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

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Date:	Wed, 26 Feb 2014 12:26:43 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	David Ahern <dsahern@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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 12:14 PM, David Ahern <dsahern@...il.com> wrote:
> On 2/26/14, 12:25 PM, Andy Lutomirski wrote:
>>
>> On Wed, Feb 26, 2014 at 11:19 AM, David Ahern <dsahern@...il.com> wrote:
>>>
>>> On 2/26/14, 11:59 AM, Andy Lutomirski wrote:
>>>
>>>> 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.
>>>
>>>
>>>
>>> yes and yes. With control over the entire stack we are making sure
>>> frame-pointers are enabled as much as possible.
>>>
>>
>> I'm curious why.
>
>
> Is there some reason not to enable frame pointers?

Speed.  FPO saves one register (a big deal on x86_32; not so important
on x86_64) but also saves a few cycles on function entry and exit,
which is a bigger deal for small functions.

>
> fp method has much less overhead than dwarf, and good, clear callchains are
> important.
>

Agreed about the good, clear callchains.  But DWARF seems to work
pretty well, and you only have the overhead when you're actually
debugging or profiling.

>
>>
>> Maybe this should be a config option.  Anyone using a standard distro
>> is running a nearly completely frame-pointer-omitted userspace these
>> days.
>
>
> Does WRL or Yocto fall into that 'standard distro' comment? Fairly easy to
> enable frame-pointers.

Fair enough :)

--Andy
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