Re: [PATCH v7 13/16] drm/scheduler: Fix hang when sched_entity released

["Christian König" <christian.koenig@amd.com>]

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.

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@amd.com>
>>>>> Reviewed-by: Christian König <christian.koenig@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);
>>>>
>>