enqueue free-job work if ready
>> - * @sched: scheduler instance
>> - */
>> -static void drm_sched_run_free_queue(struct drm_gpu_scheduler
>> *sched)
>> -{
>> -	struct drm_sched_job *job;
>> -
>> -	spin_lock(&sched->job_list_lock);
>> -	job = list_first_entry_or_null(&sched->pending_list,
>> -				       struct drm_sched_job, list);
>> -	if (job && dma_fence_is_signaled(&job->s_fence->finished))
>> -		__drm_sched_run_free_queue(sched);
>> -	spin_unlock(&sched->job_list_lock);
>> -}
>> -
>>   /**
>>    * drm_sched_job_done - complete a job
>>    * @s_job: pointer to the job which is done
>> @@ -1097,12 +1081,13 @@ drm_sched_select_entity(struct
>> drm_gpu_scheduler *sched)
>>    * drm_sched_get_finished_job - fetch the next finished job to be
>> destroyed
>>    *
>>    * @sched: scheduler instance
>> + * @have_more: are there more finished jobs on the list
>>    *
>>    * Returns the next finished job from the pending list (if there is
>> one)
>>    * ready for it to be destroyed.
>>    */
>>   static struct drm_sched_job *
>> -drm_sched_get_finished_job(struct drm_gpu_scheduler *sched)
>> +drm_sched_get_finished_job(struct drm_gpu_scheduler *sched, bool
>> *have_more)
>>   {
>>   	struct drm_sched_job *job, *next;
>>   
>> @@ -1110,22 +1095,27 @@ drm_sched_get_finished_job(struct
>> drm_gpu_scheduler *sched)
>>   
>>   	job = list_first_entry_or_null(&sched->pending_list,
>>   				       struct drm_sched_job, list);
>> -
>>   	if (job && dma_fence_is_signaled(&job->s_fence->finished)) {
>>   		/* remove job from pending_list */
>>   		list_del_init(&job->list);
>>   
>>   		/* cancel this job's TO timer */
>>   		cancel_delayed_work(&sched->work_tdr);
>> -		/* make the scheduled timestamp more accurate */
>> +
>> +		*have_more = false;
>>   		next = list_first_entry_or_null(&sched-
>>> pending_list,
>>   						typeof(*next),
>> list);
>> -
>>   		if (next) {
>> +			/* make the scheduled timestamp more
>> accurate */
>>   			if (test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT,
>>   				     &next->s_fence-
>>> scheduled.flags))
>>   				next->s_fence->scheduled.timestamp =
>>   					dma_fence_timestamp(&job-
>>> s_fence->finished);
>> +
>> +			if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
>> +				     &next->s_fence-
>>> finished.flags))
>> +				*have_more = true;
>> +
>>   			/* start TO timer for next job */
>>   			drm_sched_start_timeout(sched);
>>   		}
>> @@ -1184,12 +1174,15 @@ static void drm_sched_free_job_work(struct
>> work_struct *w)
>>   	struct drm_gpu_scheduler *sched =
>>   		container_of(w, struct drm_gpu_scheduler,
>> work_free_job);
>>   	struct drm_sched_job *job;
>> +	bool have_more;
>>   
>> -	job = drm_sched_get_finished_job(sched);
>> -	if (job)
>> +	job = drm_sched_get_finished_job(sched, &have_more);
>> +	if (job) {
>>   		sched->ops->free_job(job);
>> +		if (have_more)
>> +			__drm_sched_run_free_queue(sched);
>> +	}
>>   
>> -	drm_sched_run_free_queue(sched);
>>   	drm_sched_run_job_queue(sched);
>>   }
>>   
>