On Wed, Jun 02, 2021 at 06:50:37PM +1000, Balbir Singh wrote: > On Wed, May 26, 2021 at 12:17:18AM -0700, John Hubbard wrote: > > On 5/25/21 4:51 AM, Balbir Singh wrote: > > ... > > > > How beneficial is this code to nouveau users? I see that it permits a > > > > part of OpenCL to be implemented, but how useful/important is this in > > > > the real world? > > > > > > That is a very good question! I've not reviewed the code, but a sample > > > program with the described use case would make things easy to parse. > > > I suspect that is not easy to build at the moment? > > > > > > > The cover letter says this: > > > > This has been tested with upstream Mesa 21.1.0 and a simple OpenCL program > > which checks that GPU atomic accesses to system memory are atomic. Without > > this series the test fails as there is no way of write-protecting the page > > mapping which results in the device clobbering CPU writes. For reference > > the test is available at https://ozlabs.org/~apopple/opencl_svm_atomics/ > > > > Further testing has been performed by adding support for testing exclusive > > access to the hmm-tests kselftests. > > > > ...so that seems to cover the "sample program" request, at least. > > Thanks, I'll take a look > > > > > > I wonder how we co-ordinate all the work the mm is doing, page migration, > > > reclaim with device exclusive access? Do we have any numbers for the worst > > > case page fault latency when something is marked away for exclusive access? > > > > CPU page fault latency is approximately "terrible", if a page is resident on > > the GPU. We have to spin up a DMA engine on the GPU and have it copy the page > > over the PCIe bus, after all. > > > > > I presume for now this is anonymous memory only? SWP_DEVICE_EXCLUSIVE would > > > > Yes, for now. > > > > > only impact the address space of programs using the GPU. Should the exclusively > > > marked range live in the unreclaimable list and recycled back to active/in-active > > > to account for the fact that > > > > > > 1. It is not reclaimable and reclaim will only hurt via page faults? > > > 2. It ages the page correctly or at-least allows for that possibility when the > > > page is used by the GPU. > > > > I'm not sure that that is *necessarily* something we can conclude. It depends upon > > access patterns of each program. For example, a "reduction" parallel program sends > > over lots of data to the GPU, and only a tiny bit of (reduced!) data comes back > > to the CPU. In that case, freeing the physical page on the CPU is actually the > > best decision for the OS to make (if the OS is sufficiently prescient). > > > > With a shared device or a device exclusive range, it would be good to get the device > usage pattern and update the mm with that knowledge, so that the LRU can be better > maintained. With your comment you seem to suggest that a page used by the GPU might > be a good candidate for reclaim based on the CPU's understanding of the age of > the page should not account for use by the device > (are GPU workloads - access once and discard?) Hmm, besides the aging info, this reminded me: do we need to isolate the page from lru too when marking device exclusive access? Afaict the current patch didn't do that so I think it's reclaimable. If we still have the rmap then we'll get a mmu notify CLEAR when unmapping that special pte, so device driver should be able to drop the ownership. However we dropped the rmap when marking exclusive. Now I don't know whether and how it'll work if page reclaim runs with the page being exclusively owned if without isolating the page.. -- Peter Xu
