[PATCH v2] drm/scheduler: Fix hang when sched_entity released
Andrey Grodzovsky
andrey.grodzovsky at amd.com
Thu Feb 25 21:27:04 UTC 2021
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 at amd.com>
>>>>>>>>>>> ---
>>>>>>>>>>> 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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