Messages in this thread | | | Subject | Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64 | From | Robin Murphy <> | Date | Fri, 22 Jan 2021 12:45:55 +0000 |
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On 2021-01-22 12:13, Catalin Marinas wrote: > On Thu, Jan 21, 2021 at 06:59:37PM +0000, Robin Murphy wrote: >> On 2021-01-21 17:46, Will Deacon wrote: >>> On Thu, Jan 21, 2021 at 10:21:50PM +0530, Prathu Baronia wrote: >>>> This patch removes the unnecessary kmap calls in the hugepage zeroing path and >>>> improves the timing by 62%. >>>> >>>> I had proposed a similar change in Apr-May'20 timeframe in memory.c where I >>>> proposed to clear out a hugepage by directly calling a memset over the whole >>>> hugepage but got the opposition that the change was not architecturally neutral. >>>> >>>> Upon revisiting this now I see significant improvement by removing around 2k >>>> barrier calls from the zeroing path. So hereby I propose an arm64 specific >>>> definition of clear_user_highpage(). >>> >>> Given that barrier() is purely a thing for the compiler, wouldn't the same >>> change yield a benefit on any other architecture without HIGHMEM? In which >>> case, I think this sort of change belongs in the core code if it's actually >>> worthwhile. >> >> I would have thought it's more the constant manipulation of the preempt and >> pagefault counts, rather than the compiler barriers between them, that has >> the impact. Either way, if arm64 doesn't need to be atomic WRT preemption >> when clearing parts of hugepages then I also can't imagine that anyone else >> (at least for !HIGHMEM) would either. > > I thought the kmap_local stuff was supposed to fix this unnecessary > preemption disabling on 64-bit architectures: > > https://lwn.net/Articles/836144/ > > I guess it's not there yet.
No, it's there alright - when I pulled up the code to double-check my memory of this area, I did notice the kerneldoc and start wondering if this should simply be using kmap_local_page() for everyone.
Robin.
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