Re: [PATCH] perf: Add a new sort order: SORT_INCLUSIVE (v4)
From: Peter Zijlstra
Date: Tue Mar 27 2012 - 15:38:34 EST
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:
> > 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:
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:
b) foo, bar1
c) foo, bar2
Furthermore, the LBR is about any branch, callchains are about function
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