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

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

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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.

BTW: Could it be that the device SRCU protects more than one device and 
we deadlock because of this?

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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