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

[Rob Clark <robdclark@gmail.com>]

On Tue, Sep 21, 2021 at 8:57 AM Rob Clark <robdclark@gmail.com> wrote:
>
> 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.
>

something like:

-----
diff --git a/drivers/gpu/drm/scheduler/sched_fence.c
b/drivers/gpu/drm/scheduler/sched_fence.c
index 4fc41a71d1c7..7f2af6d1777c 100644
--- a/drivers/gpu/drm/scheduler/sched_fence.c
+++ b/drivers/gpu/drm/scheduler/sched_fence.c
@@ -132,6 +132,7 @@ 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);
+ struct dma_fence *parent;
  unsigned long flags;

  spin_lock_irqsave(&fence->lock, flags);
@@ -148,8 +149,9 @@ static void
drm_sched_fence_set_deadline_finished(struct dma_fence *f,

  spin_unlock_irqrestore(&fence->lock, flags);

- if (fence->parent)
- dma_fence_set_deadline(fence->parent, deadline);
+ parent = smp_load_acquire(&fence->parent);
+ if (parent)
+ dma_fence_set_deadline(parent, deadline);
 }

 static const struct dma_fence_ops drm_sched_fence_ops_scheduled = {
@@ -180,7 +182,7 @@ 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);
+ smp_store_release(&s_fence->parent, dma_fence_get(fence));
  if (test_bit(DMA_FENCE_FLAG_HAS_DEADLINE_BIT,
       &s_fence->finished.flags))
  dma_fence_set_deadline(fence, s_fence->deadline);
-----

BR,
-R