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.
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)
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.
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.
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,
