Re: [PATCH 0/4] Refine GPU recovery sequence to enhance its stability

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

On 2021-04-12 3:18 p.m., Christian König wrote:
> Am 12.04.21 um 21:12 schrieb Andrey Grodzovsky:
>> [SNIP]
>>>>
>>>> So what's the right approach ? How we guarantee that when running 
>>>> amdgpu_fence_driver_force_completion we will signal all the HW 
>>>> fences and not racing against some more fences insertion into that 
>>>> array ?
>>>>
>>>
>>> Well I would still say the best approach would be to insert this 
>>> between the front end and the backend and not rely on signaling 
>>> fences while holding the device srcu.
>>
>>
>> My question is, even now, when we run 
>> amdgpu_fence_driver_fini_hw->amdgpu_fence_wait_empty or 
>> amdgpu_fence_driver_fini_hw->amdgpu_fence_driver_force_completion, 
>> what there prevents a race with another fence being at the same time 
>> emitted and inserted into the fence array ? Looks like nothing.
>>
>
> Each ring can only be used by one thread at the same time, this 
> includes emitting fences as well as other stuff.
>
> During GPU reset we make sure nobody writes to the rings by stopping 
> the scheduler and taking the GPU reset lock (so that nobody else can 
> start the scheduler again).


What about direct submissions not through scheduler - 
amdgpu_job_submit_direct, I don't see how this is protected.


>
>>>
>>> BTW: Could it be that the device SRCU protects more than one device 
>>> and we deadlock because of this?
>>
>>
>> I haven't actually experienced any deadlock until now but, yes, 
>> drm_unplug_srcu is defined as static in drm_drv.c and so in the 
>> presence  of multiple devices from same or different drivers we in 
>> fact are dependent on all their critical sections i guess.
>>
>
> Shit, yeah the devil is a squirrel. So for A+I laptops we actually 
> need to sync that up with Daniel and the rest of the i915 guys.
>
> IIRC we could actually have an amdgpu device in a docking station 
> which needs hotplug and the driver might depend on waiting for the 
> i915 driver as well.


Can't we propose a patch to make drm_unplug_srcu per drm_device ? I 
don't see why it has to be global and not per device thing.

Andrey


>
> Christian.
>
>> Andrey
>>
>>
>>>
>>> Christian.
>>>
>>>> Andrey
>>>>
>>>>
>>>>>
>>>>>> Andrey
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Christian.
>>>>>>>
>>>>>>>>     /* Past this point no more fence are submitted to HW ring 
>>>>>>>> and hence we can safely call force signal on all that are 
>>>>>>>> currently there.
>>>>>>>>      * Any subsequently created  HW fences will be returned 
>>>>>>>> signaled with an error code right away
>>>>>>>>      */
>>>>>>>>
>>>>>>>>     for_each_ring(adev)
>>>>>>>>         amdgpu_fence_process(ring)
>>>>>>>>
>>>>>>>>     drm_dev_unplug(dev);
>>>>>>>>     Stop schedulers
>>>>>>>>     cancel_sync(all timers and queued works);
>>>>>>>>     hw_fini
>>>>>>>>     unmap_mmio
>>>>>>>>
>>>>>>>> }
>>>>>>>>
>>>>>>>>
>>>>>>>> Andrey
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Alternatively grabbing the reset write side and stopping 
>>>>>>>>>>>>> and then restarting the scheduler could work as well.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Christian.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> I didn't get the above and I don't see why I need to reuse 
>>>>>>>>>>>> the GPU reset rw_lock. I rely on the SRCU unplug flag for 
>>>>>>>>>>>> unplug. Also, not clear to me why are we focusing on the 
>>>>>>>>>>>> scheduler threads, any code patch to generate HW fences 
>>>>>>>>>>>> should be covered, so any code leading to amdgpu_fence_emit 
>>>>>>>>>>>> needs to be taken into account such as, direct IB 
>>>>>>>>>>>> submissions, VM flushes e.t.c
>>>>>>>>>>>
>>>>>>>>>>> You need to work together with the reset lock anyway, cause 
>>>>>>>>>>> a hotplug could run at the same time as a reset.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> For going my way indeed now I see now that I have to take 
>>>>>>>>>> reset write side lock during HW fences signalling in order to 
>>>>>>>>>> protect against scheduler/HW fences detachment and 
>>>>>>>>>> reattachment during schedulers stop/restart. But if we go 
>>>>>>>>>> with your approach  then calling drm_dev_unplug and scoping 
>>>>>>>>>> amdgpu_job_timeout with drm_dev_enter/exit should be enough 
>>>>>>>>>> to prevent any concurrent GPU resets during unplug. In fact I 
>>>>>>>>>> already do it anyway - 
>>>>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=https:%2F%2Fcgit.freedesktop.org%2F~agrodzov%2Flinux%2Fcommit%2F%3Fh%3Ddrm-misc-next%26id%3Def0ea4dd29ef44d2649c5eda16c8f4869acc36b1&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Ceefa9c90ed8c405ec3b708d8fc46daaa%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637536728550884740%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=UiNaJE%2BH45iYmbwSDnMSKZS5z0iak0fNlbbfYqKS2Jo%3D&reserved=0
>>>>>>>>>
>>>>>>>>> Yes, good point as well.
>>>>>>>>>
>>>>>>>>> Christian.
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Andrey
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Christian.
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Andrey
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Christian.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Andrey
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Andrey
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
>>>
>
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