Re: [PATCH v9 07/10] mm: Device exclusive memory access

[Peter Xu]

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