HMM fence (was Re: [PATCH 00/35] Add HMM-based SVM memory manager to KFD)

[Daniel Vetter]

On Thu, Jan 14, 2021 at 3:13 PM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
>
> Am 14.01.21 um 14:57 schrieb Daniel Vetter:
> > On Thu, Jan 14, 2021 at 2:37 PM Christian König
> > <christian.koenig at amd.com> wrote:
> >> Am 14.01.21 um 12:52 schrieb Daniel Vetter:
> >>> [SNIP]
> >>>>> I had a new idea, i wanted to think more about it but have not yet,
> >>>>> anyway here it is. Adding a new callback to dma fence which ask the
> >>>>> question can it dead lock ? Any time a GPU driver has pending page
> >>>>> fault (ie something calling into the mm) it answer yes, otherwise
> >>>>> no. The GPU shrinker would ask the question before waiting on any
> >>>>> dma-fence and back of if it gets yes. Shrinker can still try many
> >>>>> dma buf object for which it does not get a yes on associated fence.
> >>>>>
> >>>>> This does not solve the mmu notifier case, for this you would just
> >>>>> invalidate the gem userptr object (with a flag but not releasing the
> >>>>> page refcount) but you would not wait for the GPU (ie no dma fence
> >>>>> wait in that code path anymore). The userptr API never really made
> >>>>> the contract that it will always be in sync with the mm view of the
> >>>>> world so if different page get remapped to same virtual address
> >>>>> while GPU is still working with the old pages it should not be an
> >>>>> issue (it would not be in our usage of userptr for compositor and
> >>>>> what not).
> >>>> The current working idea in my mind goes into a similar direction.
> >>>>
> >>>> But instead of a callback I'm adding a complete new class of HMM fences.
> >>>>
> >>>> Waiting in the MMU notfier, scheduler, TTM etc etc is only allowed for
> >>>> the dma_fences and HMM fences are ignored in container objects.
> >>>>
> >>>> When you handle an implicit or explicit synchronization request from
> >>>> userspace you need to block for HMM fences to complete before taking any
> >>>> resource locks.
> >>> Isnt' that what I call gang scheduling? I.e. you either run in HMM
> >>> mode, or in legacy fencing mode (whether implicit or explicit doesn't
> >>> really matter I think). By forcing that split we avoid the problem,
> >>> but it means occasionally full stalls on mixed workloads.
> >>>
> >>> But that's not what Jerome wants (afaiui at least), I think his idea
> >>> is to track the reverse dependencies of all the fences floating
> >>> around, and then skip evicting an object if you have to wait for any
> >>> fence that is problematic for the current calling context. And I don't
> >>> think that's very feasible in practice.
> >>>
> >>> So what kind of hmm fences do you have in mind here?
> >> It's a bit more relaxed than your gang schedule.
> >>
> >> See the requirements are as follow:
> >>
> >> 1. dma_fences never depend on hmm_fences.
> >> 2. hmm_fences can never preempt dma_fences.
> >> 3. dma_fences must be able to preempt hmm_fences or we always reserve
> >> enough hardware resources (CUs) to guarantee forward progress of dma_fences.
> >>
> >> Critical sections are MMU notifiers, page faults, GPU schedulers and
> >> dma_reservation object locks.
> >>
> >> 4. It is valid to wait for a dma_fences in critical sections.
> >> 5. It is not valid to wait for hmm_fences in critical sections.
> >>
> >> Fence creation either happens during command submission or by adding
> >> something like a barrier or signal command to your userspace queue.
> >>
> >> 6. If we have an hmm_fence as implicit or explicit dependency for
> >> creating a dma_fence we must wait for that before taking any locks or
> >> reserving resources.
> >> 7. If we have a dma_fence as implicit or explicit dependency for
> >> creating an hmm_fence we can wait later on. So busy waiting or special
> >> WAIT hardware commands are valid.
> >>
> >> This prevents hard cuts, e.g. can mix hmm_fences and dma_fences at the
> >> same time on the hardware.
> >>
> >> In other words we can have a high priority gfx queue running jobs based
> >> on dma_fences and a low priority compute queue running jobs based on
> >> hmm_fences.
> >>
> >> Only when we switch from hmm_fence to dma_fence we need to block the
> >> submission until all the necessary resources (both memory as well as
> >> CUs) are available.
> >>
> >> This is somewhat an extension to your gang submit idea.
> > Either I'm missing something, or this is just exactly what we
> > documented already with userspace fences in general, and how you can't
> > have a dma_fence depend upon a userspace (or hmm_fence).
> >
> > My gang scheduling idea is really just an alternative for what you
> > have listed as item 3 above. Instead of requiring preempt or requiring
> > guaranteed forward progress of some other sorts we flush out any
> > pending dma_fence request. But _only_ those which would get stalled by
> > the job we're running, so high-priority sdma requests we need in the
> > kernel to shuffle buffers around are still all ok. This would be
> > needed if you're hw can't preempt, and you also have shared engines
> > between compute and gfx, so reserving CUs won't solve the problem
> > either.
> >
> > What I don't mean with my gang scheduling is a completely exclusive
> > mode between hmm_fence and dma_fence, since that would prevent us from
> > using copy engines and dma_fence in the kernel to shuffle memory
> > around for hmm jobs. And that would suck, even on compute-only
> > workloads. Maybe I should rename "gang scheduling" to "engine flush"
> > or something like that.
>
> Yeah, "engine flush" makes it much more clearer.
>
> What I wanted to emphasis is that we have to mix dma_fences and
> hmm_fences running at the same time on the same hardware fighting over
> the same resources.
>
> E.g. even on the newest hardware multimedia engines can't handle page
> faults, so video decoding/encoding will still produce dma_fences.

Well we also have to mix them so the kernel can shovel data around
using copy engines. Plus we have to mix it at the overall subsystem
level because I'm not sure SoC-class gpus will ever get here,
definitely aren't yet there for sure.

> > I think the basics of userspace or hmm_fence or whatever we'll call it
> > we've documented already here:
> >
> > https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_fence#indefinite-dma-fences
>
> This talks about the restrictions we have for dma_fences and why
> infinite fences (even as hmm_fence) will never work.
>
> But it doesn't talk about how to handle implicit or explicit
> dependencies with something like hmm_fences.
>
> In other words my proposal above allows for hmm_fences to show up in
> dma_reservation objects and are used together with all this explicit
> synchronization we still have with only a medium amount of work :)

Oh. I don't think we should put any hmm_fence or other infinite fence
into a dma_resv object. At least not into the current dma_resv object,
because then we have that infinite fences problem everywhere, and very
hard to audit.

What we could do is add new hmm_fence only slots for implicit sync,
but I think consensus is that implicit sync is bad, never do it again.
Last time around (for timeline syncobj) we've also pushed the waiting
on cross-over to userspace, and I think that's the right option, so we
need userspace to understand the hmm fence anyway. At that point we
might as well bite the bullet and do another round of wayland/dri
protocols.

So from that pov I think the kernel should at most deal with an
hmm_fence for cross-process communication and maybe some standard wait
primitives (for userspace to use, not for the kernel).

The only use case this would forbid is using page faults for legacy
implicit/explicit dma_fence synced workloads, and I think that's
perfectly ok to not allow. Especially since the motivation here for
all this is compute, and compute doesn't pass around dma_fences
anyway.

> > I think the only thing missing is clarifying a bit what you have under
> > item 3, i.e. how do we make sure there's no accidental hidden
> > dependency between hmm_fence and dma_fence. Maybe a subsection about
> > gpu page fault handling?
>
> The real improvement is item 6. The problem with it is that it requires
> auditing all occasions when we create dma_fences so that we don't
> accidentally depend on an HMM fence.

We have that rule already, it's the "dma_fence must not depend upon an
infinite fence anywhere" rule we documented last summer. So that
doesn't feel new.
-Daniel

>
> Regards,
> Christian.
>
> >
> > Or are we still talking past each another a bit here?
> > -Daniel
> >
> >
> >> Regards,
> >> Christian.
> >>
> >>> -Daniel
> >>>
> >
>


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