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

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Message-ID: <CABPqkBT2ZTC4Q3L9KwNfAs05qQvRFUZYvQ4QXJmiFeL-jwWSSw@mail.gmail.com>
Date:	Wed, 2 Jul 2014 13:08:30 +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 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.
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