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

[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.
>
> > 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
:-/

> Additional to that we have some ugly inter dependencies between tearing
> down an application (potential with a KILL signal from the OOM killer)
> and backward compatibility for some applications which render something
> and quit before the rendering is completed in the hardware.

Yeah I think that part would also be good to sort out once&for all in
drm/sched, because i915 has/had the same struggle.
-Daniel

>
> Regards,
> Christian.
>
> >
> > Regards,
> > Luben
> >
> >> Christian mentioned amdgpu tried something similar to what Lucas tried
> >> running into similar trouble, backed off and implemented it in another
> >> way - a driver specific way I guess?
> >>
> >>> Lucas, can you shed some light on,
> >>>
> >>> 1. In what way the current FIFO scheduling is unfair, and
> >>> 2. shed some details on this "scheduling algorithm with a bit
> >>> more fairness than the current one"?
> >>>
> >>> Regards,
> >>> Luben
> >>>
> >>>>> 2. Somehow make sure drm_sched_entity_destroy() does block until all
> >>>>> jobs deployed through this entity were fetched from the schedulers
> >>>>> pending list. Though, I'm pretty sure that this is not really desirable.
> >>>>>
> >>>>> 3. Just revert the change and let drivers implement tracking of GPU
> >>>>> active times themselves.
> >>>>>
> >>>> Given that we are already pretty late in the release cycle and etnaviv
> >>>> being the only driver so far making use of the scheduler elapsed time
> >>>> tracking I think the right short term solution is to either move the
> >>>> tracking into etnaviv or just revert the change for now. I'll have a
> >>>> look at this.
> >>>>
> >>>> Regards,
> >>>> Lucas
> >>>>
> >>>>> In the case of just reverting the change I'd propose to also set a jobs
> >>>>> entity pointer to NULL  once the job was taken from the entity, such
> >>>>> that in case of a future issue we fail where the actual issue resides
> >>>>> and to make it more obvious that the field shouldn't be used anymore
> >>>>> after the job was taken from the entity.
> >>>>>
> >>>>> I'm happy to implement the solution we agree on. However, it might also
> >>>>> make sense to revert the change until we have a solution in place. I'm
> >>>>> also happy to send a revert with a proper description of the problem.
> >>>>> Please let me know what you think.
> >>>>>
> >>>>> - Danilo
> >>>>>
>


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