linux-kernel - Re: regression caused by cgroups optimization in 3.17-rc2

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Date:	Tue, 2 Sep 2014 21:33:17 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Hansen <dave@...1.net>, Michal Hocko <mhocko@...e.com>,
	Hugh Dickins <hughd@...gle.com>, Tejun Heo <tj@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: regression caused by cgroups optimization in 3.17-rc2

On Tue, Sep 02, 2014 at 05:20:55PM -0700, Linus Torvalds wrote:
> On Tue, Sep 2, 2014 at 5:10 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > That looks like a partial profile, where did the page allocator, page
> > zeroing etc. go?  Because the distribution among these listed symbols
> > doesn't seem all that crazy:
> 
> Please argue this *after* the commit has been reverted. You guys can
> try to make the memcontrol batching actually work and scale later.
> It's not appropriate to argue against major regressions when reported
> and bisected by users.

I'll send a clean revert later.

> Showing the spinlock at the top of the profile is very much crazy
> (apparently taking 68% of all cpu time), when it's all useless
> make-believe work. I don't understand why you wouldn't call that
> crazy.

If you limit perf to a subset of symbols, it will show a relative
distribution between them, i.e: perf top --symbols kfree,memset during
some disk access:

   PerfTop:    1292 irqs/sec  kernel:84.4%  exact:  0.0% [4000Hz cycles],  (all, 4 CPUs)
-------------------------------------------------------------------------------

    56.23%  [kernel]      [k] kfree 
    41.86%  [kernel]      [k] memset
     1.91%  libc-2.19.so  [.] memset

kfree isn't eating 56% of "all cpu time" here, and it wasn't clear to
me whether Dave filtered symbols from only memcontrol.o, memory.o, and
mmap.o in a similar way.  I'm not arguing against the regression, I'm
just trying to make sense of the numbers from the *patched* kernel.
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