| Message ID | 20190321132140.114878-1-thellstrom@vmware.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | mm modifications / helpers for emulated GPU coherent memory | expand |
On Thu, Mar 21, 2019 at 01:22:22PM +0000, Thomas Hellstrom wrote: > Resending since last series was sent through a mis-configured SMTP server. > > Hi, > This is an early RFC to make sure I don't go too far in the wrong direction. > > Non-coherent GPUs that can't directly see contents in CPU-visible memory, > like VMWare's SVGA device, run into trouble when trying to implement > coherent memory requirements of modern graphics APIs. Examples are > Vulkan and OpenGL 4.4's ARB_buffer_storage. > > To remedy, we need to emulate coherent memory. Typically when it's detected > that a buffer object is about to be accessed by the GPU, we need to > gather the ranges that have been dirtied by the CPU since the last operation, > apply an operation to make the content visible to the GPU and clear the > the dirty tracking. > > Depending on the size of the buffer object and the access pattern there are > two major possibilities: > > 1) Use page_mkwrite() and pfn_mkwrite(). (GPU buffer objects are backed > either by PCI device memory or by driver-alloced pages). > The dirty-tracking needs to be reset by write-protecting the affected ptes > and flush tlb. This has a complexity of O(num_dirty_pages), but the > write page-fault is of course costly. > > 2) Use hardware dirty-flags in the ptes. The dirty-tracking needs to be reset > by clearing the dirty bits and flush tlb. This has a complexity of > O(num_buffer_object_pages) and dirty bits need to be scanned in full before > each gpu-access. > > So in practice the two methods need to be interleaved for best performance. > > So to facilitate this, I propose two new helpers, apply_as_wrprotect() and > apply_as_clean() ("as" stands for address-space) both inspired by > unmap_mapping_range(). Users of these helpers are in the making, but needs > some cleaning-up. To be clear this should _only be use_ for mmap of device file ? If so the API should try to enforce that as much as possible for instance by mandating the file as argument so that the function can check it is only use in that case. Also big scary comment to make sure no one just start using those outside this very limited frame. > > There's also a change to x_mkwrite() to allow dropping the mmap_sem while > waiting. This will most likely conflict with userfaultfd write protection. Maybe building your thing on top of that would be better. https://lwn.net/Articles/783571/ I will take a cursory look at the patches. Cheers, Jérôme
Hi, Jérôme, Thanks for commenting. I have a couple of questions / clarifications below. On Thu, 2019-03-21 at 09:46 -0400, Jerome Glisse wrote: > On Thu, Mar 21, 2019 at 01:22:22PM +0000, Thomas Hellstrom wrote: > > Resending since last series was sent through a mis-configured SMTP > > server. > > > > Hi, > > This is an early RFC to make sure I don't go too far in the wrong > > direction. > > > > Non-coherent GPUs that can't directly see contents in CPU-visible > > memory, > > like VMWare's SVGA device, run into trouble when trying to > > implement > > coherent memory requirements of modern graphics APIs. Examples are > > Vulkan and OpenGL 4.4's ARB_buffer_storage. > > > > To remedy, we need to emulate coherent memory. Typically when it's > > detected > > that a buffer object is about to be accessed by the GPU, we need to > > gather the ranges that have been dirtied by the CPU since the last > > operation, > > apply an operation to make the content visible to the GPU and clear > > the > > the dirty tracking. > > > > Depending on the size of the buffer object and the access pattern > > there are > > two major possibilities: > > > > 1) Use page_mkwrite() and pfn_mkwrite(). (GPU buffer objects are > > backed > > either by PCI device memory or by driver-alloced pages). > > The dirty-tracking needs to be reset by write-protecting the > > affected ptes > > and flush tlb. This has a complexity of O(num_dirty_pages), but the > > write page-fault is of course costly. > > > > 2) Use hardware dirty-flags in the ptes. The dirty-tracking needs > > to be reset > > by clearing the dirty bits and flush tlb. This has a complexity of > > O(num_buffer_object_pages) and dirty bits need to be scanned in > > full before > > each gpu-access. > > > > So in practice the two methods need to be interleaved for best > > performance. > > > > So to facilitate this, I propose two new helpers, > > apply_as_wrprotect() and > > apply_as_clean() ("as" stands for address-space) both inspired by > > unmap_mapping_range(). Users of these helpers are in the making, > > but needs > > some cleaning-up. > > To be clear this should _only be use_ for mmap of device file ? If so > the API should try to enforce that as much as possible for instance > by > mandating the file as argument so that the function can check it is > only use in that case. Also big scary comment to make sure no one > just > start using those outside this very limited frame. Fine with me. Perhaps we could BUG() / WARN() on certain VMA flags instead of mandating the file as argument. That can make sure we don't accidently hit pages we shouldn't hit. > > > There's also a change to x_mkwrite() to allow dropping the mmap_sem > > while > > waiting. > > This will most likely conflict with userfaultfd write protection. Are you referring to the x_mkwrite() usage itself or the mmap_sem dropping facilitation? > Maybe > building your thing on top of that would be better. > > ... > > I will take a cursory look at the patches. > Some more questions / clarifications on those as well. > Cheers, > Jérôme
On Thu, Mar 21, 2019 at 07:51:16PM +0000, Thomas Hellstrom wrote: > Hi, Jérôme, > > Thanks for commenting. I have a couple of questions / clarifications > below. > > On Thu, 2019-03-21 at 09:46 -0400, Jerome Glisse wrote: > > On Thu, Mar 21, 2019 at 01:22:22PM +0000, Thomas Hellstrom wrote: > > > Resending since last series was sent through a mis-configured SMTP > > > server. > > > > > > Hi, > > > This is an early RFC to make sure I don't go too far in the wrong > > > direction. > > > > > > Non-coherent GPUs that can't directly see contents in CPU-visible > > > memory, > > > like VMWare's SVGA device, run into trouble when trying to > > > implement > > > coherent memory requirements of modern graphics APIs. Examples are > > > Vulkan and OpenGL 4.4's ARB_buffer_storage. > > > > > > To remedy, we need to emulate coherent memory. Typically when it's > > > detected > > > that a buffer object is about to be accessed by the GPU, we need to > > > gather the ranges that have been dirtied by the CPU since the last > > > operation, > > > apply an operation to make the content visible to the GPU and clear > > > the > > > the dirty tracking. > > > > > > Depending on the size of the buffer object and the access pattern > > > there are > > > two major possibilities: > > > > > > 1) Use page_mkwrite() and pfn_mkwrite(). (GPU buffer objects are > > > backed > > > either by PCI device memory or by driver-alloced pages). > > > The dirty-tracking needs to be reset by write-protecting the > > > affected ptes > > > and flush tlb. This has a complexity of O(num_dirty_pages), but the > > > write page-fault is of course costly. > > > > > > 2) Use hardware dirty-flags in the ptes. The dirty-tracking needs > > > to be reset > > > by clearing the dirty bits and flush tlb. This has a complexity of > > > O(num_buffer_object_pages) and dirty bits need to be scanned in > > > full before > > > each gpu-access. > > > > > > So in practice the two methods need to be interleaved for best > > > performance. > > > > > > So to facilitate this, I propose two new helpers, > > > apply_as_wrprotect() and > > > apply_as_clean() ("as" stands for address-space) both inspired by > > > unmap_mapping_range(). Users of these helpers are in the making, > > > but needs > > > some cleaning-up. > > > > To be clear this should _only be use_ for mmap of device file ? If so > > the API should try to enforce that as much as possible for instance > > by > > mandating the file as argument so that the function can check it is > > only use in that case. Also big scary comment to make sure no one > > just > > start using those outside this very limited frame. > > Fine with me. Perhaps we could BUG() / WARN() on certain VMA flags > instead of mandating the file as argument. That can make sure we > don't accidently hit pages we shouldn't hit. You already provide the mapping as argument it should not be hard to check it is a mapping to a device file as the vma flags will not be enough to identify this case. > > > > > > There's also a change to x_mkwrite() to allow dropping the mmap_sem > > > while > > > waiting. > > > > This will most likely conflict with userfaultfd write protection. > > Are you referring to the x_mkwrite() usage itself or the mmap_sem > dropping facilitation? Both i believe, however i have not try to apply your patches on top of the userfaultfd patchset Cheers, Jérôme
Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Will Deacon <will.deacon@arm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Rik van Riel <riel@surriel.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Huang Ying <ying.huang@intel.com> Cc: Souptick Joarder <jrdr.linux@gmail.com> Cc: "Jérôme Glisse" <jglisse@redhat.com> Cc: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org Resending since last series was sent through a mis-configured SMTP server. Hi, This is an early RFC to make sure I don't go too far in the wrong direction. Non-coherent GPUs that can't directly see contents in CPU-visible memory, like VMWare's SVGA device, run into trouble when trying to implement coherent memory requirements of modern graphics APIs. Examples are Vulkan and OpenGL 4.4's ARB_buffer_storage. To remedy, we need to emulate coherent memory. Typically when it's detected that a buffer object is about to be accessed by the GPU, we need to gather the ranges that have been dirtied by the CPU since the last operation, apply an operation to make the content visible to the GPU and clear the the dirty tracking. Depending on the size of the buffer object and the access pattern there are two major possibilities: 1) Use page_mkwrite() and pfn_mkwrite(). (GPU buffer objects are backed either by PCI device memory or by driver-alloced pages). The dirty-tracking needs to be reset by write-protecting the affected ptes and flush tlb. This has a complexity of O(num_dirty_pages), but the write page-fault is of course costly. 2) Use hardware dirty-flags in the ptes. The dirty-tracking needs to be reset by clearing the dirty bits and flush tlb. This has a complexity of O(num_buffer_object_pages) and dirty bits need to be scanned in full before each gpu-access. So in practice the two methods need to be interleaved for best performance. So to facilitate this, I propose two new helpers, apply_as_wrprotect() and apply_as_clean() ("as" stands for address-space) both inspired by unmap_mapping_range(). Users of these helpers are in the making, but needs some cleaning-up. There's also a change to x_mkwrite() to allow dropping the mmap_sem while waiting. Any comments or suggestions appreciated. Thanks, Thomas