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Re: [PATCH v9 04/10] tracing: add dynamic function tracer support for MI

To: Wu Zhangjin <wuzhangjin@gmail.com>
Subject: Re: [PATCH v9 04/10] tracing: add dynamic function tracer support for MIPS
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 20 Nov 2009 17:23:27 +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 08:34:32PM +0800, Wu Zhangjin wrote:

> From: Wu Zhangjin <wuzhangjin@gmail.com>
> 
> With dynamic function tracer, by default, _mcount is defined as an
> "empty" function, it returns directly without any more action . When
> enabling it in user-space, it will jump to a real tracing
> function(ftrace_caller), and do the real job for us.
> 
> Differ from the static function tracer, dynamic function tracer provides
> two functions ftrace_make_call()/ftrace_make_nop() to enable/disable the
> tracing of some indicated kernel functions(set_ftrace_filter).
> 
> In the -v4 version, the implementation of this support is basically the same 
> as
> X86 version does: _mcount is implemented as an empty function and 
> ftrace_caller
> is implemented as a real tracing function respectively.
> 
> But in this version, to support module tracing with the help of
> -mlong-calls in arch/mips/Makefile:
> 
> MODFLAGS += -mlong-calls.
> 
> The stuff becomes a little more complex. We need to cope with two
> different type of calling to _mcount.
> 
> For the kernel part, the calling to _mcount(result of "objdump -hdr
> vmlinux"). is like this:
> 
>       108:   03e0082d        move    at,ra
>       10c:   0c000000        jal     0 <fpcsr_pending>
>                         10c: R_MIPS_26  _mcount
>                         10c: R_MIPS_NONE        *ABS*
>                         10c: R_MIPS_NONE        *ABS*
>       110:   00020021        nop
> 
> For the module with -mlong-calls, it looks like this:
> 
>       c:      3c030000        lui     v1,0x0
>                       c: R_MIPS_HI16  _mcount
>                       c: R_MIPS_NONE  *ABS*
>                       c: R_MIPS_NONE  *ABS*
>       10:     64630000        daddiu  v1,v1,0
>                       10: R_MIPS_LO16 _mcount
>                       10: R_MIPS_NONE *ABS*
>                       10: R_MIPS_NONE *ABS*
>       14:     03e0082d        move    at,ra
>       18:     0060f809        jalr    v1
> 
> In the kernel version, there is only one "_mcount" string for every
> kernel function, so, we just need to match this one in mcount_regex of
> scripts/recordmcount.pl, but in the module version, we need to choose
> one of the two to match. Herein, I choose the first one with
> "R_MIPS_HI16 _mcount".
> 
> and In the kernel verion, without module tracing support, we just need
> to replace "jal _mcount" by "jal ftrace_caller" to do real tracing, and
> filter the tracing of some kernel functions via replacing it by a nop
> instruction.
> 
> but as we have described before, the instruction "jal ftrace_caller" only left
> 32bit length for the address of ftrace_caller, it will fail when calling from
> the module space. so, herein, we must replace something else.
> 
> the basic idea is loading the address of ftrace_caller to v1 via changing 
> these
> two instructions:
> 
>       lui     v1,0x0
>       addiu   v1,v1,0
> 
> If we want to enable the tracing, we need to replace the above instructions 
> to:
> 
>       lui     v1, HI_16BIT_ftrace_caller
>       addiu   v1, v1, LOW_16BIT_ftrace_caller
> 
> If we want to stop the tracing of the indicated kernel functions, we
> just need to replace the "jalr v1" to a nop instruction. but we need to
> replace two instructions and encode the above two instructions
> oursevles.
> 
> Is there a simpler solution? Yes! Here it is, in this version, we put _mcount
> and ftrace_caller together, which means the address of _mcount and
> ftrace_caller is the same:
> 
> _mcount:
> ftrace_caller:
>       j       ftrace_stub
>        nop
> 
>       ...(do real tracing here)...
> 
> ftrace_stub:
>       jr      ra
>        move   ra, at
> 
> By default, the kernel functions call _mcount, and then jump to ftrace_stub 
> and
> return. and when we want to do real tracing, we just need to remove that "j
> ftrace_stub", and it will run through the two "nop" instructions and then do
> the real tracing job.
> 
> what about filtering job? we just need to do this:
> 
>        lui v1, hi_16bit_of_mcount        <--> b 1f (0x10000004)
>        addiu v1, v1, low_16bit_of_mcount
>        move at, ra
>        jalr v1
>        nop
>                                            1f: (rec->ip + 12)
> 
> In linux-mips64, there will be some local symbols, whose name are
> prefixed by $L, which need to be filtered. thanks goes to Steven for
> writing the mips64-specific function_regex.
> 
> In a conclusion, with RISC, things becomes easier with such a "stupid"
> trick, RISC is something like K.I.S.S, and also, there are lots of
> "simple" tricks in the whole ftrace support, thanks goes to Steven and
> the other folks for providing such a wonderful tracing framework!
> 
> 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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