On Wed, 2024-09-11 at 13:22 +0100, Tvrtko Ursulin wrote:
>
> On 10/09/2024 11:25, Philipp Stanner wrote:
> > On Mon, 2024-09-09 at 18:19 +0100, Tvrtko Ursulin wrote:
> > > From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
> > >
> > > Having removed one re-lock cycle on the entity->lock in a patch
> > > titled
> > > "drm/sched: Optimise drm_sched_entity_push_job", with only a tiny
> > > bit
> > > larger refactoring we can do the same optimisation on the rq-
> > > >lock
> > > (Currently both drm_sched_rq_add_entity() and
> > > drm_sched_rq_update_fifo_locked() take and release the same
> > > lock.)
> > >
> > > To achieve this we rename drm_sched_rq_add_entity() to
> > > drm_sched_rq_add_entity_locked(), making it expect the rq->lock
> > > to be
> > > held, and also add the same expectation to
> > > drm_sched_rq_update_fifo_locked().
> > >
> > > For more stream-lining we also add the run-queue as an explicit
> > > parameter
> > > to drm_sched_rq_remove_fifo_locked() to avoid both callers and
> > > callee
> > > having to dereference entity->rq.
> >
> > Why is dereferencing it a problem?
>
> As you have noticed below the API is a bit unsightly. Consider for
> example this call chain:
>
> drm_sched_entity_kill(entity)
> drm_sched_rq_remove_entity(entity->rq, entity);
> drm_sched_rq_remove_fifo_locked(entity);
> struct drm_sched_rq *rq = entity->rq;
>
> A bit confused, no?
>
> I thought adding rq to remove_fifo_locked at least removes one back
> and
> forth between the entity->rq and rq.
>
> And then if we cache the rq in a local variable, after having
> explicitly
> taken the correct lock, we have this other call chain example:
>
> drm_sched_entity_push_job()
> ...
> rq = entity->rq;
> spin_lock(rq->lock);
>
> drm_sched_rq_add_entity_locked(rq, entity);
> drm_sched_rq_update_fifo_locked(rq, entity, submit_ts);
>
> spin_unlock(rq->lock);
>
> To me at least this reads more streamlined.
Alright, doesn't sound to bad, but
>
> > > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
> > > Cc: Christian König <christian.koenig@xxxxxxx>
> > > Cc: Alex Deucher <alexander.deucher@xxxxxxx>
> > > Cc: Luben Tuikov <ltuikov89@xxxxxxxxx>
> > > Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> > > Cc: Philipp Stanner <pstanner@xxxxxxxxxx>
> > > ---
> > > drivers/gpu/drm/scheduler/sched_entity.c | 7 ++--
> > > drivers/gpu/drm/scheduler/sched_main.c | 41 +++++++++++++----
> > > -----
> > > --
> > > include/drm/gpu_scheduler.h | 7 ++--
> > > 3 files changed, 31 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
> > > b/drivers/gpu/drm/scheduler/sched_entity.c
> > > index b4c4f9923e0b..2102c726d275 100644
> > > --- a/drivers/gpu/drm/scheduler/sched_entity.c
> > > +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> > > @@ -614,11 +614,14 @@ void drm_sched_entity_push_job(struct
> > > drm_sched_job *sched_job)
> > > sched = rq->sched;
> > >
> > > atomic_inc(sched->score);
> > > - drm_sched_rq_add_entity(rq, entity);
> > > +
> > > + spin_lock(&rq->lock);
> > > + drm_sched_rq_add_entity_locked(rq, entity);
> > >
> > > if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
> > > - drm_sched_rq_update_fifo_locked(entity,
> > > submit_ts);
> > > + drm_sched_rq_update_fifo_locked(entity,
> > > rq,
> > > submit_ts);
> > >
> > > + spin_unlock(&rq->lock);
> > > spin_unlock(&entity->lock);
> > >
> > > drm_sched_wakeup(sched, entity);
> > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> > > b/drivers/gpu/drm/scheduler/sched_main.c
> > > index 937e7d1cfc49..1ccd2aed2d32 100644
> > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > @@ -153,41 +153,44 @@ static __always_inline bool
> > > drm_sched_entity_compare_before(struct rb_node *a,
> > > return ktime_before(ent_a->oldest_job_waiting, ent_b-
> > > > oldest_job_waiting);
> > > }
> > >
> > > -static inline void drm_sched_rq_remove_fifo_locked(struct
> > > drm_sched_entity *entity)
> > > +static void drm_sched_rq_remove_fifo_locked(struct
> > > drm_sched_entity
> > > *entity,
> > > + struct drm_sched_rq
> > > *rq)
I would then at least like to see a comment somewhere telling the
reader why rq is taken as a separate variable. One might otherwise
easily wonder why it's not obtained through the entity and what the
difference is.
> >
> > So here we'd add a new function parameter that still doesn't allow
> > for
> > getting rid of 'entity' as a parameter.
>
> We can't get rid of the entity.
>
> Maaaybe instead we could get rid of the rq in the whole chain, I mean
> from drm_sched_rq_add_entity and drm_sched_rq_remove_entity to start
> with.
Let's postpone that.
>
> But then to remove double re-lock we still (like in this patch) need
> to
> make the callers take the locks and rename the helpers with _locked
> suffix. Otherwise it would be incosistent that a lock is taken
> outside
> the helpers with no _locked suffix.
>
> I am not sure if that is better. All it achieves is remove the rq as
> explicit parameter my making the callees dereference it from the
> entity.
OK, as I see it now it would actually be desirable to have suffix
_locked indicate that the caller must hold all necessary locks. So your
patch would actually make that consistent within drm/sched/.
Looks good
P.
>
> Worst part is all these helpers have drm_sched_rq_ prefix.. which to
> me
> reads as "we operate on rq". So not passing in rq is confusing to
> start
> with.
>
> Granted, some confusion still remains with my approach since ideally,
> to
> those helpers, I wanted to add some asserts that rq == entity->rq...
>
> > The API gets larger that way and readers will immediately wonder
> > why
> > sth is passed as a separate variable that could also be obtained
> > through the pointer.
> >
> > > {
> > > - struct drm_sched_rq *rq = entity->rq;
> > > -
> > > if (!RB_EMPTY_NODE(&entity->rb_tree_node)) {
> > > rb_erase_cached(&entity->rb_tree_node, &rq-
> > > > rb_tree_root);
> > > RB_CLEAR_NODE(&entity->rb_tree_node);
> > > }
> > > }
> > >
> > > -void drm_sched_rq_update_fifo_locked(struct drm_sched_entity
> > > *entity, ktime_t ts)
> > > +void drm_sched_rq_update_fifo_locked(struct drm_sched_entity
> > > *entity,
> > > + struct drm_sched_rq *rq,
> > > + ktime_t ts)
> >
> > The function is still called _locked. That implies to the reader
> > that
> > this function takes care of locking. But it doesn't anymore.
> > Instead,
> >
> > > {
> > > lockdep_assert_held(&entity->lock);
> > > + lockdep_assert_held(&rq->lock);
> > >
> > > - spin_lock(&entity->rq->lock);
> > > -
> > > - drm_sched_rq_remove_fifo_locked(entity);
> > > + drm_sched_rq_remove_fifo_locked(entity, rq);
> > >
> > > entity->oldest_job_waiting = ts;
> > >
> > > - rb_add_cached(&entity->rb_tree_node, &entity->rq-
> > > > rb_tree_root,
> > > + rb_add_cached(&entity->rb_tree_node, &rq->rb_tree_root,
> > > drm_sched_entity_compare_before);
> > > -
> > > - spin_unlock(&entity->rq->lock);
> > > }
> > >
> > > void drm_sched_rq_update_fifo(struct drm_sched_entity *entity,
> > > ktime_t ts)
> > > {
> > > + struct drm_sched_rq *rq;
> > > +
> > > /*
> > > * Both locks need to be grabbed, one to protect from
> > > entity->rq change
> > > * for entity from within concurrent
> > > drm_sched_entity_select_rq and the
> > > * other to update the rb tree structure.
> > > */
> > > spin_lock(&entity->lock);
> > > - drm_sched_rq_update_fifo_locked(entity, ts);
> > > + rq = entity->rq;
> > > + spin_lock(&rq->lock);
> > > + drm_sched_rq_update_fifo_locked(entity, rq, ts);
> > > + spin_unlock(&rq->lock);
> >
> > its caller, drm_sched_rq_update_fifo(), now takes care of the
> > locking.
> > So if it all drm_sched_rq_update_fifo_locked() should be called
> > drm_sched_rq_update_fifo_unlocked().
> >
> > If such a change is really being done, we have to go through the
> > entire
> > scheduler and make sure that the suffix "_locked" is used
> > consistently
> > throughout the scheduler. And even better, as consistent with the
> > kernel as possible.
>
> Use of _locked follows the existing pattern of
> drm_sched_rq_remove_fifo_locked :shrug:
>
> Are you referring to drm_sched_start_timeout(_unlocked) which is the
> opposite pattern? (Although a more recent addition.)
>
> The wider kernel also uses both patterns so don't know. Would be nice
> to
> align in the scheduler but drm_sched_start_timeout is out of scope
> for
> this series.
>
> > To be honest folks, I don't think this entire "optimization" patch
> > is
> > that much of a good idea. The scheduler has real, big problems,
> > such as
> > race conditions, memory leaks and lack of documentation.
> >
> > I think we should for the forseeable future dedicate our attention
> > towards solving those problems, instead of optimizing things.
> > Especially if the optimization might decrease readability as with
> > the
> > naming here.
>
> In principle I agree, but on the other hand lets first see if this
> patch
> is really making things any worse, or is perhaps just maintaining the
> status quo in the API elegance department, while at the same time
> removing the quite lazy double re-lock from the main submission path.
>
> Regards,
>
> Tvrtko
>
> >
> > P.
> >
> >
> > > spin_unlock(&entity->lock);
> > > }
> > >
> > > @@ -210,25 +213,23 @@ static void drm_sched_rq_init(struct
> > > drm_gpu_scheduler *sched,
> > > }
> > >
> > > /**
> > > - * drm_sched_rq_add_entity - add an entity
> > > + * drm_sched_rq_add_entity_locked - add an entity
> > > *
> > > * @rq: scheduler run queue
> > > * @entity: scheduler entity
> > > *
> > > * Adds a scheduler entity to the run queue.
> > > */
> > > -void drm_sched_rq_add_entity(struct drm_sched_rq *rq,
> > > - struct drm_sched_entity *entity)
> > > +void drm_sched_rq_add_entity_locked(struct drm_sched_rq *rq,
> > > + struct drm_sched_entity
> > > *entity)
> > > {
> > > + lockdep_assert_held(&rq->lock);
> > > +
> > > if (!list_empty(&entity->list))
> > > return;
> > >
> > > - spin_lock(&rq->lock);
> > > -
> > > atomic_inc(rq->sched->score);
> > > list_add_tail(&entity->list, &rq->entities);
> > > -
> > > - spin_unlock(&rq->lock);
> > > }
> > >
> > > /**
> > > @@ -242,6 +243,8 @@ void drm_sched_rq_add_entity(struct
> > > drm_sched_rq
> > > *rq,
> > > void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
> > > struct drm_sched_entity *entity)
> > > {
> > > + lockdep_assert_held(&entity->lock);
> > > +
> > > if (list_empty(&entity->list))
> > > return;
> > >
> > > @@ -254,7 +257,7 @@ void drm_sched_rq_remove_entity(struct
> > > drm_sched_rq *rq,
> > > rq->current_entity = NULL;
> > >
> > > if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
> > > - drm_sched_rq_remove_fifo_locked(entity);
> > > + drm_sched_rq_remove_fifo_locked(entity, rq);
> > >
> > > spin_unlock(&rq->lock);
> > > }
> > > diff --git a/include/drm/gpu_scheduler.h
> > > b/include/drm/gpu_scheduler.h
> > > index 5a1e4c803b90..2ad33e2fe2d2 100644
> > > --- a/include/drm/gpu_scheduler.h
> > > +++ b/include/drm/gpu_scheduler.h
> > > @@ -591,13 +591,14 @@ bool drm_sched_dependency_optimized(struct
> > > dma_fence* fence,
> > > struct drm_sched_entity
> > > *entity);
> > > void drm_sched_fault(struct drm_gpu_scheduler *sched);
> > >
> > > -void drm_sched_rq_add_entity(struct drm_sched_rq *rq,
> > > - struct drm_sched_entity *entity);
> > > +void drm_sched_rq_add_entity_locked(struct drm_sched_rq *rq,
> > > + struct drm_sched_entity
> > > *entity);
> > > void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
> > > struct drm_sched_entity
> > > *entity);
> > >
> > > void drm_sched_rq_update_fifo(struct drm_sched_entity *entity,
> > > ktime_t ts);
> > > -void drm_sched_rq_update_fifo_locked(struct drm_sched_entity
> > > *entity, ktime_t ts);
> > > +void drm_sched_rq_update_fifo_locked(struct drm_sched_entity
> > > *entity,
> > > + struct drm_sched_rq *rq,
> > > ktime_t ts);
> > >
> > > int drm_sched_entity_init(struct drm_sched_entity *entity,
> > > enum drm_sched_priority priority,
> >
>
