Re: [PATCH v3 4/9] drm/scheduler: Add fence deadline support

[Rob Clark <robdclark@gmail.com>]

On Wed, Sep 8, 2021 at 10:45 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Fri, Sep 03, 2021 at 11:47:55AM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@chromium.org>
> >
> > As the finished fence is the one that is exposed to userspace, and
> > therefore the one that other operations, like atomic update, would
> > block on, we need to propagate the deadline from from the finished
> > fence to the actual hw fence.
> >
> > v2: Split into drm_sched_fence_set_parent() (ckoenig)
> >
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > ---
> >  drivers/gpu/drm/scheduler/sched_fence.c | 34 +++++++++++++++++++++++++
> >  drivers/gpu/drm/scheduler/sched_main.c  |  2 +-
> >  include/drm/gpu_scheduler.h             |  8 ++++++
> >  3 files changed, 43 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c
> > index bcea035cf4c6..4fc41a71d1c7 100644
> > --- a/drivers/gpu/drm/scheduler/sched_fence.c
> > +++ b/drivers/gpu/drm/scheduler/sched_fence.c
> > @@ -128,6 +128,30 @@ static void drm_sched_fence_release_finished(struct dma_fence *f)
> >       dma_fence_put(&fence->scheduled);
> >  }
> >
> > +static void drm_sched_fence_set_deadline_finished(struct dma_fence *f,
> > +                                               ktime_t deadline)
> > +{
> > +     struct drm_sched_fence *fence = to_drm_sched_fence(f);
> > +     unsigned long flags;
> > +
> > +     spin_lock_irqsave(&fence->lock, flags);
> > +
> > +     /* If we already have an earlier deadline, keep it: */
> > +     if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &f->flags) &&
> > +         ktime_before(fence->deadline, deadline)) {
> > +             spin_unlock_irqrestore(&fence->lock, flags);
> > +             return;
> > +     }
> > +
> > +     fence->deadline = deadline;
> > +     set_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT, &f->flags);
> > +
> > +     spin_unlock_irqrestore(&fence->lock, flags);
> > +
> > +     if (fence->parent)
> > +             dma_fence_set_deadline(fence->parent, deadline);
> > +}
> > +
> >  static const struct dma_fence_ops drm_sched_fence_ops_scheduled = {
> >       .get_driver_name = drm_sched_fence_get_driver_name,
> >       .get_timeline_name = drm_sched_fence_get_timeline_name,
> > @@ -138,6 +162,7 @@ static const struct dma_fence_ops drm_sched_fence_ops_finished = {
> >       .get_driver_name = drm_sched_fence_get_driver_name,
> >       .get_timeline_name = drm_sched_fence_get_timeline_name,
> >       .release = drm_sched_fence_release_finished,
> > +     .set_deadline = drm_sched_fence_set_deadline_finished,
> >  };
> >
> >  struct drm_sched_fence *to_drm_sched_fence(struct dma_fence *f)
> > @@ -152,6 +177,15 @@ struct drm_sched_fence *to_drm_sched_fence(struct dma_fence *f)
> >  }
> >  EXPORT_SYMBOL(to_drm_sched_fence);
> >
> > +void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> > +                             struct dma_fence *fence)
> > +{
> > +     s_fence->parent = dma_fence_get(fence);
> > +     if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT,
> > +                  &s_fence->finished.flags))
>
> Don't you need the spinlock here too to avoid races? test_bit is very
> unordered, so guarantees nothing. Spinlock would need to be both around
> ->parent = and the test_bit.
>
> Entirely aside, but there's discussions going on to preallocate the hw
> fence somehow. If we do that we could make the deadline forwarding
> lockless here. Having a spinlock just to set the parent is a bit annoying
> ...
>
> Alternative is that you do this locklessly with barriers and a _lot_ of
> comments. Would be good to benchmark whether the overhead matters though
> first.

So, my thinking is that very few (well no) guarantees are made to the
fence implementor that their ->set_deadline() is not called multiple
times, from multiple threads, etc.  And no guarantee that a later
deadline is set after an earlier deadline has been set.  It is all up
to the set_deadline() implementation to deal with these cases.

So that means we just need the appropriate barrier-fu to ensure
another thread calling drm_sched_fence_set_deadline_finished() sees
fence->parent set before the test_bit.  It could mean that the backend
implementation sees the same deadline set twice, but that is fine.

BR,
-R

> -Daniel
>
> > +             dma_fence_set_deadline(fence, s_fence->deadline);
> > +}
> > +
> >  struct drm_sched_fence *drm_sched_fence_alloc(struct drm_sched_entity *entity,
> >                                             void *owner)
> >  {
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > index 595e47ff7d06..27bf0ac0625f 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -978,7 +978,7 @@ static int drm_sched_main(void *param)
> >               drm_sched_fence_scheduled(s_fence);
> >
> >               if (!IS_ERR_OR_NULL(fence)) {
> > -                     s_fence->parent = dma_fence_get(fence);
> > +                     drm_sched_fence_set_parent(s_fence, fence);
> >                       r = dma_fence_add_callback(fence, &sched_job->cb,
> >                                                  drm_sched_job_done_cb);
> >                       if (r == -ENOENT)
> > diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> > index 7f77a455722c..158ddd662469 100644
> > --- a/include/drm/gpu_scheduler.h
> > +++ b/include/drm/gpu_scheduler.h
> > @@ -238,6 +238,12 @@ struct drm_sched_fence {
> >           */
> >       struct dma_fence                finished;
> >
> > +     /**
> > +      * @deadline: deadline set on &drm_sched_fence.finished which
> > +      * potentially needs to be propagated to &drm_sched_fence.parent
> > +      */
> > +     ktime_t                         deadline;
> > +
> >          /**
> >           * @parent: the fence returned by &drm_sched_backend_ops.run_job
> >           * when scheduling the job on hardware. We signal the
> > @@ -505,6 +511,8 @@ void drm_sched_entity_set_priority(struct drm_sched_entity *entity,
> >                                  enum drm_sched_priority priority);
> >  bool drm_sched_entity_is_ready(struct drm_sched_entity *entity);
> >
> > +void drm_sched_fence_set_parent(struct drm_sched_fence *s_fence,
> > +                             struct dma_fence *fence);
> >  struct drm_sched_fence *drm_sched_fence_alloc(
> >       struct drm_sched_entity *s_entity, void *owner);
> >  void drm_sched_fence_init(struct drm_sched_fence *fence,
> > --
> > 2.31.1
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch