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

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Message-ID: <20140409164852.GA22728@two.firstfloor.org>
Date:	Wed, 9 Apr 2014 18:48:53 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	David Ahern <dsahern@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Stephane Eranian <eranian@...gle.com>,
	"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>
Subject: Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support

On Wed, Apr 09, 2014 at 01:48:57PM +0200, Peter Zijlstra wrote:
> On Wed, Feb 26, 2014 at 12:26:43PM -0800, Andy Lutomirski wrote:
> > 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.
> 
> So I though that LTO was supposed to get rid of a lot of the small
> function and inline them.

It does it when it can (no indirect), thinks it's profitable and won't
increase code size too much. 

> 
> I've also heard that in practise this is very 'hard', and thus we're
> still stuck with a gazillion small functions (mostly C++ people suffer
> from this).

They need devirtualization, which we cannot do currently in the kernel.

> 
> Can anybody give a concise explanation on why LTO doesn't rid us of
> these small functions or point to a web resource that describes the
> problem?

It depends on the code of course.
On one of my LTO builds I have ~10% less functions in System.map.

Actual results will vary of course on the config.

-Andi


-- 
ak@...ux.intel.com -- Speaking for myself only.
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