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

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Date:	Wed, 2 Jul 2014 15:00:52 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 2, 2014 at 2:27 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.

He is probably talking about precise=2 for non HSW PMU.
But it is true,  you need to look at x86_pmu_hw_config()
 *br_type = PERF_SAMPLE_BRANCH_ANY;
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