linux-kernel - Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support

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Message-ID: <CALCETrVQ8SBg+YLuPmDevL+f2dzBjLJucfMvVHaB04E8QJSGXw@mail.gmail.com>
Date:	Wed, 26 Feb 2014 10:59:17 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support

On Wed, Feb 26, 2014 at 10:55 AM, Andi Kleen <andi@...stfloor.org> wrote:
>> I'm suggesting a perf event option, just like the way that PEBS works.
>
> Right now it's a somewhat experimental feature and just having
> the sysctl is fine. If it turns out that is what everyone uses
> such an option could be still added later.

I'm a bit worried that the syscall will be stuck as ABI forever,
though.  Its presence will make adding a different configuration
mechanism awkward.

>
> I suspect most people would still use FP if they can, just use
> the LBRs if that doesn't work.

I wonder if anyone who uses perf for userspace profiling *ever* uses
FP and gets away with it.  There's precious little userspace software
compiled with frame pointers these days on most architectures.

I have a concrete reason for this question: it would be nice to
compile the vDSO with frame pointers off.  IIRC there would be a
significant performance gain, and I think the only thing that would
break is perf.  But it looks like perf will have nice elfutils unwind
support in 3.15, and if FP support is useless anyway...

--Andy
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