Subject: Re: [PATCH] ftrace: On PowerPC we don't need frame pointers for CALLER_ADDRs

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 02 Feb 2009 11:35:19 +1100

On Sat, 2009-01-31 at 22:06 +0300, Anton Vorontsov wrote:
> According to this discussion:
> 
> http://lkml.org/lkml/2008/7/25/338
> http://lkml.org/lkml/2008/7/26/72
> 
> Frame pointers do nothing useful on PowerPC, so lib/Kconfig.debug
> makes CONFIG_FRAME_POINTER unselectable on PPC targets. But ftrace.h
> requires CONFIG_FRAME_POINTER for CALLER_ADDR macros. Therefore
> tracing is completely useless on PowerPC:

But we need them for -pg which ftrace uses no ? (ie, gcc forces you to
have -fno-omit-frame-pointers with -fpg iirc).

Now, regardless, I agree that on PPC, __builtin_return_address() should
always work with our without that cruft, so we may as well apply that
patch...

> [...]
>   <idle>-0       0X.h3    2us+:      0:140:R   + [000]  1733:120:S mvtsd
>   <idle>-0       0X.h3    9us+: 0 (0)
>   <idle>-0       0X..3   72us : 0 (0)
>   <idle>-0       0X..3   73us :      0:140:R ==> [000]  1733:120:R mvtsd
> 
> On PPC we can safely use __builtin_return_address(1..6) w/o frame
> pointers, and with this patch the trace output looks OK:
> 
> [...]
>   <idle>-0       0X.h3    2us+:      0:140:R   + [000]  1740:120:S mvtsd
>   <idle>-0       0X.h3    9us+: hrtimer_wakeup (__run_hrtimer)
>   <idle>-0       0X..3   87us : cpu_idle (__got2_end)
>   <idle>-0       0X..3   89us :      0:140:R ==> [000]  1740:120:R mvtsd
> 
> Signed-off-by: Anton Vorontsov <[email-masked]>
> ---
> 
> Btw, on PowerPC tracing is also broken w/o "ring-buffer: fix alignment
> problem" patch (currently collecting dust in the -tip tree, commit
> 082605de5f82eb692cc90f7fda071cc01bb5ac34). Any chance the fix go into
> Linus' tree, to not waste other people's time bisecting and debugging
> the problem? ;-)
> 
> For google: tracing, regression, "ring-buffer: move some metadata
> into buffer page", commit abc9b56d66fbd4d93302ef4bf6fa726e1b8255f9,
> answer is here.
> 
>  include/linux/ftrace.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 9c5bc6b..13eba02 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -146,7 +146,7 @@ static inline void __ftrace_enabled_restore(int enabled)
>  #endif
>  }
>  
> -#ifdef CONFIG_FRAME_POINTER
> +#if defined(CONFIG_FRAME_POINTER) || defined(CONFIG_PPC)
>  /* TODO: need to fix this for ARM */
>  # define CALLER_ADDR0 ((unsigned long)__builtin_return_address(0))
>  # define CALLER_ADDR1 ((unsigned long)__builtin_return_address(1))