[PATCH] drm/sched: Barriers are needed for entity->last_scheduled

[Daniel Vetter]

On Fri, Jul 9, 2021 at 8:58 AM Christian König <christian.koenig at amd.com> wrote:
>
>
>
> Am 08.07.21 um 23:54 schrieb Daniel Vetter:
> > It might be good enough on x86 with just READ_ONCE, but the write side
> > should then at least be WRITE_ONCE because x86 has total store order.
> >
> > It's definitely not enough on arm.
> >
> > Fix this proplery, which means
> > - explain the need for the barrier in both places
> > - point at the other side in each comment
> >
> > Also pull out the !sched_list case as the first check, so that the
> > code flow is clearer.
> >
> > While at it sprinkle some comments around because it was very
> > non-obvious to me what's actually going on here and why.
> >
> > Note that we really need full barriers here, at first I thought
> > store-release and load-acquire on ->last_scheduled would be enough,
> > but we actually requiring ordering between that and the queue state.
> >
> > v2: Put smp_rmp() in the right place and fix up comment (Andrey)
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: "Christian König" <christian.koenig at amd.com>
> > Cc: Steven Price <steven.price at arm.com>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Cc: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
> > Cc: Lee Jones <lee.jones at linaro.org>
> > Cc: Boris Brezillon <boris.brezillon at collabora.com>
> > ---
> >   drivers/gpu/drm/scheduler/sched_entity.c | 27 ++++++++++++++++++++++--
> >   1 file changed, 25 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> > index 64d398166644..6366006c0fcf 100644
> > --- a/drivers/gpu/drm/scheduler/sched_entity.c
> > +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> > @@ -439,8 +439,16 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
> >               dma_fence_set_error(&sched_job->s_fence->finished, -ECANCELED);
> >
> >       dma_fence_put(entity->last_scheduled);
> > +
> >       entity->last_scheduled = dma_fence_get(&sched_job->s_fence->finished);
> >
> > +     /*
> > +      * If the queue is empty we allow drm_sched_entity_select_rq() to
> > +      * locklessly access ->last_scheduled. This only works if we set the
> > +      * pointer before we dequeue and if we a write barrier here.
> > +      */
> > +     smp_wmb();
> > +
>
> That whole stuff needs to be inside the spsc queue, not outside.
>
> Otherwise drm_sched_entity_is_idle() won't work either and cause a lot
> of trouble during process tear down.

Nah, that just means you need another 2 comments with their barrier to
explain how things are serialized there against entity->stopped. The
queue only needs to provide store-release and load-acquire barriers
from a functional pov, if you assumie more then that's very strange.
We need barriers in the other direction here (I haven't looked at what
entity_is_idle) needs.

This is why the first rule of lockless algorithms is "don't", and the
second one is to very painstaikingly document every barrier necessary
with
- explanation what it synchronizes and why
- and a pointer to where the other side of the barrier is in the code
(there always has to be one, barrier on one side does nothing)
-Daniel

> Christian.
>
> >       spsc_queue_pop(&entity->job_queue);
> >       return sched_job;
> >   }
> > @@ -459,10 +467,25 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
> >       struct drm_gpu_scheduler *sched;
> >       struct drm_sched_rq *rq;
> >
> > -     if (spsc_queue_count(&entity->job_queue) || !entity->sched_list)
> > +     /* single possible engine and already selected */
> > +     if (!entity->sched_list)
> > +             return;
> > +
> > +     /* queue non-empty, stay on the same engine */
> > +     if (spsc_queue_count(&entity->job_queue))
> >               return;
> >
> > -     fence = READ_ONCE(entity->last_scheduled);
> > +     /*
> > +      * Only when the queue is empty are we guaranteed that the scheduler
> > +      * thread cannot change ->last_scheduled. To enforce ordering we need
> > +      * a read barrier here. See drm_sched_entity_pop_job() for the other
> > +      * side.
> > +      */
> > +     smp_rmb();
> > +
> > +     fence = entity->last_scheduled;
> > +
> > +     /* stay on the same engine if the previous job hasn't finished */
> >       if (fence && !dma_fence_is_signaled(fence))
> >               return;
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch