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

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

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On 2021-04-12 2:23 p.m., Christian König wrote:
> Am 12.04.21 um 20:18 schrieb Andrey Grodzovsky:
>>
>> On 2021-04-12 2:05 p.m., Christian König wrote:
>>
>>> Am 12.04.21 um 20:01 schrieb Andrey Grodzovsky:
>>>>
>>>> On 2021-04-12 1:44 p.m., Christian König wrote:
>>>>
>>>>>
>>>>> Am 12.04.21 um 19:27 schrieb Andrey Grodzovsky:
>>>>>> On 2021-04-10 1:34 p.m., Christian König wrote:
>>>>>>> Hi Andrey,
>>>>>>>
>>>>>>> Am 09.04.21 um 20:18 schrieb Andrey Grodzovsky:
>>>>>>>> [SNIP]
>>>>>>>>>>
>>>>>>>>>> If we use a list and a flag called 'emit_allowed' under a 
>>>>>>>>>> lock such that in amdgpu_fence_emit we lock the list, check 
>>>>>>>>>> the flag and if true add the new HW fence to list and proceed 
>>>>>>>>>> to HW emition as normal, otherwise return with -ENODEV. In 
>>>>>>>>>> amdgpu_pci_remove we take the lock, set the flag to false, 
>>>>>>>>>> and then iterate the list and force signal it. Will this not 
>>>>>>>>>> prevent any new HW fence creation from now on from any place 
>>>>>>>>>> trying to do so ?
>>>>>>>>>
>>>>>>>>> Way to much overhead. The fence processing is intentionally 
>>>>>>>>> lock free to avoid cache line bouncing because the IRQ can 
>>>>>>>>> move from CPU to CPU.
>>>>>>>>>
>>>>>>>>> We need something which at least the processing of fences in 
>>>>>>>>> the interrupt handler doesn't affect at all.
>>>>>>>>
>>>>>>>>
>>>>>>>> As far as I see in the code, amdgpu_fence_emit is only called 
>>>>>>>> from task context. Also, we can skip this list I proposed and 
>>>>>>>> just use amdgpu_fence_driver_force_completion for each ring to 
>>>>>>>> signal all created HW fences.
>>>>>>>
>>>>>>> Ah, wait a second this gave me another idea.
>>>>>>>
>>>>>>> See amdgpu_fence_driver_force_completion():
>>>>>>>
>>>>>>> amdgpu_fence_write(ring, ring->fence_drv.sync_seq);
>>>>>>>
>>>>>>> If we change that to something like:
>>>>>>>
>>>>>>> amdgpu_fence_write(ring, ring->fence_drv.sync_seq + 0x3FFFFFFF);
>>>>>>>
>>>>>>> Not only the currently submitted, but also the next 0x3FFFFFFF 
>>>>>>> fences will be considered signaled.
>>>>>>>
>>>>>>> This basically solves out problem of making sure that new fences 
>>>>>>> are also signaled without any additional overhead whatsoever.
>>>>>>
>>>>>>
>>>>>> Problem with this is that the act of setting the sync_seq to some 
>>>>>> MAX value alone is not enough, you actually have to call 
>>>>>> amdgpu_fence_process to iterate and signal the fences currently 
>>>>>> stored in ring->fence_drv.fences array and to guarantee that once 
>>>>>> you done your signalling no more HW fences will be added to that 
>>>>>> array anymore. I was thinking to do something like bellow:
>>>>>>
>>>>>
>>>>> Well we could implement the is_signaled callback once more, but 
>>>>> I'm not sure if that is a good idea.
>>>>
>>>>
>>>> This indeed could save the explicit signaling I am doing bellow but 
>>>> I also set an error code there which might be helpful to propagate 
>>>> to users
>>>>
>>>>
>>>>>
>>>>>> amdgpu_fence_emit()
>>>>>>
>>>>>> {
>>>>>>
>>>>>>     dma_fence_init(fence);
>>>>>>
>>>>>>     srcu_read_lock(amdgpu_unplug_srcu)
>>>>>>
>>>>>>     if (!adev->unplug)) {
>>>>>>
>>>>>>         seq = ++ring->fence_drv.sync_seq;
>>>>>>         emit_fence(fence);
>>>>>>
>>>>>> */* We can't wait forever as the HW might be gone at any point*/**
>>>>>>        dma_fence_wait_timeout(old_fence, 5S);*
>>>>>>
>>>>>
>>>>> You can pretty much ignore this wait here. It is only as a last 
>>>>> resort so that we never overwrite the ring buffers.
>>>>
>>>>
>>>> If device is present how can I ignore this ?
>>>>
>>
>> I think you missed my question here
>>
>
> Sorry I thought I answered that below.
>
> See this is just the last resort so that we don't need to worry about 
> ring buffer overflows during testing.
>
> We should not get here in practice and if we get here generating a 
> deadlock might actually be the best handling.
>
> The alternative would be to call BUG().
>
>>>>
>>>>>
>>>>> But it should not have a timeout as far as I can see.
>>>>
>>>>
>>>> Without timeout wait the who approach falls apart as I can't call 
>>>> srcu_synchronize on this scope because once device is physically 
>>>> gone the wait here will be forever
>>>>
>>>
>>> Yeah, but this is intentional. The only alternative to avoid 
>>> corruption is to wait with a timeout and call BUG() if that 
>>> triggers. That isn't much better.
>>>
>>>>
>>>>>
>>>>>>         ring->fence_drv.fences[seq & 
>>>>>> ring->fence_drv.num_fences_mask] = fence;
>>>>>>
>>>>>>     } else {
>>>>>>
>>>>>>         dma_fence_set_error(fence, -ENODEV);
>>>>>>         DMA_fence_signal(fence)
>>>>>>
>>>>>>     }
>>>>>>
>>>>>>     srcu_read_unlock(amdgpu_unplug_srcu)
>>>>>>     return fence;
>>>>>>
>>>>>> }
>>>>>>
>>>>>> amdgpu_pci_remove
>>>>>>
>>>>>> {
>>>>>>
>>>>>>     adev->unplug = true;
>>>>>>     synchronize_srcu(amdgpu_unplug_srcu)
>>>>>>
>>>>>
>>>>> Well that is just duplicating what drm_dev_unplug() should be 
>>>>> doing on a different level.
>>>>
>>>>
>>>> drm_dev_unplug is on a much wider scope, for everything in the 
>>>> device including 'flushing' in flight IOCTLs, this deals 
>>>> specifically with the issue of force signalling HW fences
>>>>
>>>
>>> Yeah, but it adds the same overhead as the device srcu.
>>>
>>> Christian.
>>
>>
>> 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.


>
> 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.

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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