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Re: [PATCH v2 4/6] x86: Add clear_page_nocache

To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v2 4/6] x86: Add clear_page_nocache
From: "Jan Beulich" <JBeulich@suse.com>
Date: Mon, 13 Aug 2012 13:02:57 +0100
Cc: "Andy Lutomirski" <luto@amacapital.net>, "Robert Richter" <robert.richter@amd.com>, "Johannes Weiner" <hannes@cmpxchg.org>, "Hugh Dickins" <hughd@google.com>, "Alex Shi" <alex.shu@intel.com>, "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>, <x86@kernel.org>, <linux-mm@kvack.org>, "Thomas Gleixner" <tglx@linutronix.de>, "Andrew Morton" <akpm@linux-foundation.org>, <linux-mips@linux-mips.org>, "Andi Kleen" <ak@linux.intel.com>, "Tim Chen" <tim.c.chen@linux.intel.com>, <linuxppc-dev@lists.ozlabs.org>, "Andrea Arcangeli" <aarcange@redhat.com>, "Ingo Molnar" <mingo@redhat.com>, "Mel Gorman" <mgorman@suse.de>, <linux-kernel@vger.kernel.org>, <linux-sh@vger.kernel.org>, <sparclinux@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>
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References: <1344524583-1096-1-git-send-email-kirill.shutemov@linux.intel.com> <1344524583-1096-5-git-send-email-kirill.shutemov@linux.intel.com> <5023F1BC0200007800093EF0@nat28.tlf.novell.com> <20120813114334.GA21855@otc-wbsnb-06>
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>>> On 13.08.12 at 13:43, "Kirill A. Shutemov" 
>>> <kirill.shutemov@linux.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@linux.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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