linux-kernel - Re: [PATCH V5 14/16] perf, x86: enable LBR callstack when recording callchain

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Date:	Tue, 7 Oct 2014 17:25:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"eranian@...gle.com" <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>
Subject: Re: [PATCH V5 14/16] perf, x86: enable LBR callstack when recording
 callchain

On Tue, Oct 07, 2014 at 03:00:43AM +0000, Liang, Kan wrote:
> 
> 
> >
> > On Wed, Sep 10, 2014 at 10:09:11AM -0400, kan.liang@...el.com wrote:
> > > From: Kan Liang <kan.liang@...el.com>
> > >
> > > If a task specific event wants user space callchain but does not want
> > > branch stack sampling, enable the LBR call stack facility implicitly.
> > > The LBR call stack facility can help perf to get user space callchain
> > > in case of there is no frame pointer.
> > >
> > > Note: this feature only affects how to get user callchain. The kernel
> > > callchain is always got by frame pointers.
> >
> > Yeah, don't like this either. Suppose you have sane userspace (with
> > framepointers enabled) then you're now loosing the better option.
> 
> FP is the first option. This patch tries to enable LBR call stack facility implicitly.
> Only when FP disabled or failed, we try to use LBR call stack.
> Please refer to the previous patch https://lkml.org/lkml/2014/9/10/376

Still makes for an entirely unpredictable situation. That way you never
quite know where your data came from.
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