linux-kernel - Re: profile_pc() bogus since <= 2.6.19 (x86 at least)?

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Message-Id: <47CECF9E.76E4.0078.0@novell.com>
Date:	Wed, 05 Mar 2008 15:51:42 +0000
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: profile_pc() bogus since <= 2.6.19 (x86 at least)?

>>> Ingo Molnar <mingo@...e.hu> 05.03.08 16:37 >>>
>
>* Jan Beulich <jbeulich@...ell.com> wrote:
>
>> Ingo,
>> 
>> while the comment at the top of kernel/spinlock.c states so:
>> 
>>  * Note that some architectures have special knowledge about the
>>  * stack frames of these functions in their profile_pc. If you
>>  * change anything significant here that could change the stack
>>  * frame contact the architecture maintainers.
>> 
>> the actual code doesn't seem to match this anymore. With all (and even 
>> before that, many) functions being written in C, there cannot be 
>> validly made assumptions about the stack frame layout. Indeed, if I 
>> check the disassembly framed by __lock_text_{start,end} on x86, there 
>> are a number of functions that push one or two registers (in 
>> lock_kernel() even stack variables are being allocated), which clearly 
>> breaks profile_pc()'s assumptions.
>> 
>> Since it's been this way for so long, I wonder how frequently this 
>> code is actually being exercised...
>
>yeah - i guess it's not really relevant anymore now that lockdep saves 
>full stack traces. I doubt anyone bothers to look at wchan anymore. We 
>might even remove all the __lock and __sched sections and annotations?

Since drivers/oprofile/cpu_buffer.c and kernel/profile.c both have a
reference to profile_pc(), I'm not so sure about that. Perhaps if a
replacement for these two can be found...

Jan

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