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

[Andrey Grodzovsky <andrey.grodzovsky@amd.com>]

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 ?

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);
>>>
>