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Re: [PATCH v9 10/10] tracing: make function graph tracer work with -mmco

To: Wu Zhangjin <wuzhangjin@gmail.com>
Subject: Re: [PATCH v9 10/10] tracing: make function graph tracer work with -mmcount-ra-address
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 20 Nov 2009 17:25:42 +0000
Cc: rostedt@goodmis.org, Nicholas Mc Guire <der.herr@hofr.at>, zhangfx@lemote.com, Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>, Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org, linux-mips@linux-mips.org
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On Fri, Nov 20, 2009 at 08:34:38PM +0800, Wu Zhangjin wrote:

> That thread "MIPS: Add option to pass return address location to
> _mcount" from "David Daney <ddaney@caviumnetworks.com>" have added a new
> option -mmcount-ra-address to gcc(4.5) for MIPS to transfer the location
> of the return address to _mcount.
> 
> Benefit from this new feature, function graph tracer on MIPS will be
> easier and safer to hijack the return address of the kernel function,
> which will save some overhead and make the whole thing more reliable.
> 
> In this patch, at first, try to enable the option -mmcount-ra-address in
> arch/mips/Makefile with cc-option, if gcc support it, it will be
> enabled, otherwise, no side effect.
> 
> and then, we need to support this new option of gcc 4.5 and also support
> the old gcc versions.
> 
> with _mcount in the old gcc versions, it's not easy to get the location
> of return address(tracing: add function graph tracer support for MIPS),
>    so, we do it in a C function: ftrace_get_parent_addr(ftrace.c), but
>    with -mmcount-ra-address, only several instructions need to get what
>    we want, so, I put into asm(mcount.S). and also, as the $12(t0) is
>    used by -mmcount-ra-address for transferring the localtion of return
>    address to _mcount, we need to save it into the stack and restore it
>    when enabled dynamic function tracer, 'Cause we have called
>    "ftrace_call" before "ftrace_graph_caller", which may destroy
>    $12(t0).
> 
> (Thanks to David for providing that -mcount-ra-address and giving the
>  idea of KBUILD_MCOUNT_RA_ADDRESS, both of them have made the whole
>  thing more beautiful!)
> 
> Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>

Queued for 2.6.33 but due to patch 3/3 I won't propagate this series
immediately to linux-next.

Thanks!

  Ralf

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