linux-kernel - Re: [PATCH] ftrace: cover new frame pointer semantics

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Date:	Fri, 22 Jan 2010 09:45:37 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Frysinger <vapier@...too.org>
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] ftrace: cover new frame pointer semantics

On Fri, 2010-01-22 at 08:12 -0500, Mike Frysinger wrote:
> Update the graph tracer examples to cover the new frame pointer semantics
> (in terms of passing it along).  Move the HAVE_FUNCTION_GRAPH_FP_TEST docs
> out of the Kconfig, into the right place, and expand on the details.
> 
> Signed-off-by: Mike Frysinger <vapier@...too.org>

Thanks! I'll queue this up for 34. and maybe 33 if there's a fix that is
needed. Not sure if Doc only changes can still make 33, if so, I may
send this patch by itself.

-- Steve

> ---
>  Documentation/trace/ftrace-design.txt |   26 +++++++++++++++++++++++---
>  kernel/trace/Kconfig                  |    4 +---
>  2 files changed, 24 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/trace/ftrace-design.txt b/Documentation/trace/ftrace-design.txt
> index 239f14b..6a5a579 100644
> --- a/Documentation/trace/ftrace-design.txt
> +++ b/Documentation/trace/ftrace-design.txt
> @@ -1,5 +1,6 @@
>  		function tracer guts
>  		====================
> +		By Mike Frysinger
>  
>  Introduction
>  ------------
> @@ -173,14 +174,16 @@ void ftrace_graph_caller(void)
>  
>  	unsigned long *frompc = &...;
>  	unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
> -	prepare_ftrace_return(frompc, selfpc);
> +	/* passing frame pointer up is optional -- see below */
> +	prepare_ftrace_return(frompc, selfpc, frame_pointer);
>  
>  	/* restore all state needed by the ABI */
>  }
>  #endif
>  
> -For information on how to implement prepare_ftrace_return(), simply look at
> -the x86 version.  The only architecture-specific piece in it is the setup of
> +For information on how to implement prepare_ftrace_return(), simply look at the
> +x86 version (the frame pointer passing is optional; see the next section for
> +more information).  The only architecture-specific piece in it is the setup of
>  the fault recovery table (the asm(...) code).  The rest should be the same
>  across architectures.
>  
> @@ -205,6 +208,23 @@ void return_to_handler(void)
>  #endif
>  
> 
> +HAVE_FUNCTION_GRAPH_FP_TEST
> +---------------------------
> +
> +An arch may pass in a unique value (frame pointer) to both the entering and
> +exiting of a function.  On exit, the value is compared and if it does not
> +match, then it will panic the kernel.  This is largely a sanity check for bad
> +code generation with gcc.  If gcc for your port sanely updates the frame
> +pointer under different opitmization levels, then ignore this option.
> +
> +However, adding support for it isn't terribly difficult.  In your assembly code
> +that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
> +Then in the C version of that function, do what the x86 port does and pass it
> +along to ftrace_push_return_trace() instead of a stub value of 0.
> +
> +Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
> +
> +
>  HAVE_FTRACE_NMI_ENTER
>  ---------------------
>  
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 6c22d8a..60e2ce0 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -27,9 +27,7 @@ config HAVE_FUNCTION_GRAPH_TRACER
>  config HAVE_FUNCTION_GRAPH_FP_TEST
>  	bool
>  	help
> -	 An arch may pass in a unique value (frame pointer) to both the
> -	 entering and exiting of a function. On exit, the value is compared
> -	 and if it does not match, then it will panic the kernel.
> +	  See Documentation/trace/ftrace-design.txt
>  
>  config HAVE_FUNCTION_TRACE_MCOUNT_TEST
>  	bool


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