linux-kernel - Re: [PATCH] mips: function tracer: Fix broken function tracing

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Date:	Tue, 15 Jan 2013 17:42:42 -0500
From:	Alan Cooper <alcooperx@...il.com>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, ralf@...ux-mips.org,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mips: function tracer: Fix broken function tracing

On Tue, Jan 15, 2013 at 4:34 PM, David Daney <ddaney.cavm@...il.com> wrote:
> On 01/15/2013 01:07 PM, Steven Rostedt wrote:
>>
>> On Tue, 2013-01-15 at 09:55 -0800, David Daney wrote:
>>
>>>> There's nothing that states what the ftrace caller must be. We can have
>>>> it do a proper stack update. That is, only at boot up do we need to
>>>> handle the defined mcount. After that, those instructions are just place
>>>> holders for our own algorithms. If the addiu was needed for the defined
>>>> mcount, there's no reason to keep it for our own ftrace_caller.
>>>>
>>>> Would that work?
>>>
>>>
>>> ... either do as you suggest and dynamically change the ABI of the
>>> target function.
>>
>>
>> We already change the ABI. We have it call ftrace_caller instead of
>> mcount.
>>
>> BTW, I've just compiled with gcc 4.6.3 against mips, and I don't see the
>> issue. I have:
>>
>> 0000000000000000 <account_kernel_stack>:
>>         0:       03e0082d        move    at,ra
>>         4:       0c000000        jal     0 <account_kernel_stack>
>>                          4: R_MIPS_26    _mcount
>>                          4: R_MIPS_NONE  *ABS*
>>                          4: R_MIPS_NONE  *ABS*
>>         8:       0000602d        move    t0,zero
>>         c:       2402000d        li      v0,13
>>        10:       3c030000        lui     v1,0x0
>>                          10: R_MIPS_HI16 mem_section
>>                          10: R_MIPS_NONE *ABS*
>>                          10: R_MIPS_NONE *ABS*
>>        14:       000216fc        dsll32  v0,v0,0x1b
>>        18:       64630000        daddiu  v1,v1,0
>>
>> Is it dependent on the config?
>
>
> Yes.
>
> You need to select a 32-bit kernel (which in turn may require selecting a
> board type that also supports it).
>
> The ABI is different for 32-bit and 64-bit _mcount.
>
> David Daney
>

Building for MIPS malta will show the problem.

>
>
>>
>>>
>>> Or add support to GCC for a better tracing ABI (as I already said we did
>>> for mips64).
>>
>>
>> I wouldn't waste time changing gcc for this. If you're going to change
>> gcc than please implement the -mfentry option. Look at x86_64 to
>> understand this more.
>
>
> A good point.  But I don't really plan on doing any work related to 32-bit
> mips things at this point, so any such change would have to be done by
> someone else.
>
> David Daney
>

I love the idea of removing the useless stack adjust stuff at run time!
The issue still remains for the initial writing of the 2 nops. It
looks like the initial call to write the nops is done from ftrace_init
which is called before SMP is up, so if I write the 2 nops via a
single call to a function with interrupts disabled it should be safe.
I also need to do this for modules at insmod time.

This has been GREAT feedback!

Thanks
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