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Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS

To: wuzhangjin@gmail.com
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS
From: Patrik Kluba <kpajko79@gmail.com>
Date: Sun, 25 Oct 2009 14:37:07 +0100
Cc: linux-kernel@vger.kernel.org, GCC Patches <gcc-patches@gcc.gnu.org>, Adam Nemet <anemet@caviumnetworks.com>, rostedt@goodmis.org, linux-mips@linux-mips.org, Thomas Gleixner <tglx@linutronix.de>, Ralf Baechle <ralf@linux-mips.org>, Nicholas Mc Guire <der.herr@hofr.at>, Richard Sandiford <rdsandiford@googlemail.com>, David Daney <ddaney@caviumnetworks.com>
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On Sun, Oct 25, 2009 at 11:48 AM, Wu Zhangjin <wuzhangjin@gmail.com> wrote:
>
> do you mean if enabling PROFILE_BEFORE_PROLOGUE, there will be some
> problems with module support using -mlong-calls?
>

No, there are no problems. I've tested it on friday, and function
graph tracing was working correctly.
I meant to say that 4.2.1 we use does not generate correct profile
calls from kernel modules. Maybe this issue was fixed in newer
releases, I did not check. I've applied a patch (don't remember where
have I found that, maybe it was created by you) to our toolchain
several months ago.

I was thinking about dynamic tracing, and I think a toolchain patch
can be avoided completely. We only need to make difference between
"jal _mcount" and "jalr v1"-style mcount calls when replacing them
with "nop" instructions in the code-patching function called by
ftrace_convert_nops(). This can be done in 2 ways:
1) keeping old instructions - takes extra memory, not an option
2) using 2 separate instructions to replace with. One of them could be
the normal NOP instruction, which expands to "sll r0, r0, 0". For the
other we could use "sll r0, r0, 1" but as it has already special
meaning (SSNOP) a better candidate could be something like "sll r1,
r1, 0". This way we can decide which instruction to patch in when
tracing is enabled for a function, eg. when the code patcher
encounters a "sll r0, r0, 0" it emits a function call using JAL and
when it encounters "sll r1, r1, 0" it emits a function call using
"JALR v1".

Regards,
Patrik

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