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

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Nov 19, 2020 at 10:02:28AM -0500, Andrey Grodzovsky wrote:
> 
> 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.

Yeah sprinkling drm_dev_enter/exit over the usual suspect code paths like
tdr to make the entire unloading much faster makes sense. Waiting for
enormous amounts of mmio ops to time out isn't fun. A comment might be
good for that though, to explain why we're doing that.
-Daniel

> 
> 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
> > 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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