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Date: Tue, 7 Oct 2014 17:25:28 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Liang, Kan" <kan.liang@intel.com>
Cc: "eranian@google.com" <eranian@google.com>,
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        "Yan, Zheng" <zheng.z.yan@intel.com>
Subject: Re: [PATCH V5 14/16] perf, x86: enable LBR callstack when recording
 callchain
Message-ID: <20141007152528.GD5850@twins.programming.kicks-ass.net>
References: <1410358153-421-1-git-send-email-kan.liang@intel.com>
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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@intel.com wrote:
> > > From: Kan Liang <kan.liang@intel.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.