Am 2021-02-09 um 1:37 a.m. schrieb Daniel Vetter: > On Tue, Feb 9, 2021 at 4:13 AM Bas Nieuwenhuizen > <bas@xxxxxxxxxxxxxxxxxxx> wrote: >> On Thu, Jan 28, 2021 at 4:40 PM Felix Kuehling <felix.kuehling@xxxxxxx> wrote: >>> Am 2021-01-28 um 2:39 a.m. schrieb Christian König: >>>> Am 27.01.21 um 23:00 schrieb Felix Kuehling: >>>>> Am 2021-01-27 um 7:16 a.m. schrieb Christian König: >>>>>> Am 27.01.21 um 13:11 schrieb Maarten Lankhorst: >>>>>>> Op 27-01-2021 om 01:22 schreef Felix Kuehling: >>>>>>>> Am 2021-01-21 um 2:40 p.m. schrieb Daniel Vetter: >>>>>>>>> Recently there was a fairly long thread about recoreable hardware >>>>>>>>> page >>>>>>>>> faults, how they can deadlock, and what to do about that. >>>>>>>>> >>>>>>>>> While the discussion is still fresh I figured good time to try and >>>>>>>>> document the conclusions a bit. >>>>>>>>> >>>>>>>>> References: >>>>>>>>> https://lore.kernel.org/dri-devel/20210107030127.20393-1-Felix.Kuehling@xxxxxxx/ >>>>>>>>> >>>>>>>>> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx> >>>>>>>>> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxx> >>>>>>>>> Cc: "Christian König" <christian.koenig@xxxxxxx> >>>>>>>>> Cc: Jerome Glisse <jglisse@xxxxxxxxxx> >>>>>>>>> Cc: Felix Kuehling <felix.kuehling@xxxxxxx> >>>>>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> >>>>>>>>> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx> >>>>>>>>> Cc: linux-media@xxxxxxxxxxxxxxx >>>>>>>>> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx >>>>>>>>> -- >>>>>>>>> I'll be away next week, but figured I'll type this up quickly for >>>>>>>>> some >>>>>>>>> comments and to check whether I got this all roughly right. >>>>>>>>> >>>>>>>>> Critique very much wanted on this, so that we can make sure hw which >>>>>>>>> can't preempt (with pagefaults pending) like gfx10 has a clear >>>>>>>>> path to >>>>>>>>> support page faults in upstream. So anything I missed, got wrong or >>>>>>>>> like that would be good. >>>>>>>>> -Daniel >>>>>>>>> --- >>>>>>>>> Documentation/driver-api/dma-buf.rst | 66 >>>>>>>>> ++++++++++++++++++++++++++++ >>>>>>>>> 1 file changed, 66 insertions(+) >>>>>>>>> >>>>>>>>> diff --git a/Documentation/driver-api/dma-buf.rst >>>>>>>>> b/Documentation/driver-api/dma-buf.rst >>>>>>>>> index a2133d69872c..e924c1e4f7a3 100644 >>>>>>>>> --- a/Documentation/driver-api/dma-buf.rst >>>>>>>>> +++ b/Documentation/driver-api/dma-buf.rst >>>>>>>>> @@ -257,3 +257,69 @@ fences in the kernel. This means: >>>>>>>>> userspace is allowed to use userspace fencing or long running >>>>>>>>> compute >>>>>>>>> workloads. This also means no implicit fencing for shared >>>>>>>>> buffers in these >>>>>>>>> cases. >>>>>>>>> + >>>>>>>>> +Recoverable Hardware Page Faults Implications >>>>>>>>> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >>>>>>>>> + >>>>>>>>> +Modern hardware supports recoverable page faults, which has a >>>>>>>>> lot of >>>>>>>>> +implications for DMA fences. >>>>>>>>> + >>>>>>>>> +First, a pending page fault obviously holds up the work that's >>>>>>>>> running on the >>>>>>>>> +accelerator and a memory allocation is usually required to resolve >>>>>>>>> the fault. >>>>>>>>> +But memory allocations are not allowed to gate completion of DMA >>>>>>>>> fences, which >>>>>>>>> +means any workload using recoverable page faults cannot use DMA >>>>>>>>> fences for >>>>>>>>> +synchronization. Synchronization fences controlled by userspace >>>>>>>>> must be used >>>>>>>>> +instead. >>>>>>>>> + >>>>>>>>> +On GPUs this poses a problem, because current desktop compositor >>>>>>>>> protocols on >>>>>>>>> +Linus rely on DMA fences, which means without an entirely new >>>>>>>>> userspace stack >>>>>>>>> +built on top of userspace fences, they cannot benefit from >>>>>>>>> recoverable page >>>>>>>>> +faults. The exception is when page faults are only used as >>>>>>>>> migration hints and >>>>>>>>> +never to on-demand fill a memory request. For now this means >>>>>>>>> recoverable page >>>>>>>>> +faults on GPUs are limited to pure compute workloads. >>>>>>>>> + >>>>>>>>> +Furthermore GPUs usually have shared resources between the 3D >>>>>>>>> rendering and >>>>>>>>> +compute side, like compute units or command submission engines. If >>>>>>>>> both a 3D >>>>>>>>> +job with a DMA fence and a compute workload using recoverable page >>>>>>>>> faults are >>>>>>>>> +pending they could deadlock: >>>>>>>>> + >>>>>>>>> +- The 3D workload might need to wait for the compute job to finish >>>>>>>>> and release >>>>>>>>> + hardware resources first. >>>>>>>>> + >>>>>>>>> +- The compute workload might be stuck in a page fault, because the >>>>>>>>> memory >>>>>>>>> + allocation is waiting for the DMA fence of the 3D workload to >>>>>>>>> complete. >>>>>>>>> + >>>>>>>>> +There are a few ways to prevent this problem: >>>>>>>>> + >>>>>>>>> +- Compute workloads can always be preempted, even when a page >>>>>>>>> fault is pending >>>>>>>>> + and not yet repaired. Not all hardware supports this. >>>>>>>>> + >>>>>>>>> +- DMA fence workloads and workloads which need page fault handling >>>>>>>>> have >>>>>>>>> + independent hardware resources to guarantee forward progress. >>>>>>>>> This could be >>>>>>>>> + achieved through e.g. through dedicated engines and minimal >>>>>>>>> compute unit >>>>>>>>> + reservations for DMA fence workloads. >>>>>>>>> + >>>>>>>>> +- The reservation approach could be further refined by only >>>>>>>>> reserving the >>>>>>>>> + hardware resources for DMA fence workloads when they are >>>>>>>>> in-flight. This must >>>>>>>>> + cover the time from when the DMA fence is visible to other >>>>>>>>> threads up to >>>>>>>>> + moment when fence is completed through dma_fence_signal(). >>>>>>>>> + >>>>>>>>> +- As a last resort, if the hardware provides no useful reservation >>>>>>>>> mechanics, >>>>>>>>> + all workloads must be flushed from the GPU when switching >>>>>>>>> between jobs >>>>>>>>> + requiring DMA fences or jobs requiring page fault handling: This >>>>>>>>> means all DMA >>>>>>>>> + fences must complete before a compute job with page fault >>>>>>>>> handling can be >>>>>>>>> + inserted into the scheduler queue. And vice versa, before a DMA >>>>>>>>> fence can be >>>>>>>>> + made visible anywhere in the system, all compute workloads must >>>>>>>>> be preempted >>>>>>>>> + to guarantee all pending GPU page faults are flushed. >>>>>>>> I thought of another possible workaround: >>>>>>>> >>>>>>>> * Partition the memory. Servicing of page faults will use a >>>>>>>> separate >>>>>>>> memory pool that can always be allocated from without >>>>>>>> waiting for >>>>>>>> fences. This includes memory for page tables and memory for >>>>>>>> migrating data to. You may steal memory from other processes >>>>>>>> that >>>>>>>> can page fault, so no fence waiting is necessary. Being able to >>>>>>>> steal memory at any time also means there are basically no >>>>>>>> out-of-memory situations you need to worry about. Even page >>>>>>>> tables >>>>>>>> (except the root page directory of each process) can be >>>>>>>> stolen in >>>>>>>> the worst case. >>>>>>> I think 'overcommit' would be a nice way to describe this. But I'm not >>>>>>> sure how easy this is to implement in practice. You would basically >>>>>>> need >>>>>>> to create your own memory manager for this. >>>>>> Well you would need a completely separate pool for both device as well >>>>>> as system memory. >>>>>> >>>>>> E.g. on boot we say we steal X GB system memory only for HMM. >>>>> Why? The GPU driver doesn't need to allocate system memory for HMM. >>>>> Migrations to system memory are handled by the kernel's handle_mm_fault >>>>> and page allocator and swap logic. >>>> And that one depends on dma_fence completion because you can easily >>>> need to wait for an MMU notifier callback. >>> I see, the GFX MMU notifier for userpointers in amdgpu currently waits >>> for fences. For the KFD MMU notifier I am planning to fix this by >>> causing GPU page faults instead of preempting the queues. Can we limit >>> userptrs in amdgpu to engines that can page fault. Basically make it >>> illegal to attach userptr BOs to graphics CS BO lists, so they can only >>> be used in user mode command submissions, which can page fault. Then the >>> GFX MMU notifier could invalidate PTEs and would not have to wait for >>> fences. >> sadly graphics + userptr is already exposed via Mesa. > This is not about userptr, we fake userptr entirely in software. It's > about exposing recoverable gpu page faults (which would make userptr > maybe more efficient since we could do on-demand paging). userptr > itself isn't a problem, but it is part of the reasons why this is > tricky. > > Christian/Felix, I think for kernel folks this is clear enough that I > don't need to clarify this in the text? Yeah, it's clear to me. Anyway, your latest text doesn't reference userptr directly and keeps the discussion at a fairly abstract level. So I think it's fine. It's the practical details of the proposed workarounds where it feel like walking through a mirror cabinet, bumping into unexpected obstacles with every other step. Regards, Felix > -Daniel > >>> >>>> As Maarten wrote when you want to go down this route you need a >>>> complete separate memory management parallel to the one of the kernel. >>> Not really. I'm trying to make the GPU memory management more similar to >>> what the kernel does for system memory. >>> >>> I understood Maarten's comment as "I'm creating a new memory manager and >>> not using TTM any more". This is true. The idea is that this portion of >>> VRAM would be managed more like system memory. >>> >>> Regards, >>> Felix >>> >>> >>>> Regards, >>>> Christian. >>>> >>>>> It doesn't depend on any fences, so >>>>> it cannot deadlock with any GPU driver-managed memory. The GPU driver >>>>> gets involved in the MMU notifier to invalidate device page tables. But >>>>> that also doesn't need to wait for any fences. >>>>> >>>>> And if the kernel runs out of pageable memory, you're in trouble anyway. >>>>> The OOM killer will step in, nothing new there. >>>>> >>>>> Regards, >>>>> Felix >>>>> >>>>> >>>>>>> But from a design point of view, definitely a valid solution. >>>>>> I think the restriction above makes it pretty much unusable. >>>>>> >>>>>>> But this looks good, those solutions are definitely the valid >>>>>>> options we >>>>>>> can choose from. >>>>>> It's certainly worth noting, yes. And just to make sure that nobody >>>>>> has the idea to reserve only device memory. >>>>>> >>>>>> Christian. >>>>>> >>>>>>> ~Maarten >>>>>>> >>>>> _______________________________________________ >>>>> Linaro-mm-sig mailing list >>>>> Linaro-mm-sig@xxxxxxxxxxxxxxxx >>>>> https://lists.linaro.org/mailman/listinfo/linaro-mm-sig >>>>> >>> _______________________________________________ >>> dri-devel mailing list >>> dri-devel@xxxxxxxxxxxxxxxxxxxxx >>> https://lists.freedesktop.org/mailman/listinfo/dri-devel > >
