[PATCH v5 04/16] drm/sched: Avoid double re-lock on the job free path

[Philipp Stanner]

On Fri, 2025-07-04 at 09:29 -0300, Maíra Canal wrote:
> Hi Tvrtko,
> 
> On 23/06/25 09:27, 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.
> > 
> > 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 1f077782ec12..c6c26aec07b6 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -366,22 +366,6 @@ 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
> > @@ -1102,12 +1086,13 @@ 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;
> >   
> > @@ -1115,22 +1100,27 @@ 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))
> 
> Shouldn't we use dma_fence_is_signaled() to keep the same check that
> we
> have in drm_sched_run_free_queue()?

There is a paused-ongoing discussion about this function:

https://lore.kernel.org/all/20250522112540.161411-2-phasta@kernel.org/


dma_fence_is_signaled() can have side effects by actually signaling,
instead of just checking.

Not sure if Tvrtko wanted to bypass that behavior here, though.

P.

> 
> Best Regards,
> - Maíra
> 
> > +				*have_more = true;
> > +
> >   			/* start TO timer for next job */
> >   			drm_sched_start_timeout(sched);
> >   		}
> 
>