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From: Ingo Molnar <mi...@kernel.org>
Newsgroups: linux.kernel
Subject: Re: [GIT PULL 0/8] Annotation weekly ponies delivery
Date: Mon, 14 May 2012 14:00:02 +0200
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* Peter Zijlstra <pet...@infradead.org> wrote:

> On Sat, 2012-05-12 at 13:40 -0700, Linus Torvalds wrote:
> > The whole "-e cycles:pp" doesn't work any more. I don't get any nice
> > PEBS information, I get the totally useless irq-based profiling.
> > 
> > The difference for a "make -j" profile is quite stunning:
> > 
> > Doing "perf record -f -e cycles:pp -F 20000 make -j"
> > 
> >  - my current git:
> > 
> >     [ perf record: Woken up 47 times to write data ]
> >     [ perf record: Captured and wrote 11.890 MB perf.data (~519462 samples) ]
> > 
> >  - with the above tree pulled into my current git tree (but compiling
> > the same old tree):
> > 
> >     [ perf record: Woken up 1 times to write data ]
> >     [ perf record: Captured and wrote 0.031 MB perf.data (~1375 samples) ] 
> 
> The output simply suggests we're not getting enough samples 
> not that PEBS isn't working, in fact I can could reproduce 
> without using PEBS.
> 
> This bisected to the below commit, the code has since been 
> changed again and all that evlist stuff gives me a head-ache. 
> Acme, Namhyung ?
> 
> 55261f46702cec96911a81aacfb3cba13434d304 is the first bad commit
> commit 55261f46702cec96911a81aacfb3cba13434d304
> Author: Namhyung Kim <namhyung....@lge.com>
> Date:   Mon May 7 14:08:59 2012 +0900
> 
>     perf evlist: Fix creation of cpu map

Another detail seems to be that the bug takes per-task-inherited 
profiling. Doing:

   perf record -a -e cycles:pp make -j64 bzImage

produces the expected number of events. Without the '-a' the bug 
Linus found triggers.

Similarly, 'perf top' does not trigger the bug - because it's 
using per CPU, not per task (inherited) profiling.

Thanks,

	Ingo
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