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SubjectRe: [PATCH] perf: Add a new sort order: SORT_INCLUSIVE (v4)
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On Tue, 2012-03-27 at 11:09 -0700, Arun Sharma wrote:
> 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?

Right, so the LBR is a queue not a stack. A program like:

foo() {
bar1();
bar2();
}

will, using the lbr, look like: foo->bar1->bar2 (if you filter returns),
or foo->bar1->foo+x->bar2 if you include returns.

A callchain is a pure stack, a return pops the top most entry, the above
program can only give 3 possible callchains:

a) foo
b) foo, bar1
c) foo, bar2

Furthermore, the LBR is about any branch, callchains are about function
calls.


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