linux-kernel - Re: Should we be using unlikely() around tests of GFP

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Message-ID: <AANLkTinz52Ky5BhU-gHq8vx9=1uoN+iuDn1f0C8fnSjQ@mail.gmail.com>
Date:	Mon, 3 Jan 2011 09:40:57 +0200
From:	Pekka Enberg <penberg@...nel.org>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	David Rientjes <rientjes@...gle.com>, npiggin@...nel.dk
Subject: Re: Should we be using unlikely() around tests of GFP_ZERO?

Hi,

On Mon, Jan 3, 2011 at 8:48 AM, Theodore Ts'o <tytso@....edu> wrote:
>> Given the patches being busily submitted by trivial patch submitters to
>> make use kmem_cache_zalloc(), et. al, I believe we should remove the
>> unlikely() tests around the (gfp_flags & __GFP_ZERO) tests, such as:
>>
>> -       if (unlikely((flags & __GFP_ZERO) && objp))
>> +       if ((flags & __GFP_ZERO) && objp)
>>                memset(objp, 0, obj_size(cachep));
>>
>> Agreed?  If so, I'll send a patch...

On Mon, Jan 3, 2011 at 5:46 AM, Minchan Kim <minchan.kim@...il.com> wrote:
> I support it.

I guess the rationale here is that if you're going to take the hit of
memset() you can take the hit of unlikely() as well. We're optimizing
for hot call-sites that allocate a small amount of memory and
initialize everything themselves. That said, I don't think the
unlikely() annotation matters much either way and am for removing it
unless people object to that.

On Mon, Jan 3, 2011 at 5:46 AM, Minchan Kim <minchan.kim@...il.com> wrote:
> Recently Steven tried to gather the information.
> http://thread.gmane.org/gmane.linux.kernel/1072767
> Maybe he might have a number for that.

That would be interesting, sure.
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