linux-kernel - Re: [PATCH] perf: Add a new sort order: SORT

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Message-ID: <4F72026C.3050604@fb.com>
Date:	Tue, 27 Mar 2012 11:09:48 -0700
From:	Arun Sharma <asharma@...com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"Arnaldo Carvalho de Melo" <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	"Paul Mackerras" <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Stephane Eranian" <eranian@...gle.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Tom Zanussi" <tzanussi@...il.com>,
	<linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH] perf: Add a new sort order: SORT_INCLUSIVE (v4)

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
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