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header.d=none;lists.linaro.org; dmarc=none action=none header.from=amd.com; Received: from MN2PR12MB3775.namprd12.prod.outlook.com (2603:10b6:208:159::19) by MN2PR12MB4095.namprd12.prod.outlook.com (2603:10b6:208:1d1::11) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3784.15; Wed, 27 Jan 2021 12:17:06 +0000 Received: from MN2PR12MB3775.namprd12.prod.outlook.com ([fe80::44f:9f01:ece7:f0e5]) by MN2PR12MB3775.namprd12.prod.outlook.com ([fe80::44f:9f01:ece7:f0e5%3]) with mapi id 15.20.3784.019; Wed, 27 Jan 2021 12:17:06 +0000 Subject: Re: [PATCH] RFC: dma-fence: Document recoverable page fault implications To: Maarten Lankhorst , Felix Kuehling , Daniel Vetter , DRI Development Cc: =?UTF-8?Q?Thomas_Hellstr=c3=b6m?= , Jerome Glisse , Daniel Vetter , Sumit Semwal , linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org References: <20210121194056.1734409-1-daniel.vetter@ffwll.ch> <6d373177-2645-1d67-9c14-dcad87c4f4d9@amd.com> 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HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: ND7YPA1wrzyQyU1kf6pQxSNV7gZ+wNqnxbY3E6oGBchmgFXPYUEYa+VmeeqA2I5w X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR12MB4095 Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20210107030127.20393-1-Felix.Kuehling%40amd.com%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7Cbee0aeff80f440bcc52108d8c2bcc11f%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637473463245588199%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=ncr%2Fqv5lw0ONrYxFvfdcFAXAZ%2BXcJJa6UY%2BxGfcKGVM%3D&reserved=0 >>> Cc: Maarten Lankhorst >>> Cc: Thomas Hellström >>> Cc: "Christian König" >>> Cc: Jerome Glisse >>> Cc: Felix Kuehling >>> Signed-off-by: Daniel Vetter >>> Cc: Sumit Semwal >>> Cc: linux-media@vger.kernel.org >>> Cc: linaro-mm-sig@lists.linaro.org >>> -- >>> 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. > 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 >