[PATCH v7 13/16] drm/scheduler: Fix hang when sched_entity released
Andrey Grodzovsky
andrey.grodzovsky at amd.com
Tue May 18 18:09:49 UTC 2021
On 2021-05-18 2:02 p.m., Christian König wrote:
> Am 18.05.21 um 19:43 schrieb Andrey Grodzovsky:
>> On 2021-05-18 12:33 p.m., Christian König wrote:
>>> Am 18.05.21 um 18:17 schrieb Andrey Grodzovsky:
>>>>
>>>>
>>>> On 2021-05-18 11:15 a.m., Christian König wrote:
>>>>> Am 18.05.21 um 17:03 schrieb Andrey Grodzovsky:
>>>>>>
>>>>>> On 2021-05-18 10:07 a.m., Christian König wrote:
>>>>>>> In a separate discussion with Daniel we once more iterated over
>>>>>>> the dma_resv requirements and I came to the conclusion that this
>>>>>>> approach here won't work reliable.
>>>>>>>
>>>>>>> The problem is as following:
>>>>>>> 1. device A schedules some rendering with into a buffer and
>>>>>>> exports it as DMA-buf.
>>>>>>> 2. device B imports the DMA-buf and wants to consume the
>>>>>>> rendering, for the the fence of device A is replaced with a new
>>>>>>> operation.
>>>>>>> 3. device B is hot plugged and the new operation canceled/newer
>>>>>>> scheduled.
>>>>>>>
>>>>>>> The problem is now that we can't do this since the operation of
>>>>>>> device A is still running and by signaling our fences we run into
>>>>>>> the problem of potential memory corruption.
>>>>
>>>> By signaling s_fence->finished of the canceled operation from the
>>>> removed device B we in fact cause memory corruption for the uncompleted
>>>> job still running on device A ? Because there is someone waiting to
>>>> read write from the imported buffer on device B and he only waits for
>>>> the s_fence->finished of device B we signaled
>>>> in drm_sched_entity_kill_jobs ?
>>>
>>> Exactly that, yes.
>>>
>>> In other words when you have a dependency chain like A->B->C then
>>> memory management only waits for C before freeing up the memory for
>>> example.
>>>
>>> When C now signaled because the device is hot-plugged before A or B
>>> are finished they are essentially accessing freed up memory.
>>
>> But didn't C imported the BO form B or A in this case ? Why would he be
>> the one releasing that memory ? He would be just dropping his reference
>> to the BO, no ?
>
> Well freeing the memory was just an example. The BO could also move back
> to VRAM because of the dropped reference.
>
>> Also in the general case,
>> drm_sched_entity_fini->drm_sched_entity_kill_jobs which is
>> the one who signals the 'C' fence with error code are as far
>> as I looked called from when the user of that BO is stopping
>> the usage anyway (e.g. drm_driver.postclose callback for when use
>> process closes his device file) who would then access and corrupt
>> the exported memory on device A where the job hasn't completed yet ?
>
> Key point is that memory management only waits for the last added fence,
> that is the design of the dma_resv object. How that happens is irrelevant.
>
> Because of this we at least need to wait for all dependencies of the job
> before signaling the fence, even if we cancel the job for some reason.
>
> Christian.
Would this be the right way to do it ?
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
b/drivers/gpu/drm/scheduler/sched_entity.c
index 2e93e881b65f..10f784874b63 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -223,10 +223,14 @@ static void drm_sched_entity_kill_jobs(struct
drm_sched_entity *entity)
{
struct drm_sched_job *job;
int r;
+ struct dma_fence *f;
while ((job =
to_drm_sched_job(spsc_queue_pop(&entity->job_queue)))) {
struct drm_sched_fence *s_fence = job->s_fence;
+ while (f = sched->ops->dependency(sched_job, entity))
+ dma_fence_wait(f);
+
drm_sched_fence_scheduled(s_fence);
dma_fence_set_error(&s_fence->finished, -ESRCH);
Andrey
>
>>
>> Andrey
>>
>>>
>>> Christian.
>>>
>>>>
>>>> Andrey
>>>>
>>>>>>
>>>>>>
>>>>>> I am not sure this problem you describe above is related to this
>>>>>> patch.
>>>>>
>>>>> Well it is kind of related.
>>>>>
>>>>>> Here we purely expand the criteria for when sched_entity is
>>>>>> considered idle in order to prevent a hang on device remove.
>>>>>
>>>>> And exactly that is problematic. See the jobs on the entity need to
>>>>> cleanly wait for their dependencies before they can be completed.
>>>>>
>>>>> drm_sched_entity_kill_jobs() is also not handling that correctly at
>>>>> the moment, we only wait for the last scheduled fence but not for
>>>>> the dependencies of the job.
>>>>>
>>>>>> Were you addressing the patch from yesterday in which you commented
>>>>>> that you found a problem with how we finish fences ? It was
>>>>>> '[PATCH v7 12/16] drm/amdgpu: Fix hang on device removal.'
>>>>>>
>>>>>> Also, in the patch series as it is now we only signal HW fences
>>>>>> for the
>>>>>> extracted device B, we are not touching any other fences. In fact
>>>>>> as you
>>>>>> may remember, I dropped all new logic to forcing fence completion in
>>>>>> this patch series and only call amdgpu_fence_driver_force_completion
>>>>>> for the HW fences of the extracted device as it's done today anyway.
>>>>>
>>>>> Signaling hardware fences is unproblematic since they are emitted
>>>>> when the software scheduling is already completed.
>>>>>
>>>>> Christian.
>>>>>
>>>>>>
>>>>>> Andrey
>>>>>>
>>>>>>>
>>>>>>> Not sure how to handle that case. One possibility would be to
>>>>>>> wait for all dependencies of unscheduled jobs before signaling
>>>>>>> their fences as canceled.
>>>>>>>
>>>>>>> Christian.
>>>>>>>
>>>>>>> Am 12.05.21 um 16:26 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.
>>>>>>>>
>>>>>>>> v3:
>>>>>>>> Drop drm_sched_rq_remove_entity, only modify entity->stopped
>>>>>>>> and check for it in drm_sched_entity_is_idle
>>>>>>>>
>>>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>>>>>>> Reviewed-by: Christian König <christian.koenig at amd.com>
>>>>>>>> ---
>>>>>>>> drivers/gpu/drm/scheduler/sched_entity.c | 3 ++-
>>>>>>>> drivers/gpu/drm/scheduler/sched_main.c | 24
>>>>>>>> ++++++++++++++++++++++++
>>>>>>>> 2 files changed, 26 insertions(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
>>>>>>>> b/drivers/gpu/drm/scheduler/sched_entity.c
>>>>>>>> index 0249c7450188..2e93e881b65f 100644
>>>>>>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>>>>>>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>>>>>>>> @@ -116,7 +116,8 @@ static bool drm_sched_entity_is_idle(struct
>>>>>>>> drm_sched_entity *entity)
>>>>>>>> rmb(); /* for list_empty to work without lock */
>>>>>>>> if (list_empty(&entity->list) ||
>>>>>>>> - spsc_queue_count(&entity->job_queue) == 0)
>>>>>>>> + spsc_queue_count(&entity->job_queue) == 0 ||
>>>>>>>> + entity->stopped)
>>>>>>>> return true;
>>>>>>>> return false;
>>>>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>> b/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>> index 8d1211e87101..a2a953693b45 100644
>>>>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>>>>>> @@ -898,9 +898,33 @@ EXPORT_SYMBOL(drm_sched_init);
>>>>>>>> */
>>>>>>>> void drm_sched_fini(struct drm_gpu_scheduler *sched)
>>>>>>>> {
>>>>>>>> + struct drm_sched_entity *s_entity;
>>>>>>>> + int i;
>>>>>>>> +
>>>>>>>> if (sched->thread)
>>>>>>>> kthread_stop(sched->thread);
>>>>>>>> + 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;
>>>>>>>> +
>>>>>>>> + spin_lock(&rq->lock);
>>>>>>>> + list_for_each_entry(s_entity, &rq->entities, list)
>>>>>>>> + /*
>>>>>>>> + * Prevents reinsertion and marks job_queue as idle,
>>>>>>>> + * it will removed from rq in drm_sched_entity_fini
>>>>>>>> + * eventually
>>>>>>>> + */
>>>>>>>> + s_entity->stopped = true;
>>>>>>>> + 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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