From: Jason Gunthorpe <jgg@nvidia.com>
To: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: "Zeng, Oak" <oak.zeng@intel.com>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"intel-xe@lists.freedesktop.org" <intel-xe@lists.freedesktop.org>,
"Brost, Matthew" <matthew.brost@intel.com>,
"Welty, Brian" <brian.welty@intel.com>,
"Ghimiray, Himal Prasad" <himal.prasad.ghimiray@intel.com>,
"Bommu, Krishnaiah" <krishnaiah.bommu@intel.com>,
"Vishwanathapura,
Niranjana" <niranjana.vishwanathapura@intel.com>,
Leon Romanovsky <leon@kernel.org>
Subject: Re: [PATCH 06/23] drm/xe/svm: Introduce a helper to build sg table from hmm range
Date: Fri, 26 Apr 2024 09:00:47 -0300 [thread overview]
Message-ID: <20240426120047.GX941030@nvidia.com> (raw)
In-Reply-To: <65cb3984309d377d6e7d57cb6567473c8a83ed78.camel@linux.intel.com>
On Fri, Apr 26, 2024 at 11:55:05AM +0200, Thomas Hellström wrote:
> First, the gpu_vma structure is something that partitions the gpu_vm
> that holds gpu-related range metadata, like what to mirror, desired gpu
> caching policies etc. These are managed (created, removed and split)
> mainly from user-space. These are stored and looked up from an rb-tree.
Except we are talking about SVA here, so all of this should not be
exposed to userspace.
> Now, when we hit a fault, we want to use hmm_range_fault() to re-
> populate the faulting PTE, but also to pre-fault a range. Using a range
> here (let's call this a prefault range for clarity) rather than to
> insert a single PTE is for multiple reasons:
I've never said to do a single range, everyone using hmm_range_fault()
has some kind of prefetch populate around algorithm.
> This is why we've been using dma_map_sg() for these ranges, since it is
> assumed the benefits gained from
This doesn't logically follow. You need to use dma_map_page page by
page and batch that into your update mechanism.
If you use dma_map_sg you get into the world of wrongness where you
have to track ranges and invalidation has to wipe an entire range -
because you cannot do a dma unmap of a single page from a dma_map_sg
mapping. This is all the wrong way to use hmm_range_fault.
hmm_range_fault() is page table mirroring, it fundamentally must be
page-by-page. The target page table structure must have similar
properties to the MM page table - especially page by page
validate/invalidate. Meaning you cannot use dma_map_sg().
> Second, when pre-faulting a range like this, the mmu interval notifier
> seqno comes into play, until the gpu ptes for the prefault range are
> safely in place. Now if an invalidation happens in a completely
> separate part of the mirror range, it will bump the seqno and force us
> to rerun the fault processing unnecessarily.
This is how hmm_range_fault() works. Drivers should not do hacky
things to try to "improve" this. SVA granuals should be large, maybe
not the entire MM, but still quite large. 2M is far to small.
There is a tradeoff here of slowing down the entire MM vs risking an
iteration during fault processing. We want to err toward making fault
processing slowing because fault procesing is already really slow.
> Hence, for this purpose we
> ideally just want to get a seqno bump covering the prefault range.
Ideally, but this is not something we can get for free.
> That's why finer-granularity mmu_interval notifiers might be beneficial
> (and then cached for future re-use of the same prefault range). This
> leads me to the next question:
It is not the design, please don't invent crazy special Intel things
on top of hmm_range_fault.
> You mention that mmu_notifiers are expensive to register. From looking
> at the code it seems *mmu_interval* notifiers are cheap unless there
> are ongoing invalidations in which case using a gpu_vma-wide notifier
> would block anyway? Could you clarify a bit more the cost involved
> here?
The rb tree insertions become expensive the larger the tree is. If you
have only a couple of notifiers it is reasonable.
> If we don't register these smaller-range interval notifiers, do
> you think the seqno bumps from unrelated subranges would be a real
> problem?
I don't think it is, you'd need to have a workload which was
agressively manipulating the CPU mm (which is also pretty slow). If
the workload is doing that then it also really won't like being slowed
down by the giant rb tree.
You can't win with an argument that collisions are likely due to an
app pattern that makes alot of stress on the MM so the right response
is to make the MM slower.
> Finally the size of the pre-faulting range is something we need to
> tune.
Correct.
> Currently it is cpu vma - wide. I understand you strongly suggest
> this should be avoided. Could you elaborate a bit on why this is such a
> bad choice?
Why would a prefetch have anything to do with a VMA? Ie your app calls
malloc() and gets a little allocation out of a giant mmap() arena -
you want to prefault the entire arena? Does that really make any
sense?
Mirroring is a huge PITA, IMHO it should be discouraged in favour of
SVA. Sadly too many CPUs still canot implement SVA.
With mirroring there is no good way for the system to predict what the
access pattern is. The only way to make this actually performant is
for userspace to explicitly manage the mirroring with some kind of
prefetching scheme to avoid faulting its accesses except in
extrodinary cases.
VMA is emphatically not a hint about what to prefetch. You should
balance your prefetching based on HW performance and related. If it is
tidy for HW to fault around a 2M granual then just do that.
Jason
next prev parent reply other threads:[~2024-04-26 12:01 UTC|newest]
Thread overview: 198+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-17 22:12 [PATCH 00/23] XeKmd basic SVM support Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 01/23] drm/xe/svm: Add SVM document Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 02/23] drm/xe/svm: Add svm key data structures Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 03/23] drm/xe/svm: create xe svm during vm creation Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 04/23] drm/xe/svm: Trace svm creation Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 05/23] drm/xe/svm: add helper to retrieve svm range from address Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 06/23] drm/xe/svm: Introduce a helper to build sg table from hmm range Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-04-05 0:39 ` Jason Gunthorpe
2024-04-05 3:33 ` Zeng, Oak
2024-04-05 12:37 ` Jason Gunthorpe
2024-04-05 16:42 ` Zeng, Oak
2024-04-05 18:02 ` Jason Gunthorpe
2024-04-09 16:45 ` Zeng, Oak
2024-04-09 17:24 ` Jason Gunthorpe
2024-04-23 21:17 ` Zeng, Oak
2024-04-24 2:31 ` Matthew Brost
2024-04-24 13:57 ` Jason Gunthorpe
2024-04-24 16:35 ` Matthew Brost
2024-04-24 16:44 ` Jason Gunthorpe
2024-04-24 16:56 ` Matthew Brost
2024-04-24 17:48 ` Jason Gunthorpe
2024-04-24 13:48 ` Jason Gunthorpe
2024-04-24 23:59 ` Zeng, Oak
2024-04-25 1:05 ` Jason Gunthorpe
2024-04-26 9:55 ` Thomas Hellström
2024-04-26 12:00 ` Jason Gunthorpe [this message]
2024-04-26 14:49 ` Thomas Hellström
2024-04-26 16:35 ` Jason Gunthorpe
2024-04-29 8:25 ` Thomas Hellström
2024-04-30 17:30 ` Jason Gunthorpe
2024-04-30 18:57 ` Daniel Vetter
2024-05-01 0:09 ` Jason Gunthorpe
2024-05-02 8:04 ` Daniel Vetter
2024-05-02 9:11 ` Thomas Hellström
2024-05-02 12:46 ` Jason Gunthorpe
2024-05-02 15:01 ` Thomas Hellström
2024-05-02 19:25 ` Zeng, Oak
2024-05-03 13:37 ` Jason Gunthorpe
2024-05-03 14:43 ` Zeng, Oak
2024-05-03 16:28 ` Jason Gunthorpe
2024-05-03 20:29 ` Zeng, Oak
2024-05-04 1:03 ` Dave Airlie
2024-05-06 13:04 ` Daniel Vetter
2024-05-06 23:50 ` Matthew Brost
2024-05-07 11:56 ` Jason Gunthorpe
2024-05-06 13:33 ` Jason Gunthorpe
2024-04-09 17:33 ` Matthew Brost
2024-01-17 22:12 ` [PATCH 07/23] drm/xe/svm: Add helper for binding hmm range to gpu Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 08/23] drm/xe/svm: Add helper to invalidate svm range from GPU Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 09/23] drm/xe/svm: Remap and provide memmap backing for GPU vram Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 10/23] drm/xe/svm: Introduce svm migration function Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 11/23] drm/xe/svm: implement functions to allocate and free device memory Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 12/23] drm/xe/svm: Trace buddy block allocation and free Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 13/23] drm/xe/svm: Handle CPU page fault Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 14/23] drm/xe/svm: trace svm range migration Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 15/23] drm/xe/svm: Implement functions to register and unregister mmu notifier Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 16/23] drm/xe/svm: Implement the mmu notifier range invalidate callback Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 17/23] drm/xe/svm: clean up svm range during process exit Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 18/23] drm/xe/svm: Move a few structures to xe_gt.h Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 19/23] drm/xe/svm: migrate svm range to vram Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 20/23] drm/xe/svm: Populate svm range Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 21/23] drm/xe/svm: GPU page fault support Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-23 2:06 ` Welty, Brian
2024-01-23 2:06 ` Welty, Brian
2024-01-23 3:09 ` Zeng, Oak
2024-01-23 3:09 ` Zeng, Oak
2024-01-23 3:21 ` Making drm_gpuvm work across gpu devices Zeng, Oak
2024-01-23 3:21 ` Zeng, Oak
2024-01-23 11:13 ` Christian König
2024-01-23 11:13 ` Christian König
2024-01-23 19:37 ` Zeng, Oak
2024-01-23 19:37 ` Zeng, Oak
2024-01-23 20:17 ` Felix Kuehling
2024-01-23 20:17 ` Felix Kuehling
2024-01-25 1:39 ` Zeng, Oak
2024-01-25 1:39 ` Zeng, Oak
2024-01-23 23:56 ` Danilo Krummrich
2024-01-23 23:56 ` Danilo Krummrich
2024-01-24 3:57 ` Zeng, Oak
2024-01-24 3:57 ` Zeng, Oak
2024-01-24 4:14 ` Zeng, Oak
2024-01-24 4:14 ` Zeng, Oak
2024-01-24 6:48 ` Christian König
2024-01-24 6:48 ` Christian König
2024-01-25 22:13 ` Danilo Krummrich
2024-01-25 22:13 ` Danilo Krummrich
2024-01-24 8:33 ` Christian König
2024-01-24 8:33 ` Christian König
2024-01-25 1:17 ` Zeng, Oak
2024-01-25 1:17 ` Zeng, Oak
2024-01-25 1:25 ` David Airlie
2024-01-25 1:25 ` David Airlie
2024-01-25 5:25 ` Zeng, Oak
2024-01-25 5:25 ` Zeng, Oak
2024-01-26 10:09 ` Christian König
2024-01-26 10:09 ` Christian König
2024-01-26 20:13 ` Zeng, Oak
2024-01-26 20:13 ` Zeng, Oak
2024-01-29 10:10 ` Christian König
2024-01-29 10:10 ` Christian König
2024-01-29 20:09 ` Zeng, Oak
2024-01-29 20:09 ` Zeng, Oak
2024-01-25 11:00 ` 回复：Making " 周春明(日月)
2024-01-25 11:00 ` 周春明(日月)
2024-01-25 17:00 ` Zeng, Oak
2024-01-25 17:00 ` Zeng, Oak
2024-01-25 17:15 ` Making " Felix Kuehling
2024-01-25 17:15 ` Felix Kuehling
2024-01-25 18:37 ` Zeng, Oak
2024-01-25 18:37 ` Zeng, Oak
2024-01-26 13:23 ` Christian König
2024-01-26 13:23 ` Christian König
2024-01-25 16:42 ` Zeng, Oak
2024-01-25 16:42 ` Zeng, Oak
2024-01-25 18:32 ` Daniel Vetter
2024-01-25 18:32 ` Daniel Vetter
2024-01-25 21:02 ` Zeng, Oak
2024-01-25 21:02 ` Zeng, Oak
2024-01-26 8:21 ` Thomas Hellström
2024-01-26 8:21 ` Thomas Hellström
2024-01-26 12:52 ` Christian König
2024-01-26 12:52 ` Christian König
2024-01-27 2:21 ` Zeng, Oak
2024-01-27 2:21 ` Zeng, Oak
2024-01-29 10:19 ` Christian König
2024-01-29 10:19 ` Christian König
2024-01-30 0:21 ` Zeng, Oak
2024-01-30 0:21 ` Zeng, Oak
2024-01-30 8:39 ` Christian König
2024-01-30 8:39 ` Christian König
2024-01-30 22:29 ` Zeng, Oak
2024-01-30 22:29 ` Zeng, Oak
2024-01-30 23:12 ` David Airlie
2024-01-30 23:12 ` David Airlie
2024-01-31 9:15 ` Daniel Vetter
2024-01-31 9:15 ` Daniel Vetter
2024-01-31 20:17 ` Zeng, Oak
2024-01-31 20:17 ` Zeng, Oak
2024-01-31 20:59 ` Zeng, Oak
2024-01-31 20:59 ` Zeng, Oak
2024-02-01 8:52 ` Christian König
2024-02-01 8:52 ` Christian König
2024-02-29 18:22 ` Zeng, Oak
2024-03-08 4:43 ` Zeng, Oak
2024-03-08 10:07 ` Christian König
2024-01-30 8:43 ` Thomas Hellström
2024-01-30 8:43 ` Thomas Hellström
2024-01-29 15:03 ` Felix Kuehling
2024-01-29 15:03 ` Felix Kuehling
2024-01-29 15:33 ` Christian König
2024-01-29 15:33 ` Christian König
2024-01-29 16:24 ` Felix Kuehling
2024-01-29 16:24 ` Felix Kuehling
2024-01-29 16:28 ` Christian König
2024-01-29 16:28 ` Christian König
2024-01-29 17:52 ` Felix Kuehling
2024-01-29 17:52 ` Felix Kuehling
2024-01-29 19:03 ` Christian König
2024-01-29 19:03 ` Christian König
2024-01-29 20:24 ` Felix Kuehling
2024-01-29 20:24 ` Felix Kuehling
2024-02-23 20:12 ` Zeng, Oak
2024-02-27 6:54 ` Christian König
2024-02-27 15:58 ` Zeng, Oak
2024-02-28 19:51 ` Zeng, Oak
2024-02-29 9:41 ` Christian König
2024-02-29 16:05 ` Zeng, Oak
2024-02-29 17:12 ` Thomas Hellström
2024-03-01 7:01 ` Christian König
2024-01-17 22:12 ` [PATCH 22/23] drm/xe/svm: Add DRM_XE_SVM kernel config entry Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-17 22:12 ` [PATCH 23/23] drm/xe/svm: Add svm memory hints interface Oak Zeng
2024-01-17 22:12 ` Oak Zeng
2024-01-18 2:45 ` ✓ CI.Patch_applied: success for XeKmd basic SVM support Patchwork
2024-01-18 2:46 ` ✗ CI.checkpatch: warning " Patchwork
2024-01-18 2:46 ` ✗ CI.KUnit: failure " Patchwork
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