Re: [Regression] drm/scheduler: track GPU active time per entity

[Daniel Vetter]

On Thu, 6 Apr 2023 at 17:59, Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Thu, Apr 06, 2023 at 05:33:18PM +0200, Christian König wrote:
> > Am 06.04.23 um 17:24 schrieb Lucas Stach:
> > > Am Donnerstag, dem 06.04.2023 um 16:21 +0200 schrieb Christian König:
> > > > Am 06.04.23 um 12:45 schrieb Lucas Stach:
> > > > > Am Donnerstag, dem 06.04.2023 um 10:27 +0200 schrieb Daniel Vetter:
> > > > > > On Thu, 6 Apr 2023 at 10:22, Christian König <christian.koenig@xxxxxxx> wrote:
> > > > > > > Am 05.04.23 um 18:09 schrieb Luben Tuikov:
> > > > > > > > On 2023-04-05 10:05, Danilo Krummrich wrote:
> > > > > > > > > On 4/4/23 06:31, Luben Tuikov wrote:
> > > > > > > > > > On 2023-03-28 04:54, Lucas Stach wrote:
> > > > > > > > > > > Hi Danilo,
> > > > > > > > > > >
> > > > > > > > > > > Am Dienstag, dem 28.03.2023 um 02:57 +0200 schrieb Danilo Krummrich:
> > > > > > > > > > > > Hi all,
> > > > > > > > > > > >
> > > > > > > > > > > > Commit df622729ddbf ("drm/scheduler: track GPU active time per entity")
> > > > > > > > > > > > tries to track the accumulated time that a job was active on the GPU
> > > > > > > > > > > > writing it to the entity through which the job was deployed to the
> > > > > > > > > > > > scheduler originally. This is done within drm_sched_get_cleanup_job()
> > > > > > > > > > > > which fetches a job from the schedulers pending_list.
> > > > > > > > > > > >
> > > > > > > > > > > > Doing this can result in a race condition where the entity is already
> > > > > > > > > > > > freed, but the entity's newly added elapsed_ns field is still accessed
> > > > > > > > > > > > once the job is fetched from the pending_list.
> > > > > > > > > > > >
> > > > > > > > > > > > After drm_sched_entity_destroy() being called it should be safe to free
> > > > > > > > > > > > the structure that embeds the entity. However, a job originally handed
> > > > > > > > > > > > over to the scheduler by this entity might still reside in the
> > > > > > > > > > > > schedulers pending_list for cleanup after drm_sched_entity_destroy()
> > > > > > > > > > > > already being called and the entity being freed. Hence, we can run into
> > > > > > > > > > > > a UAF.
> > > > > > > > > > > >
> > > > > > > > > > > Sorry about that, I clearly didn't properly consider this case.
> > > > > > > > > > >
> > > > > > > > > > > > In my case it happened that a job, as explained above, was just picked
> > > > > > > > > > > > from the schedulers pending_list after the entity was freed due to the
> > > > > > > > > > > > client application exiting. Meanwhile this freed up memory was already
> > > > > > > > > > > > allocated for a subsequent client applications job structure again.
> > > > > > > > > > > > Hence, the new jobs memory got corrupted. Luckily, I was able to
> > > > > > > > > > > > reproduce the same corruption over and over again by just using
> > > > > > > > > > > > deqp-runner to run a specific set of VK test cases in parallel.
> > > > > > > > > > > >
> > > > > > > > > > > > Fixing this issue doesn't seem to be very straightforward though (unless
> > > > > > > > > > > > I miss something), which is why I'm writing this mail instead of sending
> > > > > > > > > > > > a fix directly.
> > > > > > > > > > > >
> > > > > > > > > > > > Spontaneously, I see three options to fix it:
> > > > > > > > > > > >
> > > > > > > > > > > > 1. Rather than embedding the entity into driver specific structures
> > > > > > > > > > > > (e.g. tied to file_priv) we could allocate the entity separately and
> > > > > > > > > > > > reference count it, such that it's only freed up once all jobs that were
> > > > > > > > > > > > deployed through this entity are fetched from the schedulers pending list.
> > > > > > > > > > > >
> > > > > > > > > > > My vote is on this or something in similar vain for the long term. I
> > > > > > > > > > > have some hope to be able to add a GPU scheduling algorithm with a bit
> > > > > > > > > > > more fairness than the current one sometime in the future, which
> > > > > > > > > > > requires execution time tracking on the entities.
> > > > > > > > > > Danilo,
> > > > > > > > > >
> > > > > > > > > > Using kref is preferable, i.e. option 1 above.
> > > > > > > > > I think the only real motivation for doing that would be for generically
> > > > > > > > > tracking job statistics within the entity a job was deployed through. If
> > > > > > > > > we all agree on tracking job statistics this way I am happy to prepare a
> > > > > > > > > patch for this option and drop this one:
> > > > > > > > > https://lore.kernel.org/all/20230331000622.4156-1-dakr@xxxxxxxxxx/T/#u
> > > > > > > > Hmm, I never thought about "job statistics" when I preferred using kref above.
> > > > > > > > The reason kref is attractive is because one doesn't need to worry about
> > > > > > > > it--when the last user drops the kref, the release is called to do
> > > > > > > > housekeeping. If this never happens, we know that we have a bug to debug.
> > > > > > > Yeah, reference counting unfortunately have some traps as well. For
> > > > > > > example rarely dropping the last reference from interrupt context or
> > > > > > > with some unexpected locks help when the cleanup function doesn't expect
> > > > > > > that is a good recipe for problems as well.
> > > > > > >
> > > > > Fully agreed.
> > > > >
> > > > > > > > Regarding the patch above--I did look around the code, and it seems safe,
> > > > > > > > as per your analysis, I didn't see any reference to entity after job submission,
> > > > > > > > but I'll comment on that thread as well for the record.
> > > > > > > Reference counting the entities was suggested before. The intentionally
> > > > > > > avoided that so far because the entity might be the tip of the iceberg
> > > > > > > of stuff you need to keep around.
> > > > > > >
> > > > > > > For example for command submission you also need the VM and when you
> > > > > > > keep the VM alive you also need to keep the file private alive....
> > > > > > Yeah refcounting looks often like the easy way out to avoid
> > > > > > use-after-free issue, until you realize you've just made lifetimes
> > > > > > unbounded and have some enourmous leaks: entity keeps vm alive, vm
> > > > > > keeps all the bo alives, somehow every crash wastes more memory
> > > > > > because vk_device_lost means userspace allocates new stuff for
> > > > > > everything.
> > > > > >
> > > > > > If possible a lifetime design where lifetimes have hard bounds and you
> > > > > > just borrow a reference under a lock (or some other ownership rule) is
> > > > > > generally much more solid. But also much harder to design correctly
> > > > > > :-/
> > > > > >
> > > > > The use we are discussing here is to keep the entity alive as long as
> > > > > jobs from that entity are still active on the HW. While there are no
> > > > > hard bounds on when a job will get inactive, at least it's not
> > > > > unbounded. On a crash/fault the job will be removed from the hardware
> > > > > pretty soon.
> > > > >
> > > > > Well behaved jobs after a application shutdown might take a little
> > > > > longer, but I don't really see the new problem with keeping the entity
> > > > > alive? As long as a job is active on the hardware, we can't throw out
> > > > > the VM or BOs, no difference whether the entity is kept alive or not.
> > > > Exactly that's the problem. VM & BOs are dropped as soon as the process
> > > > is destroyed, we *don't* wait for the hw to finish before doing so.
> > > >
> > > > Just the backing store managed by all the house keeping objects isn't
> > > > freed until the hw is idle preventing a crash or accessing freed memory.
> > > >
> > > > This behavior is rather important for the OOM killer since we need to be
> > > > able to tear down the process as fast as possible in that case.
> > > >
> > > Are you talking about the dropping of pending jobs in
> > > drm_sched_entity_kill? I'm certainly not trying to change that in any
> > > way. Those aren't put onto the hardware yet, so we can always safely
> > > drop them and do so as fast as possible.
> > >
> > > What I'm concerned about are the jobs that are already scheduled on the
> > > HW. At least with Vivante hardware there is no race free way to get rid
> > > of jobs once they are put on the ring. So whatever the scheduler or DRM
> > > core is doing, we have to hold on to the BOs and GPU memory management
> > > structures to keep the hardware from operating on freed memory.
> > >
> > > That's already a lot of memory, so I don't really see the issue with
> > > keeping the entity around in a quiescent state until all the currently
> > > queued jobs have left the HW.
> > >
> > > > Changing that is possible, but that's quite a huge change I'm not really
> > > > willing to do just for tracking the time spend.
> > > >
> > > Yea, it's a big change and whether it's a good idea really depends on
> > > what we a gaining from it. You seem to see quite low value in "just
> > > tracking the time spent" and that might be true, but it also forces all
> > > drivers that want to implement fdinfo to roll their own time tracking.
> > > I would rather see more of this moved to the scheduler and thus shared
> > > between drivers.
> >
> > That's generally a good idea, but if that means that we need to restructure
> > the whole entity handling then I would object. That's simply not worth it
> > when we can implement it differently.
> >
> > What we could do is to keep the submitted fences around in the entity.
> > Similar to the tracking amdgpu does, see struct amdgpu_ctx_entity.
> >
> > This way the entity doesn't needs to stay around after it delivered the job
> > to the hw.
>
> I've done a _very_ cursory look, but from that the design seems to be that
> we only keep something very small around to avoid oopsing (just
> drm_sched_entity) and the overall gpu ctx goes away synchronously (more or
> less) when userspace destroys it.
>
> And then the actual fdinfo reporting or any reporting is a pure pull model
> where you come from drm_file -> gpu ctx -> amdgpu_ctx_entity and have
> borrowed references by holding enough locks.
>
> The push model, where the scheduler job actively pushes the stats all the
> way to the userspace/uapi ctx object is imo impossible to get right.
> i915-gem tried a few times, and solutions start with sprinkling rcu
> everywhere and only get worse. Imo absolute no-go.
>
> If I understand this right then yes I think extracting the
> amgpu_ctx_entity split into a bit of helper. Might want to extract the
> context xarray and lookup too and put it into drm_file, including the
> amdgpu_ctx_mgr->lock and iterating over stats for fdinfo maybe too to make
> this really worth it.
>
> Of all the failed attempts I've seen for exposing ctx stats this (amdgpu
> push model) is imo the only solid one.

amdgpu _pull_ model. Yay for me introducing some concepts and then
mixing them up in the same email :-/

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



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