linux-kernel - Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS

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Message-ID: <4AF99848.9090000@caviumnetworks.com>
Date:	Tue, 10 Nov 2009 08:43:52 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	wuzhangjin@...il.com
CC:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	zhangfx@...ote.com, zhouqg@...il.com,
	Ralf Baechle <ralf@...ux-mips.org>, rostedt@...dmis.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nicholas Mc Guire <der.herr@...r.at>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Patrik Kluba <kpajko79@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Simek <monstr@...str.eu>
Subject: Re: [PATCH v7 04/17] tracing: add static function tracer support
 for MIPS

Wu Zhangjin wrote:
> Hi,
> 
> On Mon, 2009-11-09 at 16:26 -0800, David Daney wrote: 
>> Wu Zhangjin wrote:
>>
>>>  
>>> +ifndef CONFIG_FUNCTION_TRACER
>>>  cflags-y := -ffunction-sections
>>> +else
>>> +cflags-y := -mlong-calls
>>> +endif
>>>  cflags-y += $(call cc-option, -mno-check-zero-division)
>>>  
>> That doesn't make sense to me.  Modules are already compiled with 
>> -mlong-calls.  The only time you would need the entire kernel compiled 
>> with -mlong-calls is if the tracer were in a module.  The logic here 
>> doesn't seem to reflect that.
> 
> Yes, I knew the module have gotten the -mlong-calls, Here we just want
> to use -mlong-calls for the whole kernel, and then we get the same
> _mcount stuff in the whole kernel, at last, we can use the same
> scripts/recordmcount.pl and ftrace_make_nop & ftrace_make_call for the
> dynamic ftracer.
> 

-mlong-calls really degrades performance.  I have seen things like 6% 
drop in network packet forwarding rates with -mlong-calls.

It would be better to fix all the tools so that they could handle both 
-mlong-calls and -mno-long-calls code.


David Daney
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