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

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

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 ?
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.

Andrey


>
>> -Daniel
>>
>>> Will update.
>>>
>>> Andrey
>>>
>>>
>>>> For race free cancel_work_sync you need:
>>>> 1. make sure whatever is calling schedule_work is guaranteed to no longer
>>>> call schedule_work.
>>>> 2. call cancel_work_sync
>>>>
>>>> Anything else is cargo-culted work cleanup:
>>>>
>>>> - 1. without 2. means if a work got scheduled right before it'll still be
>>>>     a problem.
>>>> - 2. without 1. means a schedule_work right after makes you calling
>>>>     cancel_work_sync pointless.
>>>>
>>>> So either both or nothing.
>>>> -Daniel
>>>>
>>>>> Andrey
>>>>>
>>>>>
>>>>>> -Daniel
>>>>>>
>>>>>>> Andrey
>>>>>>>
>>>>>>>
>>>>>>>>> +
>>>>>>>>>      static void
>>>>>>>>>      amdgpu_pci_remove(struct pci_dev *pdev)
>>>>>>>>>      {
>>>>>>>>>           struct drm_device *dev = pci_get_drvdata(pdev);
>>>>>>>>> +        struct amdgpu_device *adev = dev->dev_private;
>>>>>>>>>           drm_dev_unplug(dev);
>>>>>>>>> +        amdgpu_cancel_all_tdr(adev);
>>>>>>>>> ttm_bo_unmap_virtual_address_space(&adev->mman.bdev);
>>>>>>>>>           amdgpu_driver_unload_kms(dev);
>>>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c 
>>>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
>>>>>>>>> index 4720718..87ff0c0 100644
>>>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
>>>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c
>>>>>>>>> @@ -28,6 +28,8 @@
>>>>>>>>>      #include "amdgpu.h"
>>>>>>>>>      #include "amdgpu_trace.h"
>>>>>>>>> +#include <drm/drm_drv.h>
>>>>>>>>> +
>>>>>>>>>      static void amdgpu_job_timedout(struct drm_sched_job *s_job)
>>>>>>>>>      {
>>>>>>>>>           struct amdgpu_ring *ring = to_amdgpu_ring(s_job->sched);
>>>>>>>>> @@ -37,6 +39,12 @@ static void amdgpu_job_timedout(struct 
>>>>>>>>> drm_sched_job *s_job)
>>>>>>>>>           memset(&ti, 0, sizeof(struct amdgpu_task_info));
>>>>>>>>> +        if (drm_dev_is_unplugged(adev->ddev)) {
>>>>>>>>> +                DRM_INFO("ring %s timeout, but device unplugged, 
>>>>>>>>> skipping.\n",
>>>>>>>>> + s_job->sched->name);
>>>>>>>>> +                return;
>>>>>>>>> +        }
>>>>>>>>> +
>>>>>>>>>           if (amdgpu_ring_soft_recovery(ring, job->vmid, 
>>>>>>>>> s_job->s_fence->parent)) {
>>>>>>>>>                   DRM_ERROR("ring %s timeout, but soft recovered\n",
>>>>>>>>> s_job->sched->name);
>>>>>>>>> -- 
>>>>>>>>> 2.7.4
>>>>>>>>>
>>
>>
>
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