Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()

[Nick Piggin <npiggin@suse.de>]

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...