On 2021-02-25 1:42 p.m., Christian König wrote:
Am 25.02.21 um 17:03 schrieb Andrey Grodzovsky:
On 2021-02-25 2:53 a.m., Christian König wrote:
Am 24.02.21 um 16:13 schrieb Andrey Grodzovsky:
Ping
Sorry, I've been on vacation this week.
Andrey
On 2021-02-20 7:12 a.m., Andrey Grodzovsky wrote:
On 2/20/21 3:38 AM, Christian König wrote:
Am 18.02.21 um 17:41 schrieb Andrey Grodzovsky:
On 2/18/21 10:15 AM, Christian König wrote:
Am 18.02.21 um 16:05 schrieb Andrey Grodzovsky:
On 2/18/21 3:07 AM, Christian König wrote:
Am 17.02.21 um 22:59 schrieb Andrey Grodzovsky:
Problem: If scheduler is already stopped by the time
sched_entity
is released and entity's job_queue not empty I encountred
a hang in drm_sched_entity_flush. This is because
drm_sched_entity_is_idle
never becomes false.
Fix: In drm_sched_fini detach all sched_entities from the
scheduler's run queues. This will satisfy
drm_sched_entity_is_idle.
Also wakeup all those processes stuck in sched_entity flushing
as the scheduler main thread which wakes them up is stopped
by now.
v2:
Reverse order of drm_sched_rq_remove_entity and marking
s_entity as stopped to prevent reinserion back to rq due
to race.
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
---
drivers/gpu/drm/scheduler/sched_main.c | 31
+++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)
diff --git a/drivers/gpu/drm/scheduler/sched_main.c
b/drivers/gpu/drm/scheduler/sched_main.c
index 908b0b5..c6b7947 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -897,9 +897,40 @@ EXPORT_SYMBOL(drm_sched_init);
*/
void drm_sched_fini(struct drm_gpu_scheduler *sched)
{
+ int i;
+ struct drm_sched_entity *s_entity;
BTW: Please order that so that i is declared last.
if (sched->thread)
kthread_stop(sched->thread);
+ /* Detach all sched_entites from this scheduler once
it's stopped */
+ for (i = DRM_SCHED_PRIORITY_COUNT - 1; i >=
DRM_SCHED_PRIORITY_MIN; i--) {
+ struct drm_sched_rq *rq = &sched->sched_rq[i];
+
+ if (!rq)
+ continue;
+
+ /* Loop this way because rq->lock is taken in
drm_sched_rq_remove_entity */
+ spin_lock(&rq->lock);
+ while ((s_entity =
list_first_entry_or_null(&rq->entities,
+ struct drm_sched_entity,
+ list))) {
+ spin_unlock(&rq->lock);
+
+ /* Prevent reinsertion and remove */
+ spin_lock(&s_entity->rq_lock);
+ s_entity->stopped = true;
+ drm_sched_rq_remove_entity(rq, s_entity);
+ spin_unlock(&s_entity->rq_lock);
Well this spin_unlock/lock dance here doesn't look correct at
all now.
Christian.
In what way ? It's in the same same order as in other call
sites (see drm_sched_entity_push_job and drm_sched_entity_flush).
If i just locked rq->lock and did list_for_each_entry_safe
while manually deleting entity->list instead of calling
drm_sched_rq_remove_entity this still would not be possible as
the order of lock acquisition between s_entity->rq_lock
and rq->lock would be reverse compared to the call sites
mentioned above.
Ah, now I understand. You need this because
drm_sched_rq_remove_entity() will grab the rq lock again!
Problem is now what prevents the entity from being destroyed
while you remove it?
Christian.
Right, well, since (unfortunately) sched_entity is part of
amdgpu_ctx_entity and amdgpu_ctx_entity is refcounted
there is a problem here that we don't increment
amdgpu_ctx.refcount when assigning sched_entity
to new rq (e.g. before drm_sched_rq_add_entity) and not
decrement before removing. We do it for
amdgpu_cs_parser.entity for example (in amdgpu_cs_parser_init
and amdgpu_cs_parser_fini by
calling amdgpu_ctx_get and amdgpu_ctx_put). But this seems a bit
tricky due to all the drm_sched_entity_select_rq
logic.
Another, kind of a band aid fix, would probably be just locking
amdgpu_ctx_mgr.lock around drm_sched_fini
when finalizing the fence driver and around idr iteration in
amdgpu_ctx_mgr_fini (which should be lock protected
anyway as I see from other idr usages in the code) ... This
should prevent this use after free.
Puh, that's rather complicated as well. Ok let's look at it from
the other side for a moment.
Why do we have to remove the entities from the rq in the first
place?
Wouldn't it be sufficient to just set all of them to stopped?
Christian.
And adding it as another condition in drm_sched_entity_is_idle ?
Yes, I think that should work.
In this case reverse locking order is created, In
drm_sched_entity_push_job and drm_sched_entity_flush lock
entity->rq_lock locked first and rq->lock locked second. In
drm_sched_fini on the other hand, I need to lock rq->lock first to
iterate rq->entities and then lock s_entity->rq_lock for each entity
to modify s_entity->stopped. I guess we could change
s_entity->stopped to atomic ?
Good point. But I think the memory barrier inserted by
wake_up_all(&sched->job_scheduled); should be sufficient.
If I see this correctly we have the entity->rq_lock mainly to protect
concurrent changes of entity->rq.
But when two CPUs both set entity->stopped to true at the same time we
don't really care about it as long drm_sched_entity_is_idle() sees it.
Regards,
Christian.
I was more thinking about integrity of reading/writing entity->stopped
from different threads. I guess since it's bool it's guaranteed to be
atomic from HW perspective anyway ?
Will send V3 soon.
Andrey
Andrey
Christian.
Andrey
Andrey
Andrey
+
+ spin_lock(&rq->lock);
+ }
+ spin_unlock(&rq->lock);
+
+ }
+
+ /* Wakeup everyone stuck in drm_sched_entity_flush for
this scheduler */
+ wake_up_all(&sched->job_scheduled);
+
/* Confirm no work left behind accessing device
structures */
cancel_delayed_work_sync(&sched->work_tdr);
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