linux-kernel - Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size

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Date:	Fri, 16 Dec 2011 11:46:03 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ingo Molnar" <mingo@...e.hu>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Alexander van Heukelum" <heukelum@...tmail.fm>,
	<fweisbec@...il.com>, "Arjan van de Ven" <arjan@...radead.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text
 size

>>> On 16.12.11 at 10:23, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> On 12/16/2011 12:53 AM, Ingo Molnar wrote:
>> * Ingo Molnar <mingo@...e.hu> wrote:
>>
>>> [...]
>>>
>>> The call-chains are still intact for quality backtraces and 
>>> for call-chain profiling (perf record -g), as the backtrace 
>>> walker can deduct the full backtrace from the RIP of a leaf 
>>> function and the parent chain.

Are you sure about that even if the leaf function uses rBP for a
different purpose?

>> Hm, noticed one complication while looking at annotated assembly 
>> code in perf top. Code doing function calls from within asm() is 
>> incorrectly marked 'leaf' by GCC:
>>
>> ffffffff812b82d8 <arch_local_save_flags>:
>> ffffffff812b82d8:       ff 14 25 00 d9 c1 81    callq  *0xffffffff81c1d900
>> ffffffff812b82df:       c3                      retq   
>>
>> So all the paravirt details will have to be fixed, so that GCC 
>> is able to see that there's a real function call done inside. 
>> Jeremy, Konrad?

If the above is not a problem, wouldn't this simply result in a skipped
function layer?

Also, iirc it's not just pv-ops that uses calls within asm()-s.

> Um.  So the issue is that a function that contains only pvops looks like
> it's a leaf to gcc and it does some leaf-function optimisation?
> 
> How can we tell gcc the asm contains a call, or otherwise suppress the
> "leaf function" classification?

I'm afraid you can't without adding code (i.e. a dummy function call).

Jan

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