Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

[Robin Murphy]

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.