[PATCH v7 13/16] drm/scheduler: Fix hang when sched_entity released
Christian König
christian.koenig at amd.com
Tue May 18 18:13:43 UTC 2021
Am 18.05.21 um 20:09 schrieb Andrey Grodzovsky:
> 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 ?
Yes, it is at least a start. Question is if we can wait blocking here or
not.
We install a callback a bit lower to avoid blocking, so I'm pretty sure
that won't work as expected.
Christian.
>
> 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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