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Re: [PATCH v7 04/17] tracing: add static function tracer support for MIP

To: wuzhangjin@gmail.com
Subject: Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS
From: David Daney <ddaney@caviumnetworks.com>
Date: Tue, 10 Nov 2009 08:43:52 -0800
Cc: linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, zhangfx@lemote.com, zhouqg@gmail.com, Ralf Baechle <ralf@linux-mips.org>, rostedt@goodmis.org, Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>, Nicholas Mc Guire <der.herr@hofr.at>, Richard Sandiford <rdsandiford@googlemail.com>, Patrik Kluba <kpajko79@gmail.com>, Thomas Gleixner <tglx@linutronix.de>, Michal Simek <monstr@monstr.eu>
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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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