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

[Felix Kuehling]

Am 2021-01-14 um 11:51 a.m. schrieb Jerome Glisse:
> On Thu, Jan 14, 2021 at 02:37:36PM +0100, Christian König 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.
> What is hmm_fence ? You should not have fence with hmm at all.
> So i am kind of scare now.

I kind of had the same question trying to follow Christian and Daniel's
discussion. I think an HMM fence would be any fence resulting from the
completion of a user mode operation in a context with HMM-based memory
management that may stall indefinitely due to page faults.

But on a hardware engine that cannot preempt page-faulting work and has
not reserved resources to guarantee forward progress for kernel jobs, I
think all fences will need to be HMM fences, because any work submitted
to such an engine can stall by getting stuck behind a stalled user mode
operation.

So for example, you have a DMA engine that can preempt during page
faults, but a graphics engine that cannot. Then work submitted to the
DMA engine can use dma_fence. But work submitted to the graphics engine
must use hmm_fence. To avoid deadlocks, dma_fences must never depend on
hmm_fences and resolution of page faults must never depend on hmm_fences.

Regards,
  Felix


>
> Cheers,
> Jérôme
>