linux-kernel - Re: [PATCH 0/2] perfcounter: callchains with perf report

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Date:	Sun, 28 Jun 2009 23:05:56 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 0/2] perfcounter: callchains with perf report

On Sat, Jun 27, 2009 at 11:12:47AM +1000, Paul Mackerras wrote:
> Frederic Weisbecker writes:
> 
> > Here is a first shot for the sorted callchains per entries handling
> > with per report.
> > 
> > I'll continue to improve it:
> > 
> > - symbol resolution
> > - profit we have a tree to display a better graph hierarchy
> > - let the user provide a limit for hit percentage, depth, number of
> >   backtraces, etc...
> > - better output
> > - colors
> > - and so on....
> 
> Nice!
> 
> I have just about finished doing the kernel piece of callchain support
> on powerpc.  Because of the way function calls and returns work on
> powerpc, working out the first one or two return addresses can be
> tricky.  We potentially have a valid return address in the link
> register (LR), or in the LR save area in the second stack frame, or
> both, and you need extra information such as DWARF unwind tables to
> work out which of those three possibilities you have, in general.
> This is the case at each point where an interrupt or signal has
> occurred.
> 
> Because I didn't want to go trawling through CFI tables at interrupt
> time, particularly for user code, I made the kernel save both possible
> return addresses in the callchain.  For the kernel part of the
> callchain, I check those two addresses to see if they're valid kernel
> addresses and set them to 0 if not, or if they're equal.
> 
> That means I need to make some changes to builtin-report.c to ignore
> zero addresses.  I may need to add stuff to look for and use unwind
> tables as well, if we want completely accurate call chains.


Well, I guess I can ignore them in my further patches.
But wouldn't it be better to discard them from the kernel?
Unless it's somewhat useful to know we had an unknown entry?

 
> The other thing I did is to put PERF_CONTEXT_KERNEL markers in the
> callchain every time we find an interrupt frame, and PERF_CONTEXT_USER
> markers every time we find a signal frame, so that userspace knows
> when it needs to do the unwinding.
> 
> Oh, and a third point is that on powerpc the sampled IP recorded if
> you ask for PERF_SAMPLE_IP won't in general be the same as the first
> IP in the callchain.  The reason is that the PERF_SAMPLE_IP value
> points to the instruction that caused the counter overflow whereas the
> first IP in the callchain tells you where the CPU took the interrupt.
> That is almost always a few instructions further on, and can be quite
> a way further on if interrupts were disabled when the counter overflow
> occur.  In fact we regularly see the PERF_SAMPLE_IP value being in the
> hypervisor but the first IP in the callchain being in the kernel.


Ok.

> 
> Paul.

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