linux-kernel - Re: [PATCH] mm, memcg: Optionally disable memcg by default using Kconfig

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Date:	Tue, 19 May 2015 17:04:04 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH] mm, memcg: Optionally disable memcg by default using
 Kconfig

On Tue, May 19, 2015 at 04:41:19PM +0100, Mel Gorman wrote:
> On Tue, May 19, 2015 at 05:27:10PM +0200, Michal Hocko wrote:
> > On Tue 19-05-15 16:13:02, Mel Gorman wrote:
> > [...]
> > >                :ffffffff811c160f:       je     ffffffff811c1630 <mem_cgroup_try_charge+0x40>
> > >                :ffffffff811c1611:       xor    %eax,%eax
> > >                :ffffffff811c1613:       xor    %ebx,%ebx
> > >      1 1.7e-05 :ffffffff811c1615:       mov    %rbx,(%r12)
> > >      7 1.2e-04 :ffffffff811c1619:       add    $0x10,%rsp
> > >   1211  0.0203 :ffffffff811c161d:       pop    %rbx
> > >      5 8.4e-05 :ffffffff811c161e:       pop    %r12
> > >      5 8.4e-05 :ffffffff811c1620:       pop    %r13
> > >   1249  0.0210 :ffffffff811c1622:       pop    %r14
> > >      7 1.2e-04 :ffffffff811c1624:       pop    %rbp
> > >      5 8.4e-05 :ffffffff811c1625:       retq   
> > >                :ffffffff811c1626:       nopw   %cs:0x0(%rax,%rax,1)
> > >    295  0.0050 :ffffffff811c1630:       mov    (%rdi),%rax
> > > 160703  2.6973 :ffffffff811c1633:       mov    %edx,%r13d
> > 
> > Huh, what? Even if this was off by one and the preceding instruction has
> > consumed the time. This would be reading from page->flags but the page
> > should be hot by the time we got here, no?
> > 
> 
> I would have expected so but it's not the first time I've seen cases where
> examining the flags was a costly instruction. I suspect it's due to an
> ordering issue or more likely, a frequent branch mispredict that is being
> accounted for against this instruction.
> 

Which is plausible as forward branches are statically predicted false but
in this particular load that could be a close to a 100% mispredict.

-- 
Mel Gorman
SUSE Labs
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