Re: [PATCH v2 8/8] drm/amdgpu: Prevent any job recoveries after device is unplugged.

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

On 11/19/20 2:55 AM, Christian König wrote:
> Am 18.11.20 um 17:20 schrieb Andrey Grodzovsky:
>>
>> On 11/18/20 7:01 AM, Christian König wrote:
>>> Am 18.11.20 um 08:39 schrieb Daniel Vetter:
>>>> On Tue, Nov 17, 2020 at 9:07 PM Andrey Grodzovsky
>>>> <Andrey.Grodzovsky@amd.com> wrote:
>>>>>
>>>>> On 11/17/20 2:49 PM, Daniel Vetter wrote:
>>>>>> On Tue, Nov 17, 2020 at 02:18:49PM -0500, Andrey Grodzovsky wrote:
>>>>>>> On 11/17/20 1:52 PM, Daniel Vetter wrote:
>>>>>>>> On Tue, Nov 17, 2020 at 01:38:14PM -0500, Andrey Grodzovsky wrote:
>>>>>>>>> On 6/22/20 5:53 AM, Daniel Vetter wrote:
>>>>>>>>>> On Sun, Jun 21, 2020 at 02:03:08AM -0400, Andrey Grodzovsky wrote:
>>>>>>>>>>> No point to try recovery if device is gone, just messes up things.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>>>>>>>>>> ---
>>>>>>>>>>>      drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c | 16 ++++++++++++++++
>>>>>>>>>>>      drivers/gpu/drm/amd/amdgpu/amdgpu_job.c | 8 ++++++++
>>>>>>>>>>>      2 files changed, 24 insertions(+)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c 
>>>>>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>>>>>> index 6932d75..5d6d3d9 100644
>>>>>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>>>>>> @@ -1129,12 +1129,28 @@ static int amdgpu_pci_probe(struct pci_dev 
>>>>>>>>>>> *pdev,
>>>>>>>>>>>           return ret;
>>>>>>>>>>>      }
>>>>>>>>>>> +static void amdgpu_cancel_all_tdr(struct amdgpu_device *adev)
>>>>>>>>>>> +{
>>>>>>>>>>> +        int i;
>>>>>>>>>>> +
>>>>>>>>>>> +        for (i = 0; i < AMDGPU_MAX_RINGS; ++i) {
>>>>>>>>>>> +                struct amdgpu_ring *ring = adev->rings[i];
>>>>>>>>>>> +
>>>>>>>>>>> +                if (!ring || !ring->sched.thread)
>>>>>>>>>>> +                        continue;
>>>>>>>>>>> +
>>>>>>>>>>> + cancel_delayed_work_sync(&ring->sched.work_tdr);
>>>>>>>>>>> +        }
>>>>>>>>>>> +}
>>>>>>>>>> I think this is a function that's supposed to be in drm/scheduler, not
>>>>>>>>>> here. Might also just be your cleanup code being ordered wrongly, or 
>>>>>>>>>> your
>>>>>>>>>> split in one of the earlier patches not done quite right.
>>>>>>>>>> -Daniel
>>>>>>>>> This function iterates across all the schedulers per amdgpu device and 
>>>>>>>>> accesses
>>>>>>>>> amdgpu specific structures , drm/scheduler deals with single scheduler 
>>>>>>>>> at most
>>>>>>>>> so looks to me like this is the right place for this function
>>>>>>>> I guess we could keep track of all schedulers somewhere in a list in
>>>>>>>> struct drm_device and wrap this up. That was kinda the idea.
>>>>>>>>
>>>>>>>> Minimally I think a tiny wrapper with docs for the
>>>>>>>> cancel_delayed_work_sync(&sched->work_tdr); which explains what you must
>>>>>>>> observe to make sure there's no race.
>>>>>>> Will do
>>>>>>>
>>>>>>>
>>>>>>>> I'm not exactly sure there's no
>>>>>>>> guarantee here we won't get a new tdr work launched right afterwards at
>>>>>>>> least, so this looks a bit like a hack.
>>>>>>> Note that for any TDR work happening post amdgpu_cancel_all_tdr
>>>>>>> amdgpu_job_timedout->drm_dev_is_unplugged
>>>>>>> will return true and so it will return early. To make it water proof tight
>>>>>>> against race
>>>>>>> i can switch from drm_dev_is_unplugged to drm_dev_enter/exit
>>>>>> Hm that's confusing. You do a work_cancel_sync, so that at least looks
>>>>>> like "tdr work must not run after this point"
>>>>>>
>>>>>> If you only rely on drm_dev_enter/exit check with the tdr work, then
>>>>>> there's no need to cancel anything.
>>>>>
>>>>> Agree, synchronize_srcu from drm_dev_unplug should play the role
>>>>> of 'flushing' any earlier (in progress) tdr work which is
>>>>> using drm_dev_enter/exit pair. Any later arising tdr will terminate early 
>>>>> when
>>>>> drm_dev_enter
>>>>> returns false.
>>>> Nope, anything you put into the work itself cannot close this race.
>>>> It's the schedule_work that matters here. Or I'm missing something ...
>>>> I thought that the tdr work you're cancelling here is launched by
>>>> drm/scheduler code, not by the amd callback?
>>
>>
>> My bad, you are right, I am supposed to put drm_dev_enter.exit pair into 
>> drm_sched_job_timedout
>>
>>
>>>
>>> Yes that is correct. Canceling the work item is not the right approach at 
>>> all, nor is adding dev_enter/exit pair in the recovery handler.
>>
>>
>> Without adding the dev_enter/exit guarding pair in the recovery handler you 
>> are ending up with GPU reset starting while
>> the device is already unplugged, this leads to multiple errors and general mess.
>>
>>
>>>
>>> What we need to do here is to stop the scheduler thread and then wait for 
>>> any timeout handling to have finished.
>>>
>>> Otherwise it can scheduler a new timeout just after we have canceled this one.
>>>
>>> Regards,
>>> Christian.
>>
>>
>> Schedulers are stopped from amdgpu_driver_unload_kms which indeed happens 
>> after drm_dev_unplug
>> so yes, there is still a chance for new work being scheduler and timeout 
>> armed after but, once i fix the code
>> to place drm_dev_enter/exit pair into drm_sched_job_timeout I don't see why 
>> that not a good solution ?
>
> Yeah that should work as well, but then you also don't need to cancel the work 
> item from the driver.


Indeed, as Daniel pointed out no need and I dropped it. One correction - I 
previously said that w/o
dev_enter/exit guarding pair in scheduler's TO handler you will get GPU reset 
starting while device already gone -
of course this is not fully preventing this as the device can be extracted at 
any moment just after we
already entered GPU recovery. But it does saves us processing a futile  GPU 
recovery which always
starts once you unplug the device if there are active gobs in progress at the 
moment and so I think it's
still justifiable to keep the dev_enter/exit guarding pair there.

Andrey


>
>
>> Any tdr work started after drm_dev_unplug finished will simply abort on entry 
>> to drm_sched_job_timedout
>> because drm_dev_enter will be false and the function will return without 
>> rearming the timeout timer and
>> so will have no impact.
>>
>> The only issue i see here now is of possible use after free if some late tdr 
>> work will try to execute after
>> drm device already gone, for this we probably should add 
>> cancel_delayed_work_sync(sched.work_tdr)
>> to drm_sched_fini after sched->thread is stopped there.
>
> Good point, that is indeed missing as far as I can see.
>
> Christian.
>
>>
>> Andrey
>
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