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

From: Catalin Marinas <catalin.marinas@arm.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Prathu Baronia <prathu.baronia@oneplus.com>,
	linux-kernel@vger.kernel.org,
	Prathu Baronia <prathubaronia2011@gmail.com>,
	chintan.pandya@oneplus.com,
	"glider@google.com" <glider@google.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64
Date: Fri, 22 Jan 2021 12:13:35 +0000	[thread overview]
Message-ID: <20210122121334.GC8567@gaia> (raw)
In-Reply-To: <de782758-a7bc-d5a5-832e-c09ce8fe7c00@arm.com>

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

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