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Subject: Re: [PATCH 04/12] perf_events: sync branch stack sampling with X86
 precise_sampling
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, acme@redhat.com,
        ming.m.lin@intel.com, andi@firstfloor.org, robert.richter@amd.com,
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Date: Fri, 07 Oct 2011 12:37:59 +0200
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On Fri, 2011-10-07 at 12:34 +0200, Stephane Eranian wrote:
> On Thu, Oct 6, 2011 at 7:25 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, 2011-10-06 at 16:49 +0200, Stephane Eranian wrote:
> >>
> >> On Intel X86 PERF_SAMPLE_BRANCH_STACK is implemented using LBR,
> >> therefore both features must be coordinated as they may not
> >> configure LBR the same way.
> >
> > Differently, you mean? Both wanting the same configuration seems fine.
> >
> No, I meant you can allow LBR + precise_sampling>1 ONLY when
> users set LBR to record ALL branches and at the same priv levels. In
> other words,
> you're exposing the LBR content used by the fixup code.

Right. 

> One could argue, that if LBR is set to filter certain branches, it may
> also be okay,
> it's just that you won't necessarily get the same number of successful
> fixups. The
> samples are tagged when fixups were successful, so that may also be an viable
> option. Best effort given the content of the LBR. Depending on the
> code, that might
> be slightly better than dropping to precise_sampling=1 (no fixups).

Possible, but I imagine it might surprise some people.