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Date: Tue, 27 Mar 2012 11:09:48 -0700
From: Arun Sharma <asharma@fb.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>
CC: <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
        "Arnaldo Carvalho de Melo" <acme@redhat.com>,
        Mike Galbraith <efault@gmx.de>, "Paul Mackerras" <paulus@samba.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Stephane Eranian" <eranian@google.com>,
        Namhyung Kim <namhyung.kim@lge.com>,
        "Tom Zanussi" <tzanussi@gmail.com>, <linux-perf-users@vger.kernel.org>
Subject: Re: [PATCH] perf: Add a new sort order: SORT_INCLUSIVE (v4)
References: <1331746607-6706-1-git-send-email-asharma@fb.com> <20120315141402.GA550@somewhere.redhat.com> <4F622DCE.4090608@fb.com> <20120319155742.GF2660@somewhere> <4F691289.7080505@fb.com> <20120325021451.GA14971@somewhere.redhat.com>
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On 3/24/12 7:14 PM, Frederic Weisbecker wrote:

>> The other problem in branch stacks/LBR is that they're
>> sampled branches. Just because I got a sample with:
>>
>> a ->  b
>> b ->  c
>>
>> doesn't necessarily mean that the callchain was a ->  b ->  c.
>
> Not sure what you mean. If you have a ->  b, b ->  c in single
> LBR sample it means you got a ->  b ->  c.
>

I was going by Stephane's commit message here:

http://article.gmane.org/gmane.linux.kernel/1236999

 > Statistical sampling of taken branch should not be confused
 > for branch tracing. Not all branches are necessarily captured

Stephane, could you please explain if the 16 filtered branches in LBR 
are guaranteed to be from a given callchain to the leaf function? My 
understanding is that it's not.

callchain1: a -> b -> d -> e (sample a->b)
callchain2: a -> c -> b -> f (sample b->f)

on PMU interrupt can we end up with:

b -> f <- top of stack
a -> b
...

even though a -> b -> f can never happen in the actual program flow?

  -Arun