linux-kernel - Re: [RFC PATCH 4/9] perf: Don't record frame pointer based user stacktraces if we dump stack and regs

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Message-ID: <20101016001900.GA6512@nowhere>
Date:	Sat, 16 Oct 2010 02:19:02 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Robert Richter <robert.richter@....com>
Subject: Re: [RFC PATCH 4/9] perf: Don't record frame pointer based user
	stacktraces if we dump stack and regs

On Wed, Oct 13, 2010 at 09:23:06AM +0200, Peter Zijlstra wrote:
> On Wed, 2010-10-13 at 07:06 +0200, Frederic Weisbecker wrote:
> > User and kernel stack might be selected for other uses than callchain
> > in the future, this probably shouldn't mess with the regular callchain
> > code. Instead we should probably have an exclude_callchain_user
> > attribute, that could be also useful to filter out user callchains
> > when people don't want them. 
> 
> Probably ;-)

There is another solution that would solve my vdso problem
in the meantime.

The problem with vdso is that if we entered the kernel with
a syscall, the first user entry in the callchain will be a
vdso address. But vdso doesn't have dwarf informations so we
can't unwind further.

One solution would be having max_callchain_user as an attribute.
If we do a normal frame pointer based callchain, set it to -1
and you won't have limitations in your callchain. Or you
can set it to n so that your callchains are bound to a maximum
depth.
If you don't want user callchains, set it to 0.
If you do a dwarf based callchain, set it to 2, so that if userspace
was in a vdso, we just deref the frame pointer to find what called
the vdso, and then we can start the dwarf unwinding from there.

I think I'll try that.

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