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Date: Fri, 7 Jan 2011 18:17:09 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Jan Beulich <JBeulich@novell.com>,
        "H. Peter Anvin" <a.p.zijlstra@chello.nl>,
        Stephane Eranian <eranian@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Soeren Sandmann Pedersen <sandmann@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq entry
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On Fri, Jan 07, 2011 at 05:58:43PM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > On Fri, Jan 07, 2011 at 04:13:58PM +0000, Jan Beulich wrote:
> > > >>> On 07.01.11 at 17:05, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > > Once I'll have perf callchain based on CFI ready, we'll perhaps find some 
> > > > issues
> > > > there. Although I guess there are already tools that can make use of that.
> > > 
> > > Is this to read that you're planning to do a re-spin of the CFI
> > > unwinding code (which I'm not allowed to submit another time,
> > > but which we've been using for years in SuSE distros) then?
> > 
> > An in-kernel CFI unwinder?
> > 
> > No the intended CFI unwinding that I'm working on for perf is made
> > on post-processing, on top of partial stack and regs snapshots.
> > 
> > The true unwinding is computed in userspace.
> 
> I think that design will be fundamentally more robust and more flexible than an 
> in-kernel unwinder.

But in fact for the kernel unwinding it still uses frame pointers which is certainly
the fastest and lightest way to unwind a stack. Very nice for our profiling.

Now one may argue about the loss of a register and its associated overhead, not
sure it's always noticeable though...