linux-kernel - Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page

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Message-ID: <20090927192025.GA6327@wotan.suse.de>
Date:	Sun, 27 Sep 2009 21:20:25 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()

On Sun, Sep 27, 2009 at 06:47:39PM +0800, Wu Fengguang wrote:
> > 
> > And standard deviation is 0.04%, much larger than the difference 0.008% ..
> 
> Sorry that's not correct. I improved the accounting by treating
> function0+function1 from two CPUs as an integral entity:
> 
>                  total time      add_to_page_cache_lru   percent  stddev
>          before  3880166848.722  9683329.610             0.250%   0.014%
>          after   3828516894.376  9778088.870             0.256%   0.012%
>          delta                                           0.006%

I don't understand why you're doing this NFS workload to measure?
I see significant nfs, networking protocol and device overheads in
your profiles, also you're hitting some locks or something which
is causing massive context switching. So I don't think this is a
good test. But anyway as Hugh points out, you need to compare with
a *completely* fixed kernel, which includes auditing all users of
page flags non-atomically (slab, notably, but possibly also other
places).

One other thing to keep in mind that I will mention is that I am
going to push in a patch to the page allocator to allow callers
to avoid the refcounting (atomic_dec_and_test) in page lifetime,
which is especially important for SLUB and takes more cycles off
the page allocator...

I don't know exactly what you're going to do after that to get a
stable reference to slab pages. I guess you can read the page
flags and speculatively take some slab locks and recheck etc...

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