| Message ID | cover.1730298502.git.leon@kernel.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | Provide a new two step DMA mapping API | expand |
On 10/30/24 9:12 AM, Leon Romanovsky wrote: > Changelog: > v1: > * Squashed two VFIO patches into one > * Added Acked-by/Reviewed-by tags > * Fix docs spelling errors > * Simplified dma_iova_sync() API > * Added extra check in dma_iova_destroy() if mapped size to make code more clear > * Fixed checkpatch warnings in p2p patch > * Changed implementation of VFIO mlx5 mlx5vf_add_migration_pages() to > be more general > * Reduced the number of changes in VFIO patch > v0: https://lore.kernel.org/all/cover.1730037276.git.leon@kernel.org > > ---------------------------------------------------------------------------- > The code can be downloaded from: > https://git.kernel.org/pub/scm/linux/kernel/git/leon/linux-rdma.git tag:dma-split-oct-30 On Christoph's request, I tested this series last week and saw some pretty significant performance regressions on my box. I don't know what the status is in terms of that, just want to make sure something like this doesn't get merged until that is both fully understood and sorted out.
On Wed, Oct 30, 2024 at 07:44:13PM -0600, Jens Axboe wrote: > On Christoph's request, I tested this series last week and saw some > pretty significant performance regressions on my box. I don't know what > the status is in terms of that, just want to make sure something like > this doesn't get merged until that is both fully understood and sorted > out. Working on it, but I have way too many things going on at once. Note that the weird thing about your setup was that we apparently dropped into the slow path, which still puzzles me. But I should probably also look into making that path a little less slow.
On Thu, Oct 31, 2024 at 09:34:50AM +0100, Christoph Hellwig wrote: > On Wed, Oct 30, 2024 at 07:44:13PM -0600, Jens Axboe wrote: > > On Christoph's request, I tested this series last week and saw some > > pretty significant performance regressions on my box. I don't know what > > the status is in terms of that, just want to make sure something like > > this doesn't get merged until that is both fully understood and sorted > > out. This series is a subset of the series you tested and doesn't include the block layer changes which most likely were the cause of the performance regression. This is why I separated the block layer changes from the rest of the series and marked them as RFC. The current patch set is viable for HMM and VFIO. Can you please retest only this series and leave the block layer changes for later till Christoph finds the answer for the performance regression? Thanks > > Working on it, but I have way too many things going on at once. Note > that the weird thing about your setup was that we apparently dropped into > the slow path, which still puzzles me. But I should probably also look > into making that path a little less slow. > >
On Thu, Oct 31, 2024 at 11:05:30AM +0200, Leon Romanovsky wrote: > This series is a subset of the series you tested and doesn't include the > block layer changes which most likely were the cause of the performance > regression. > > This is why I separated the block layer changes from the rest of the series > and marked them as RFC. > > The current patch set is viable for HMM and VFIO. Can you please retest > only this series and leave the block layer changes for later till Christoph > finds the answer for the performance regression? As the subset doesn't touch block code or code called by block I don't think we need Jens to benchmark it, unless he really wants to.
On Thu, Oct 31, 2024 at 10:21:13AM +0100, Christoph Hellwig wrote: > On Thu, Oct 31, 2024 at 11:05:30AM +0200, Leon Romanovsky wrote: > > This series is a subset of the series you tested and doesn't include the > > block layer changes which most likely were the cause of the performance > > regression. > > > > This is why I separated the block layer changes from the rest of the series > > and marked them as RFC. > > > > The current patch set is viable for HMM and VFIO. Can you please retest > > only this series and leave the block layer changes for later till Christoph > > finds the answer for the performance regression? > > As the subset doesn't touch block code or code called by block I don't > think we need Jens to benchmark it, unless he really wants to. He wrote this sentence in his email, while responding on subset which doesn't change anything in block layer: "just want to make sure something like this doesn't get merged until that is both fully understood and sorted out." This series works like a charm for RDMA (HMM) and VFIO. Thanks
On 10/31/24 2:34 AM, Christoph Hellwig wrote: > On Wed, Oct 30, 2024 at 07:44:13PM -0600, Jens Axboe wrote: >> On Christoph's request, I tested this series last week and saw some >> pretty significant performance regressions on my box. I don't know what >> the status is in terms of that, just want to make sure something like >> this doesn't get merged until that is both fully understood and sorted >> out. > > Working on it, but I have way too many things going on at once. Note > that the weird thing about your setup was that we apparently dropped into > the slow path, which still puzzles me. But I should probably also look > into making that path a little less slow. That's fine, just wanted to ensure that no push was being done on this before that was resolved.
On 10/31/24 3:37 AM, Leon Romanovsky wrote: > On Thu, Oct 31, 2024 at 10:21:13AM +0100, Christoph Hellwig wrote: >> On Thu, Oct 31, 2024 at 11:05:30AM +0200, Leon Romanovsky wrote: >>> This series is a subset of the series you tested and doesn't include the >>> block layer changes which most likely were the cause of the performance >>> regression. >>> >>> This is why I separated the block layer changes from the rest of the series >>> and marked them as RFC. >>> >>> The current patch set is viable for HMM and VFIO. Can you please retest >>> only this series and leave the block layer changes for later till Christoph >>> finds the answer for the performance regression? >> >> As the subset doesn't touch block code or code called by block I don't >> think we need Jens to benchmark it, unless he really wants to. > > He wrote this sentence in his email, while responding on subset which > doesn't change anything in block layer: "just want to make sure > something like this doesn't get merged until that is both fully > understood and sorted out." > > This series works like a charm for RDMA (HMM) and VFIO. I don't care about rdma/vfio, nor do I test it, so you guys can do whatever you want there, as long as it doesn't regress the iommu side. The block series is separate, so we'll deal with that when we get there. I don't know why you CC'ed linux-block on the series.
On Thu, Oct 31, 2024 at 11:43:50AM -0600, Jens Axboe wrote: > On 10/31/24 3:37 AM, Leon Romanovsky wrote: > > On Thu, Oct 31, 2024 at 10:21:13AM +0100, Christoph Hellwig wrote: > >> On Thu, Oct 31, 2024 at 11:05:30AM +0200, Leon Romanovsky wrote: > >>> This series is a subset of the series you tested and doesn't include the > >>> block layer changes which most likely were the cause of the performance > >>> regression. > >>> > >>> This is why I separated the block layer changes from the rest of the series > >>> and marked them as RFC. > >>> > >>> The current patch set is viable for HMM and VFIO. Can you please retest > >>> only this series and leave the block layer changes for later till Christoph > >>> finds the answer for the performance regression? > >> > >> As the subset doesn't touch block code or code called by block I don't > >> think we need Jens to benchmark it, unless he really wants to. > > > > He wrote this sentence in his email, while responding on subset which > > doesn't change anything in block layer: "just want to make sure > > something like this doesn't get merged until that is both fully > > understood and sorted out." > > > > This series works like a charm for RDMA (HMM) and VFIO. > > I don't care about rdma/vfio, nor do I test it, so you guys can do > whatever you want there, as long as it doesn't regress the iommu side. > The block series is separate, so we'll deal with that when we get there. > > I don't know why you CC'ed linux-block on the series. Because of the second part, which is marked as RFC and based on this one. I think that it is better to present whole picture to everyone interested in the discussion. Thanks > > -- > Jens Axboe >
On 30/10/2024 3:12 pm, Leon Romanovsky wrote: > Changelog: > v1: > * Squashed two VFIO patches into one > * Added Acked-by/Reviewed-by tags > * Fix docs spelling errors > * Simplified dma_iova_sync() API > * Added extra check in dma_iova_destroy() if mapped size to make code more clear > * Fixed checkpatch warnings in p2p patch > * Changed implementation of VFIO mlx5 mlx5vf_add_migration_pages() to > be more general > * Reduced the number of changes in VFIO patch > v0: https://lore.kernel.org/all/cover.1730037276.git.leon@kernel.org > > ---------------------------------------------------------------------------- > The code can be downloaded from: > https://git.kernel.org/pub/scm/linux/kernel/git/leon/linux-rdma.git tag:dma-split-oct-30 > > ---------------------------------------------------------------------------- > Currently the only efficient way to map a complex memory description through > the DMA API is by using the scatterlist APIs. It's really not efficient... In most cases they're just wrappers for a bunch of dma_map_page() etc. calls for the convenience of callers who are using a scatterlist for their own reasons anyway. Even with iommu-dma, I expect that approach would likely perform better for most users as well, given that typical individual segment sizes are much more likely to be in scope of the IOVA caches. The hilarious amount of work that iommu_dma_map_sg() does is pretty much entirely for the benefit of v4l2 and dma-buf importers who *depend* on being able to linearise a scatterlist in DMA address space. TBH I doubt there are many actual scatter-gather-capable devices with significant enough limitations to meaningfully benefit from DMA segment combining these days - I've often thought that by now it might be a good idea to turn that behaviour off by default and add an attribute for callers to explicitly request it. > The SG APIs are unique in that > they efficiently combine the two fundamental operations of sizing and allocating > a large IOVA window from the IOMMU and processing all the per-address > swiotlb/flushing/p2p/map details. Except that's obviously not unique when the page APIs also combine the exact same operations? :/ > This uniqueness has been a long standing pain point as the scatterlist API > is mandatory, but expensive to use. Huh? When and where has anything ever called it mandatory? Nobody's getting sent to DMA jail for open-coding: for_each_sg(...) my_dma_addr = dma_map_page(..., sg_page()); if they do know the map_sg operation is unnecessarily expensive for their needs. > It prevents any kind of optimization or > feature improvement (such as avoiding struct page for P2P) due to the impossibility > of improving the scatterlist. > > Several approaches have been explored to expand the DMA API with additional > scatterlist-like structures (BIO, rlist), instead split up the DMA API > to allow callers to bring their own data structure. And this line of reasoning is still "2 + 2 = Thursday" - what is to say those two notions in any way related? We literally already have one generic DMA operation which doesn't operate on struct page, yet needed nothing "split up" to be possible. Fair enough if callers want some alternative interfaces for mapping memory as well, but to be a common DMA API it has to be usable everywhere and cover all the DMA operations that the current page-based APIs provide, otherwise those callers obviously can't stop using struct pages. What precludes a straightforward dma_map_phys() etc. to parallel the existing API? What's the justification for an IOMMU-specific design when surely if anyone can benefit from more memory-efficient structures across drivers and subsystems it's the little embedded platforms, not the big servers already happy to spend tens to hundreds of megabytes on IOMMU pagetables? > The API is split up into parts: > - Allocate IOVA space: > To do any pre-allocation required. This is done based on the caller > supplying some details about how much IOMMU address space it would need > in worst case. > - Map and unmap relevant structures to pre-allocated IOVA space: > Perform the actual mapping into the pre-allocated IOVA. This is very > similar to dma_map_page(). > > In this and the next series [1], examples of three different users are converted > to the new API to show the benefits and its versatility. Each user has a unique > flow: > 1. RDMA ODP is an example of "SVA mirroring" using HMM that needs to > dynamically map/unmap large numbers of single pages. This becomes > significantly faster in the IOMMU case as the map/unmap is now just > a page table walk, the IOVA allocation is pre-computed once. Significant > amounts of memory are saved as there is no longer a need to store the > dma_addr_t of each page. I particularly enjoy the comment in patch #11 calling out how this "unique flow" is fundamentally incompatible with the API it's supposed to show off and has to rely on a sketchy hack to abuse its "versatility". Great stuff. > 2. VFIO PCI live migration code is building a very large "page list" > for the device. Instead of allocating a scatter list entry per allocated > page it can just allocate an array of 'struct page *', saving a large > amount of memory. VFIO already assumes a coherent device with (realistically) an IOMMU which it explicitly manages - why is it even pretending to need a generic DMA API? > 3. NVMe PCI demonstrates how a BIO can be converted to a HW scatter > list without having to allocate then populate an intermediate SG table. As above, given that a bio_vec still deals in struct pages, that could seemingly already be done by just mapping the pages, so how is it proving any benefit of a fragile new interface? Heck, not that I really want to encourage it, but we also already have network drivers who don't have the space to stash both a DMA address *and* a page address in their descriptors, and economise on shadow storage by instead grovelling into the default IOMMU domain with iova_to_phys(). I mean, I'd _kinda_ like to send them to DMA jail, but it's not an absolutely unreasonable trick to play... also DMA jail doesn't exist. > To make the use of the new API easier, HMM and block subsystems are extended > to hide the optimization details from the caller. Among these optimizations: > * Memory reduction as in most real use cases there is no need to store mapped > DMA addresses and unmap them. > * Reducing the function call overhead by removing the need to call function > pointers and use direct calls instead. > > This step is first along a path to provide alternatives to scatterlist and > solve some of the abuses and design mistakes, for instance in DMABUF's P2P > support. My big concern here is that a thin and vaguely-defined wrapper around the IOMMU API is itself a step which smells strongly of "abuse and design mistake", given that the basic notion of allocating DMA addresses in advance clearly cannot generalise. Thus it really demands some considered justification beyond "We must do something; This is something; Therefore we must do this." to be convincing. Thanks, Robin. > > Thanks > > [1] This still points to v0, as the change is just around handling dma_iova_sync(): > https://lore.kernel.org/all/cover.1730037261.git.leon@kernel.org > > Christoph Hellwig (6): > PCI/P2PDMA: Refactor the p2pdma mapping helpers > dma-mapping: move the PCI P2PDMA mapping helpers to pci-p2pdma.h > iommu: generalize the batched sync after map interface > iommu/dma: Factor out a iommu_dma_map_swiotlb helper > dma-mapping: add a dma_need_unmap helper > docs: core-api: document the IOVA-based API > > Leon Romanovsky (11): > dma-mapping: Add check if IOVA can be used > dma: Provide an interface to allow allocate IOVA > dma-mapping: Implement link/unlink ranges API > mm/hmm: let users to tag specific PFN with DMA mapped bit > mm/hmm: provide generic DMA managing logic > RDMA/umem: Store ODP access mask information in PFN > RDMA/core: Convert UMEM ODP DMA mapping to caching IOVA and page > linkage > RDMA/umem: Separate implicit ODP initialization from explicit ODP > vfio/mlx5: Explicitly use number of pages instead of allocated length > vfio/mlx5: Rewrite create mkey flow to allow better code reuse > vfio/mlx5: Convert vfio to use DMA link API > > Documentation/core-api/dma-api.rst | 70 ++++ > drivers/infiniband/core/umem_odp.c | 250 +++++---------- > drivers/infiniband/hw/mlx5/mlx5_ib.h | 12 +- > drivers/infiniband/hw/mlx5/odp.c | 65 ++-- > drivers/infiniband/hw/mlx5/umr.c | 12 +- > drivers/iommu/dma-iommu.c | 459 +++++++++++++++++++++++---- > drivers/iommu/iommu.c | 65 ++-- > drivers/pci/p2pdma.c | 38 +-- > drivers/vfio/pci/mlx5/cmd.c | 373 +++++++++++----------- > drivers/vfio/pci/mlx5/cmd.h | 35 +- > drivers/vfio/pci/mlx5/main.c | 87 +++-- > include/linux/dma-map-ops.h | 54 ---- > include/linux/dma-mapping.h | 85 +++++ > include/linux/hmm-dma.h | 32 ++ > include/linux/hmm.h | 16 + > include/linux/iommu.h | 4 + > include/linux/pci-p2pdma.h | 84 +++++ > include/rdma/ib_umem_odp.h | 25 +- > kernel/dma/direct.c | 44 +-- > kernel/dma/mapping.c | 20 ++ > mm/hmm.c | 231 +++++++++++++- > 21 files changed, 1377 insertions(+), 684 deletions(-) > create mode 100644 include/linux/hmm-dma.h >
Changelog: v1: * Squashed two VFIO patches into one * Added Acked-by/Reviewed-by tags * Fix docs spelling errors * Simplified dma_iova_sync() API * Added extra check in dma_iova_destroy() if mapped size to make code more clear * Fixed checkpatch warnings in p2p patch * Changed implementation of VFIO mlx5 mlx5vf_add_migration_pages() to be more general * Reduced the number of changes in VFIO patch v0: https://lore.kernel.org/all/cover.1730037276.git.leon@kernel.org ---------------------------------------------------------------------------- The code can be downloaded from: https://git.kernel.org/pub/scm/linux/kernel/git/leon/linux-rdma.git tag:dma-split-oct-30 ---------------------------------------------------------------------------- Currently the only efficient way to map a complex memory description through the DMA API is by using the scatterlist APIs. The SG APIs are unique in that they efficiently combine the two fundamental operations of sizing and allocating a large IOVA window from the IOMMU and processing all the per-address swiotlb/flushing/p2p/map details. This uniqueness has been a long standing pain point as the scatterlist API is mandatory, but expensive to use. It prevents any kind of optimization or feature improvement (such as avoiding struct page for P2P) due to the impossibility of improving the scatterlist. Several approaches have been explored to expand the DMA API with additional scatterlist-like structures (BIO, rlist), instead split up the DMA API to allow callers to bring their own data structure. The API is split up into parts: - Allocate IOVA space: To do any pre-allocation required. This is done based on the caller supplying some details about how much IOMMU address space it would need in worst case. - Map and unmap relevant structures to pre-allocated IOVA space: Perform the actual mapping into the pre-allocated IOVA. This is very similar to dma_map_page(). In this and the next series [1], examples of three different users are converted to the new API to show the benefits and its versatility. Each user has a unique flow: 1. RDMA ODP is an example of "SVA mirroring" using HMM that needs to dynamically map/unmap large numbers of single pages. This becomes significantly faster in the IOMMU case as the map/unmap is now just a page table walk, the IOVA allocation is pre-computed once. Significant amounts of memory are saved as there is no longer a need to store the dma_addr_t of each page. 2. VFIO PCI live migration code is building a very large "page list" for the device. Instead of allocating a scatter list entry per allocated page it can just allocate an array of 'struct page *', saving a large amount of memory. 3. NVMe PCI demonstrates how a BIO can be converted to a HW scatter list without having to allocate then populate an intermediate SG table. To make the use of the new API easier, HMM and block subsystems are extended to hide the optimization details from the caller. Among these optimizations: * Memory reduction as in most real use cases there is no need to store mapped DMA addresses and unmap them. * Reducing the function call overhead by removing the need to call function pointers and use direct calls instead. This step is first along a path to provide alternatives to scatterlist and solve some of the abuses and design mistakes, for instance in DMABUF's P2P support. Thanks [1] This still points to v0, as the change is just around handling dma_iova_sync(): https://lore.kernel.org/all/cover.1730037261.git.leon@kernel.org Christoph Hellwig (6): PCI/P2PDMA: Refactor the p2pdma mapping helpers dma-mapping: move the PCI P2PDMA mapping helpers to pci-p2pdma.h iommu: generalize the batched sync after map interface iommu/dma: Factor out a iommu_dma_map_swiotlb helper dma-mapping: add a dma_need_unmap helper docs: core-api: document the IOVA-based API Leon Romanovsky (11): dma-mapping: Add check if IOVA can be used dma: Provide an interface to allow allocate IOVA dma-mapping: Implement link/unlink ranges API mm/hmm: let users to tag specific PFN with DMA mapped bit mm/hmm: provide generic DMA managing logic RDMA/umem: Store ODP access mask information in PFN RDMA/core: Convert UMEM ODP DMA mapping to caching IOVA and page linkage RDMA/umem: Separate implicit ODP initialization from explicit ODP vfio/mlx5: Explicitly use number of pages instead of allocated length vfio/mlx5: Rewrite create mkey flow to allow better code reuse vfio/mlx5: Convert vfio to use DMA link API Documentation/core-api/dma-api.rst | 70 ++++ drivers/infiniband/core/umem_odp.c | 250 +++++---------- drivers/infiniband/hw/mlx5/mlx5_ib.h | 12 +- drivers/infiniband/hw/mlx5/odp.c | 65 ++-- drivers/infiniband/hw/mlx5/umr.c | 12 +- drivers/iommu/dma-iommu.c | 459 +++++++++++++++++++++++---- drivers/iommu/iommu.c | 65 ++-- drivers/pci/p2pdma.c | 38 +-- drivers/vfio/pci/mlx5/cmd.c | 373 +++++++++++----------- drivers/vfio/pci/mlx5/cmd.h | 35 +- drivers/vfio/pci/mlx5/main.c | 87 +++-- include/linux/dma-map-ops.h | 54 ---- include/linux/dma-mapping.h | 85 +++++ include/linux/hmm-dma.h | 32 ++ include/linux/hmm.h | 16 + include/linux/iommu.h | 4 + include/linux/pci-p2pdma.h | 84 +++++ include/rdma/ib_umem_odp.h | 25 +- kernel/dma/direct.c | 44 +-- kernel/dma/mapping.c | 20 ++ mm/hmm.c | 231 +++++++++++++- 21 files changed, 1377 insertions(+), 684 deletions(-) create mode 100644 include/linux/hmm-dma.h