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

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

On 2021-04-13 3:10 a.m., Christian König wrote:
> Am 12.04.21 um 22:01 schrieb Andrey Grodzovsky:
>>
>> 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.
>
> Those only happen during startup and GPU reset.


Ok, but then looks like I am missing something, see the following steps 
in amdgpu_pci_remove -

1) Use disable_irq API function to stop and flush all in flight HW 
interrupts handlers

2) Grab the reset lock and stop all the schedulers

After above 2 steps the HW fences array is idle, no more insertions and 
no more extractions from the array

3) Run one time amdgpu_fence_process to signal all current HW fences

4) Set drm_dev_unplug (will 'flush' all in flight IOCTLs), release the 
GPU reset lock and go on with the rest of the sequence (cancel timers, 
work items e.t.c)

What's problematic in this sequence ?

Andrey


>
>
>>>
>>>>>
>>>>> 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.
>
> I'm really wondering the same thing for quite a while now.
>
> Adding Daniel as well, maybe he knows why the drm_unplug_srcu is global.
>
> Regards,
> Christian.
>
>>
>> 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%7Cc7fc6cb505c34aedfe6d08d8fe4b3947%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637538946324857369%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=64362PRC8xTgR2Uj2R256bMegVm8YWq1KI%2BAjzeYXv4%3D&reserved=0
>>>>>>>>>>>
>>>>>>>>>>> Yes, good point as well.
>>>>>>>>>>>
>>>>>>>>>>> Christian.
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Andrey
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Christian.
>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Andrey
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Christian.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Andrey
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Andrey
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
>>>
>
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