linux-kernel - Re: [RFC PATCH 1/1] proc: introduce /proc/<pid>/lbr

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Message-ID: <20150227175434.GI27767@tassilo.jf.intel.com>
Date:	Fri, 27 Feb 2015 09:54:34 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kan.liang@...el.com, linux-kernel@...r.kernel.org,
	mingo@...hat.com, acme@...nel.org
Subject: Re: [RFC PATCH 1/1] proc: introduce /proc/<pid>/lbr_stack

On Fri, Feb 27, 2015 at 08:58:29AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 23, 2015 at 09:44:48AM -0800, Andi Kleen wrote:
> > On Mon, Feb 23, 2015 at 05:49:57PM +0100, Peter Zijlstra wrote:
> > > On Mon, Feb 23, 2015 at 03:43:41AM +0000, kan.liang@...el.com wrote:
> > > > From: Kan Liang <kan.liang@...el.com>
> > > > 
> > > > Haswell has a new feature that utilizes the existing Last Branch Record
> > > > facility to record call chains. It has been implemented in perf. The
> > > > call chains information is saved during perf event context.
> > > > 
> > > > This patch exposes a /proc/<pid>/lbr_stack file that shows the saved LBR
> > > > call chain information.
> > > 
> > > But why? I mean, this thing is only useful if you have a concurrently
> > > running perf record that selects the LBR-stack stuff.
> > > 
> > > And if you have that, you might as well look at its output instead. Why
> > > add this unconditional proc file that doesn't function on its own?
> > 
> > perf record doesn't show where you're currently blocked.
> 
> Of course it does; look at perf inject -s.

Trace points don't support the LBR stack.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only
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