On Tuesday, 25 May 2021 11:31:17 AM AEST John Hubbard wrote:
On 5/24/21 3:11 PM, Andrew Morton wrote:
...
Documentation/vm/hmm.rst | 17 ++++ include/linux/mmu_notifier.h | 6 ++ include/linux/rmap.h | 4 + include/linux/swap.h | 7 +- include/linux/swapops.h | 44 ++++++++- mm/hmm.c | 5 + mm/memory.c | 128 +++++++++++++++++++++++- mm/mprotect.c | 8 ++ mm/page_vma_mapped.c | 9 +- mm/rmap.c | 186 +++++++++++++++++++++++++++++++++++ 10 files changed, 405 insertions(+), 9 deletions(-)
This is quite a lot of code added to core MM for a single driver.
Is there any expectation that other drivers will use this code?
Yes! This should work for GPUs (and potentially, other devices) that support OpenCL SVM atomic accesses on the device. I haven't looked into how amdgpu works in any detail, but that's certainly at the top of the list of likely additional callers.
Is there a way of reducing the impact (code size, at least) for systems which don't need this code?
All of the code added to mm/rmap.c is specific to implementing this feature and not depended on by other core MM code so could be put behind something like CONFIG_DEVICE_PRIVATE to reduce the code size impact (I realise now it currently isn't but should be).
The impact on compiled code size in mm/memory.c also ends up being minimised by the compiler because all of it is of the form:
if (is_device_exclusive_entry(...)) { [...] }
Meaning it should get thrown away when the feature is not configured given is_device_exclusive_entry() is a static inline always returning false in that case.
I'll leave this question to others for the moment, in order to answer the "do we need it at all" points.
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?
So this is interesting. Right now, OpenCL support in Nouveau is rather new and so probably not a huge impact yet. However, we've built up enough experience with CUDA and OpenCL to learn that atomic operations, as part of the user space programming model, are a super big deal. Atomic operations are so useful and important that I'd expect many OpenCL SVM users to be uninterested in programming models that lack atomic operations for GPU compute programs.
Again, this doesn't rule out future, non-GPU accelerator devices that may come along.
Atomic ops are just a really important piece of high-end multi-threaded programming, it turns out. So this is the beginning of support for an important building block for general purpose programming on devices that have GPU-like memory models.
thanks,