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

[Matthew Brost <matthew.brost@intel.com>]

On Mon, May 12, 2025 at 02:49:55PM +0200, 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'm torn. It's rare that one returns a status through a boolean by
> reference.
> 

I'd say no to this (mirco optimization) and to freeing / running more
than job per worker invocation. The later was rejected in original work
queue conversion.

Matt

> 
> 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.
> 
> 
> P.
> 
> > 
> > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Danilo Krummrich <dakr@kernel.org>
> > Cc: Matthew Brost <matthew.brost@intel.com>
> > Cc: Philipp Stanner <phasta@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);
> >  }
> >  
>