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

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Date:   Fri, 22 Jan 2021 12:13:35 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Will Deacon <will@...nel.org>,
        Prathu Baronia <prathubaronia2011@...il.com>,
        Prathu Baronia <prathu.baronia@...plus.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-kernel@...r.kernel.org, chintan.pandya@...plus.com,
        "glider@...gle.com" <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

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