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. -- Catalin