linux-kernel - Re: [PATCH v2 4/6] x86: Add clear_page

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Date:	Mon, 13 Aug 2012 13:02:57 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	"Andy Lutomirski" <luto@...capital.net>,
	"Robert Richter" <robert.richter@....com>,
	"Johannes Weiner" <hannes@...xchg.org>,
	"Hugh Dickins" <hughd@...gle.com>, "Alex Shi" <alex.shu@...el.com>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	<x86@...nel.org>, <linux-mm@...ck.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<linux-mips@...ux-mips.org>, "Andi Kleen" <ak@...ux.intel.com>,
	"Tim Chen" <tim.c.chen@...ux.intel.com>,
	<linuxppc-dev@...ts.ozlabs.org>,
	"Andrea Arcangeli" <aarcange@...hat.com>,
	"Ingo Molnar" <mingo@...hat.com>, "Mel Gorman" <mgorman@...e.de>,
	<linux-kernel@...r.kernel.org>, <linux-sh@...r.kernel.org>,
	<sparclinux@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 4/6] x86: Add clear_page_nocache

>>> On 13.08.12 at 13:43, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> On Thu, Aug 09, 2012 at 04:22:04PM +0100, Jan Beulich wrote:
>> >>> On 09.08.12 at 17:03, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>  wrote:
> 
> ...
> 
>> > ---
>> >  arch/x86/include/asm/page.h          |    2 ++
>> >  arch/x86/include/asm/string_32.h     |    5 +++++
>> >  arch/x86/include/asm/string_64.h     |    5 +++++
>> >  arch/x86/lib/Makefile                |    1 +
>> >  arch/x86/lib/clear_page_nocache_32.S |   30 ++++++++++++++++++++++++++++++
>> >  arch/x86/lib/clear_page_nocache_64.S |   29 +++++++++++++++++++++++++++++
>> 
>> Couldn't this more reasonably go into clear_page_{32,64}.S?
> 
> We don't have clear_page_32.S.

Sure, but you're introducing a file anyway. Fold the new code into
the existing file for 64-bit, and create a new, similarly named one
for 32-bit.

>> >+	xorl   %eax,%eax
>> >+	movl   $4096/64,%ecx
>> >+	.p2align 4
>> >+.Lloop:
>> >+	decl	%ecx
>> >+#define PUT(x) movnti %eax,x*8(%edi) ; movnti %eax,x*8+4(%edi)
>> 
>> Is doing twice as much unrolling as on 64-bit really worth it?
> 
> Moving 64 bytes per cycle is faster on Sandy Bridge, but slower on
> Westmere. Any preference? ;)

If it's not a clear win, I'd favor the 8-stores-per-cycle variant,
matching x86-64.

Jan

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