linux-kernel - Re: [PATCH V4 10/16] perf, core: simplify need branch stack check

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Message-ID: <20140702122757.GB19379@twins.programming.kicks-ass.net>
Date:	Wed, 2 Jul 2014 14:27:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH V4 10/16] perf, core: simplify need branch stack check

On Wed, Jul 02, 2014 at 01:08:30PM +0200, Stephane Eranian wrote:
> On Wed, Jul 2, 2014 at 12:57 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, Jun 30, 2014 at 04:50:47PM +0800, Yan, Zheng wrote:
> >> event->attr.branch_sample_type is non-zero no matter branch stack
> >> is enabled explicitly or is enabled implicitly. we can use it to
> >> replace intel_pmu_needs_lbr_smpl(). This avoids duplicating code
> >> that implicitly enables the LBR.
> >
> > How so? I grep'ed for a minute but could not find what you're referring
> > to.
> 
> If !(attr->sample_type & PERF_SAMPLE_BRANCH_STACK), then
> event->attr.branch_type = 0. If the sample_type is set, then branch_type
> is necessarily non-zero.

Does not compute.. and that appears to contradict whatever Yan Zheng
tries to say. He says that we have branch_type != 0 for PEBS events, and
I couldn't find where that lived.

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