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

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

[-- Attachment #1.1: Type: text/plain, Size: 16839 bytes --]


On 2021-04-07 6:28 a.m., Christian König wrote:
> Hi Andrey,
>
> Am 06.04.21 um 23:22 schrieb Andrey Grodzovsky:
>>
>> Hey Christian, Denis, see bellow -
>>
>> On 2021-04-06 6:34 a.m., Christian König wrote:
>>> Hi Andrey,
>>>
>>> well good question. My job is to watch over the implementation and 
>>> design and while I always help I can adjust anybodies schedule.
>>>
>>> Is the patch to print a warning when the hardware is accessed 
>>> without holding the locks merged yet? If not then that would 
>>> probably be a good starting point.
>>
>>
>> It's merged into amd-staging-drm-next and since I work on 
>> drm-misc-next I will cherry-pick it into there.
>>
>
> Ok good to know, I haven't tracked that one further.
>
>>
>>>
>>> Then we would need to unify this with the SRCU to make sure that we 
>>> have both the reset lock as well as block the hotplug code from 
>>> reusing the MMIO space.
>>
>> In my understanding there is a significant difference between 
>> handling of GPU reset and unplug - while GPU reset use case requires 
>> any HW accessing code to block and wait for the reset to finish and 
>> then proceed, hot-unplug
>> is permanent and hence no need to wait and proceed but rather abort 
>> at once.
>>
>
> Yes, absolutely correct.
>
>> This why I think that in any place we already check for device reset 
>> we should also add a check for hot-unplug but the handling would be 
>> different
>> in that for hot-unplug we would abort instead of keep waiting.
>>
>
> Yes, that's the rough picture in my head as well.
>
> Essentially Daniels patch of having an 
> amdgpu_device_hwaccess_begin()/_end() was the right approach. You just 
> can't do it in the top level IOCTL handler, but rather need it 
> somewhere between front end and backend.


Can you point me to what patch was it ? Can't find.


>
>> Similar to handling device reset for unplug we obviously also need to 
>> stop and block any MMIO accesses once device is unplugged and, as 
>> Daniel Vetter mentioned - we have to do it before finishing 
>> pci_remove (early device fini)
>> and not later (when last device reference is dropped from user space) 
>> in order to prevent reuse of MMIO space we still access by other hot 
>> plugging devices. As in device reset case we need to cancel all delay 
>> works, stop drm schedule, complete all unfinished fences(both HW and 
>> scheduler fences). While you stated strong objection to force 
>> signalling scheduler fences from GPU reset, quote:
>>
>> "you can't signal the dma_fence waiting. Waiting for a dma_fence also 
>> means you wait for the GPU reset to finish. When we would signal the 
>> dma_fence during the GPU reset then we would run into memory 
>> corruption because the hardware jobs running after the GPU reset 
>> would access memory which is already freed."
>> To my understating this is a key difference with hot-unplug, the 
>> device is gone, all those concerns are irrelevant and hence we can 
>> actually force signal scheduler fences (setting and error to them 
>> before) to force completion of any
>> waiting clients such as possibly IOCTLs or async page flips e.t.c.
>>
>
> Yes, absolutely correct. That's what I also mentioned to Daniel. When 
> we are able to nuke the device and any memory access it might do we 
> can also signal the fences.
>
>> Beyond blocking all delayed works and scheduler threads we also need 
>> to guarantee no  IOCTL can access MMIO post device unplug OR in 
>> flight IOCTLs are done before we finish pci_remove (amdgpu_pci_remove 
>> for us).
>> For this I suggest we do something like what we worked on with 
>> Takashi Iwai the ALSA maintainer recently when he helped implementing 
>> PCI BARs move support for snd_hda_intel. Take a look at
>> https://cgit.freedesktop.org/~agrodzov/linux/commit/?h=yadro/pcie_hotplug/movable_bars_v9.1&id=cbaa324799718e2b828a8c7b5b001dd896748497 
>> and
>> https://cgit.freedesktop.org/~agrodzov/linux/commit/?h=yadro/pcie_hotplug/movable_bars_v9.1&id=e36365d9ab5bbc30bdc221ab4b3437de34492440
>> We also had same issue there, how to prevent MMIO accesses while the 
>> BARs are migrating. What was done there is a refcount was added to 
>> count all IOCTLs in flight, for any in flight  IOCTL the BAR 
>> migration handler would
>> block for the refcount to drop to 0 before it would proceed, for any 
>> later IOCTL it stops and wait if device is in migration state. We 
>> even don't need the wait part, nothing to wait for, we just return 
>> with -ENODEV for this case.
>>
>
> This is essentially what the DRM SRCU is doing as well.
>
> For the hotplug case we could do this in the toplevel since we can 
> signal the fence and don't need to block memory management.


To make SRCU 'wait for' all IOCTLs in flight we would need to wrap every 
IOCTL ( practically - just drm_ioctl function) with 
drm_dev_enter/drm_dev_exit - can we do it ?


>
> But I'm not sure, maybe we should handle it the same way as reset or 
> maybe we should have it at the top level.


If by top level you mean checking for device unplugged and bailing out 
at the entry to IOCTL or right at start of any work_item/timer function 
we have then seems to me it's better and more clear. Once we flushed all 
of them in flight there is no reason for them to execute any more when 
device is unplugged.

Andrey


>
> Regards,
> Christian.
>
>> The above approach should allow us to wait for all the IOCTLs in 
>> flight, together with stopping scheduler threads and cancelling and 
>> flushing all in flight work items and timers i think It should give 
>> as full solution for the hot-unplug case
>> of preventing any MMIO accesses post device pci_remove.
>>
>> Let me know what you think guys.
>>
>> Andrey
>>
>>
>>>
>>> And then testing, testing, testing to see if we have missed something.
>>>
>>> Christian.
>>>
>>> Am 05.04.21 um 19:58 schrieb Andrey Grodzovsky:
>>>>
>>>> Denis, Christian, are there any updates in the plan on how to move 
>>>> on with this ? As you know I need very similar code for my 
>>>> up-streaming of device hot-unplug. My latest solution 
>>>> (https://lists.freedesktop.org/archives/amd-gfx/2021-January/058606.html) 
>>>> was not acceptable because of low level guards on the register 
>>>> accessors level which was hurting performance. Basically I need a 
>>>> way to prevent any MMIO write accesses from kernel driver after 
>>>> device is removed (UMD accesses are taken care of by page faulting 
>>>> dummy page). We are using now hot-unplug code for Freemont program 
>>>> and so up-streaming became more of a priority then before. This 
>>>> MMIO access issue is currently my main blocker from up-streaming. 
>>>> Is there any way I can assist in pushing this on ?
>>>>
>>>> Andrey
>>>>
>>>> On 2021-03-18 5:51 a.m., Christian König wrote:
>>>>> Am 18.03.21 um 10:30 schrieb Li, Dennis:
>>>>>>
>>>>>> >>> The GPU reset doesn't complete the fences we wait for. It 
>>>>>> only completes the hardware fences as part of the reset.
>>>>>>
>>>>>> >>> So waiting for a fence while holding the reset lock is 
>>>>>> illegal and needs to be avoided.
>>>>>>
>>>>>> I understood your concern. It is more complex for DRM GFX, 
>>>>>> therefore I abandon adding lock protection for DRM ioctls now. 
>>>>>> Maybe we can try to add all kernel  dma_fence waiting in a list, 
>>>>>> and signal all in recovery threads. Do you have same concern for 
>>>>>> compute cases?
>>>>>>
>>>>>
>>>>> Yes, compute (KFD) is even harder to handle.
>>>>>
>>>>> See you can't signal the dma_fence waiting. Waiting for a 
>>>>> dma_fence also means you wait for the GPU reset to finish.
>>>>>
>>>>> When we would signal the dma_fence during the GPU reset then we 
>>>>> would run into memory corruption because the hardware jobs running 
>>>>> after the GPU reset would access memory which is already freed.
>>>>>
>>>>>> >>> Lockdep also complains about this when it is used correctly. 
>>>>>> The only reason it doesn't complain here is because you use an 
>>>>>> atomic+wait_event instead of a locking primitive.
>>>>>>
>>>>>> Agree. This approach will escape the monitor of lockdep.  Its 
>>>>>> goal is to block other threads when GPU recovery thread start. 
>>>>>> But I couldn’t find a better method to solve this problem. Do you 
>>>>>> have some suggestion?
>>>>>>
>>>>>
>>>>> Well, completely abandon those change here.
>>>>>
>>>>> What we need to do is to identify where hardware access happens 
>>>>> and then insert taking the read side of the GPU reset lock so that 
>>>>> we don't wait for a dma_fence or allocate memory, but still 
>>>>> protect the hardware from concurrent access and reset.
>>>>>
>>>>> Regards,
>>>>> Christian.
>>>>>
>>>>>> Best Regards
>>>>>>
>>>>>> Dennis Li
>>>>>>
>>>>>> *From:* Koenig, Christian <Christian.Koenig@amd.com>
>>>>>> *Sent:* Thursday, March 18, 2021 4:59 PM
>>>>>> *To:* Li, Dennis <Dennis.Li@amd.com>; 
>>>>>> amd-gfx@lists.freedesktop.org; Deucher, Alexander 
>>>>>> <Alexander.Deucher@amd.com>; Kuehling, Felix 
>>>>>> <Felix.Kuehling@amd.com>; Zhang, Hawking <Hawking.Zhang@amd.com>
>>>>>> *Subject:* AW: [PATCH 0/4] Refine GPU recovery sequence to 
>>>>>> enhance its stability
>>>>>>
>>>>>> Exactly that's what you don't seem to understand.
>>>>>>
>>>>>> The GPU reset doesn't complete the fences we wait for. It only 
>>>>>> completes the hardware fences as part of the reset.
>>>>>>
>>>>>> So waiting for a fence while holding the reset lock is illegal 
>>>>>> and needs to be avoided.
>>>>>>
>>>>>> Lockdep also complains about this when it is used correctly. The 
>>>>>> only reason it doesn't complain here is because you use an 
>>>>>> atomic+wait_event instead of a locking primitive.
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> Christian.
>>>>>>
>>>>>> ------------------------------------------------------------------------
>>>>>>
>>>>>> *Von:*Li, Dennis <Dennis.Li@amd.com <mailto:Dennis.Li@amd.com>>
>>>>>> *Gesendet:* Donnerstag, 18. März 2021 09:28
>>>>>> *An:* Koenig, Christian <Christian.Koenig@amd.com 
>>>>>> <mailto:Christian.Koenig@amd.com>>; amd-gfx@lists.freedesktop.org 
>>>>>> <mailto:amd-gfx@lists.freedesktop.org> 
>>>>>> <amd-gfx@lists.freedesktop.org 
>>>>>> <mailto:amd-gfx@lists.freedesktop.org>>; Deucher, Alexander 
>>>>>> <Alexander.Deucher@amd.com <mailto:Alexander.Deucher@amd.com>>; 
>>>>>> Kuehling, Felix <Felix.Kuehling@amd.com 
>>>>>> <mailto:Felix.Kuehling@amd.com>>; Zhang, Hawking 
>>>>>> <Hawking.Zhang@amd.com <mailto:Hawking.Zhang@amd.com>>
>>>>>> *Betreff:* RE: [PATCH 0/4] Refine GPU recovery sequence to 
>>>>>> enhance its stability
>>>>>>
>>>>>> >>> Those two steps need to be exchanged or otherwise it is 
>>>>>> possible that new delayed work items etc are started before the 
>>>>>> lock is taken.
>>>>>> What about adding check for adev->in_gpu_reset in work item? If 
>>>>>> exchange the two steps, it maybe introduce the deadlock. For 
>>>>>> example, the user thread hold the read lock and waiting for the 
>>>>>> fence, if recovery thread try to hold write lock and then 
>>>>>> complete fences, in this case, recovery thread will always be 
>>>>>> blocked.
>>>>>>
>>>>>>
>>>>>> Best Regards
>>>>>> Dennis Li
>>>>>> -----Original Message-----
>>>>>> From: Koenig, Christian <Christian.Koenig@amd.com 
>>>>>> <mailto:Christian.Koenig@amd.com>>
>>>>>> Sent: Thursday, March 18, 2021 3:54 PM
>>>>>> To: Li, Dennis <Dennis.Li@amd.com <mailto:Dennis.Li@amd.com>>; 
>>>>>> amd-gfx@lists.freedesktop.org 
>>>>>> <mailto:amd-gfx@lists.freedesktop.org>; Deucher, Alexander 
>>>>>> <Alexander.Deucher@amd.com <mailto:Alexander.Deucher@amd.com>>; 
>>>>>> Kuehling, Felix <Felix.Kuehling@amd.com 
>>>>>> <mailto:Felix.Kuehling@amd.com>>; Zhang, Hawking 
>>>>>> <Hawking.Zhang@amd.com <mailto:Hawking.Zhang@amd.com>>
>>>>>> Subject: Re: [PATCH 0/4] Refine GPU recovery sequence to enhance 
>>>>>> its stability
>>>>>>
>>>>>> Am 18.03.21 um 08:23 schrieb Dennis Li:
>>>>>> > We have defined two variables in_gpu_reset and reset_sem in 
>>>>>> adev object. The atomic type variable in_gpu_reset is used to 
>>>>>> avoid recovery thread reenter and make lower functions return 
>>>>>> more earlier when recovery start, but couldn't block recovery 
>>>>>> thread when it access hardware. The r/w semaphore reset_sem is 
>>>>>> used to solve these synchronization issues between recovery 
>>>>>> thread and other threads.
>>>>>> >
>>>>>> > The original solution locked registers' access in lower 
>>>>>> functions, which will introduce following issues:
>>>>>> >
>>>>>> > 1) many lower functions are used in both recovery thread and 
>>>>>> others. Firstly we must harvest these functions, it is easy to 
>>>>>> miss someones. Secondly these functions need select which lock 
>>>>>> (read lock or write lock) will be used, according to the thread 
>>>>>> it is running in. If the thread context isn't considered, the 
>>>>>> added lock will easily introduce deadlock. Besides that, in most 
>>>>>> time, developer easily forget to add locks for new functions.
>>>>>> >
>>>>>> > 2) performance drop. More lower functions are more frequently 
>>>>>> called.
>>>>>> >
>>>>>> > 3) easily introduce false positive lockdep complaint, because 
>>>>>> write lock has big range in recovery thread, but low level 
>>>>>> functions will hold read lock may be protected by other locks in 
>>>>>> other threads.
>>>>>> >
>>>>>> > Therefore the new solution will try to add lock protection for 
>>>>>> ioctls of kfd. Its goal is that there are no threads except for 
>>>>>> recovery thread or its children (for xgmi) to access hardware 
>>>>>> when doing GPU reset and resume. So refine recovery thread as the 
>>>>>> following:
>>>>>> >
>>>>>> > Step 0: atomic_cmpxchg(&adev->in_gpu_reset, 0, 1)
>>>>>> >     1). if failed, it means system had a recovery thread 
>>>>>> running, current thread exit directly;
>>>>>> >     2). if success, enter recovery thread;
>>>>>> >
>>>>>> > Step 1: cancel all delay works, stop drm schedule, complete all 
>>>>>> unreceived fences and so on. It try to stop or pause other threads.
>>>>>> >
>>>>>> > Step 2: call down_write(&adev->reset_sem) to hold write lock, 
>>>>>> which will block recovery thread until other threads release read 
>>>>>> locks.
>>>>>>
>>>>>> Those two steps need to be exchanged or otherwise it is possible 
>>>>>> that new delayed work items etc are started before the lock is taken.
>>>>>>
>>>>>> Just to make it clear until this is fixed the whole patch set is 
>>>>>> a NAK.
>>>>>>
>>>>>> Regards,
>>>>>> Christian.
>>>>>>
>>>>>> >
>>>>>> > Step 3: normally, there is only recovery threads running to 
>>>>>> access hardware, it is safe to do gpu reset now.
>>>>>> >
>>>>>> > Step 4: do post gpu reset, such as call all ips' resume functions;
>>>>>> >
>>>>>> > Step 5: atomic set adev->in_gpu_reset as 0, wake up other 
>>>>>> threads and release write lock. Recovery thread exit normally.
>>>>>> >
>>>>>> > Other threads call the amdgpu_read_lock to synchronize with 
>>>>>> recovery thread. If it finds that in_gpu_reset is 1, it should 
>>>>>> release read lock if it has holden one, and then blocks itself to 
>>>>>> wait for recovery finished event. If thread successfully hold 
>>>>>> read lock and in_gpu_reset is 0, it continues. It will exit 
>>>>>> normally or be stopped by recovery thread in step 1.
>>>>>> >
>>>>>> > Dennis Li (4):
>>>>>> >    drm/amdgpu: remove reset lock from low level functions
>>>>>> >    drm/amdgpu: refine the GPU recovery sequence
>>>>>> >    drm/amdgpu: instead of using down/up_read directly
>>>>>> >    drm/amdkfd: add reset lock protection for kfd entry functions
>>>>>> >
>>>>>> > drivers/gpu/drm/amd/amdgpu/amdgpu.h |   6 +
>>>>>> > drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c   | 14 +-
>>>>>> > drivers/gpu/drm/amd/amdgpu/amdgpu_device.c    | 173 
>>>>>> +++++++++++++-----
>>>>>> > .../gpu/drm/amd/amdgpu/amdgpu_ras_eeprom.c |   8 -
>>>>>> > drivers/gpu/drm/amd/amdgpu/gmc_v10_0.c |   4 +-
>>>>>> > drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c |   9 +-
>>>>>> > drivers/gpu/drm/amd/amdgpu/mxgpu_ai.c |   5 +-
>>>>>> > drivers/gpu/drm/amd/amdgpu/mxgpu_nv.c |   5 +-
>>>>>> > drivers/gpu/drm/amd/amdkfd/kfd_chardev.c      | 172 
>>>>>> ++++++++++++++++-
>>>>>> > drivers/gpu/drm/amd/amdkfd/kfd_priv.h |   3 +-
>>>>>> > drivers/gpu/drm/amd/amdkfd/kfd_process.c |   4 +
>>>>>> > .../amd/amdkfd/kfd_process_queue_manager.c    | 17 ++
>>>>>> >   12 files changed, 345 insertions(+), 75 deletions(-)
>>>>>> >
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> amd-gfx mailing list
>>>>> amd-gfx@lists.freedesktop.org
>>>>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>>>
>>
>> _______________________________________________
>> amd-gfx mailing list
>> amd-gfx@lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>

[-- Attachment #1.2: Type: text/html, Size: 37398 bytes --]

[-- Attachment #2: Type: text/plain, Size: 154 bytes --]

_______________________________________________
amd-gfx mailing list
amd-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/amd-gfx