linux-kernel - Re: [PATCH 1/4] Add support for userspace stacktraces in tracing/iter

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Date:	Sun, 26 Oct 2008 15:38:45 +0200
From:	Török Edwin <edwintorok@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	"Frank Ch. Eigler" <fche@...hat.com>, fweisbec@...il.com,
	mingo@...e.hu, srostedt@...hat.com, sandmann@...mi.au.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] Add support for userspace stacktraces in tracing/iter_ctrl

On 2008-10-26 15:29, Peter Zijlstra wrote:
> On Sun, 2008-10-26 at 09:15 -0400, Frank Ch. Eigler wrote:
>   
>> "=?ISO-8859-1?Q?Fr=E9d=E9ric_Weisbecker?=" <fweisbec@...il.com> writes:
>>
>>     
>>> [...]
>>>       
>>>> +/* Userspace stacktrace - based on kernel/trace/trace_sysprof.c */
>>>> +
>>>> +struct stack_frame {
>>>> +       const void __user       *next_fp;
>>>> +       unsigned long           return_address;
>>>> +};
>>>>         
>>> [...]
>>>       
>> To what extent does this actually work, and on which architectures?
>> While the kernel appears willing to sacrifice some performance for
>> functionality by building with frame pointers at the drop of a tracing
>> hat, userspace is not like that.
>>     
>
> Aww, common, Gentoo is brilliant for that :-)
>
> CFLAGS+=-fframe-pointer
>
> emerge -uDNe world
>
> Then again, you'd better not have all that desktop bloat installed,
> otherwise that will take ages.. KDE/OOo compile times anyone?
>
> /me runs

It should be enough to rebuild the application you are tracing[1] +
libraries, or at least libc.
In userspace I can get a stacktrace using DWARF unwind info, but doing
that in the kernel would be too expensive, right?

[1] assuming you are tracing latencies in a single application

Best regards,
--Edwin
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