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

[Philipp Stanner <phasta@mailbox.org>]

On Fri, 2025-07-18 at 08:13 +0100, Tvrtko Ursulin wrote:
> 
> On 16/07/2025 21:44, Maíra Canal wrote:
> > Hi Tvrtko,
> > 
> > On 16/07/25 11:46, Tvrtko Ursulin wrote:
> > > 
> > > On 16/07/2025 15:30, Maíra Canal wrote:
> > > > Hi Tvrtko,
> > > > 
> > > > On 16/07/25 10:49, Tvrtko Ursulin wrote:
> > > > > 
> > > > > On 16/07/2025 14:31, Maíra Canal wrote:
> > > > > > Hi Tvrtko,
> > > > > > 
> > > > > > On 16/07/25 05:51, 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 be freed 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@igalia.com>
> > > > > > > Cc: Christian König <christian.koenig@amd.com>
> > > > > > > Cc: Danilo Krummrich <dakr@kernel.org>
> > > > > > > Cc: Maíra Canal <mcanal@igalia.com>
> > > > > > > Cc: Matthew Brost <matthew.brost@intel.com>
> > > > > > > Cc: Philipp Stanner <phasta@kernel.org>
> > > > > > > ---
> > > > > > > v2:
> > > > > > >   * Improve commit text and kerneldoc. (Philipp)
> > > > > > >   * Rename run free work helper. (Philipp)
> > > > > > > 
> > > > > > > v3:
> > > > > > >   * Rebase on top of Maira's changes.
> > > > > > > ---
> > > > > > >   drivers/gpu/drm/scheduler/sched_main.c | 53 +++++++++ 
> > > > > > > +----------------
> > > > > > >   1 file changed, 21 insertions(+), 32 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/ 
> > > > > > > drm/ scheduler/sched_main.c
> > > > > > > index e2cda28a1af4..5a550fd76bf0 100644
> > > > > > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > @@ -349,34 +349,13 @@ static void drm_sched_run_job_queue(struct 
> > > > > > > drm_gpu_scheduler *sched)
> > > > > > >   }
> > > > > > >   /**
> > > > > > > - * __drm_sched_run_free_queue - enqueue free-job work
> > > > > > > - * @sched: scheduler instance
> > > > > > > - */
> > > > > > > -static void __drm_sched_run_free_queue(struct drm_gpu_scheduler 
> > > > > > > *sched)
> > > > > > > -{
> > > > > > > -    if (!READ_ONCE(sched->pause_submit))
> > > > > > > -        queue_work(sched->submit_wq, &sched->work_free_job);
> > > > > > > -}
> > > > > > > -
> > > > > > > -/**
> > > > > > > - * drm_sched_run_free_queue - enqueue free-job work if ready
> > > > > > > + * drm_sched_run_free_queue - enqueue free-job work
> > > > > > >    * @sched: scheduler instance
> > > > > > >    */
> > > > > > >   static void drm_sched_run_free_queue(struct drm_gpu_scheduler 
> > > > > > > *sched)
> > > > > > >   {
> > > > > > > -    struct drm_sched_job *job;
> > > > > > > -
> > > > > > > -    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);
> > > > > > 
> > > > > > I believe we'd still need this chunk for DRM_GPU_SCHED_STAT_NO_HANG
> > > > > > (check the comment in drm_sched_job_reinsert_on_false_timeout()). How
> > > > > 
> > > > > You mean the "is there a signaled job in the list check" is needed 
> > > > > for drm_sched_job_reinsert_on_false_timeout()? Hmm why? Worst case 
> > > > > is a false positive wakeup on the free worker, no?
> > > > 
> > > > Correct me if I'm mistaken, we would also have a false positive wake-up
> > > > on the run_job worker, which I believe it could be problematic in the
> > > > cases that we skipped the reset because the job is still running.
> > > 
> > > Run job worker exits when it sees no free credits so I don't think 
> > > there is a problem. What am I missing?
> > > 
> > 
> > I was the one missing the code in `drm_sched_can_queue()`. Sorry for the
> > misleading comments. This is:
> > 
> > Reviewed-by: Maíra Canal <mcanal@igalia.com>
> 
> No worries, and thanks!
> 
> Philipp - are you okay with this version? V2 was done to address your
> feedback so that should be good now.

Was just giving it another spin when you wrote. (a [PATCH v3] would've
been neat for identification, though – I almost pulled the wrong patch
from the archive *wink*)

LGTM, improves things, can be merged.

However, we had to merge Lin Cao's bug fix [1] recently. That one is
now in drm-misc-fixes, and your patch should go to drm-misc-next. This
would cause a conflict once the two branches meet.

So I suggest that we wait with this non-urgent patch until drm-misc-
fixes / Linus's -rc gets merged into drm-misc-next, and then we apply
it. Should be next week or the week after AFAIK.

Unless somebody has a better idea, of course?

Remind me in case I forget.


P.

[1] https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/15f77764e90a713ee3916ca424757688e4f565b9


> 
> Regards,
> 
> Tvrtko
>