Re: [PATCH 2/2] ftrace: document basic ftracer/ftracer graph needs
From: Steven Rostedt
Date: Wed Jun 10 2009 - 13:05:10 EST
On Wed, 10 Jun 2009, Mike Frysinger wrote:
> While implementing ftracer and ftracer graph support, I found the exact
> arch implementation details to be a bit lacking (and my x86 foo ain't
> great). So after pounding out support for the Blackfin arch, document
> the requirements.
> Signed-off-by: Mike Frysinger <vapier@xxxxxxxxxx>
> Documentation/trace/ftrace.txt | 7 ++++++
> kernel/trace/Kconfig | 43 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 50 insertions(+), 0 deletions(-)
> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
> index 5ad2ded..e9f0527 100644
> --- a/Documentation/trace/ftrace.txt
> +++ b/Documentation/trace/ftrace.txt
> @@ -27,6 +27,13 @@ disabled, and more (ftrace allows for tracer plugins, which
> means that the list of tracers can always grow).
> +Implementation Details
> +See kernel/trace/Kconfig for details about different options and
> +the required architecture pieces.
> The File System
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 417d198..be936c9 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -14,9 +14,46 @@ config HAVE_FTRACE_NMI_ENTER
> config HAVE_FUNCTION_TRACER
> + help
> + Implement support for the _mcount() function (or whatever your
> + toolchain names it), and the ftrace_stub function (which only
> + does a return).
> + The _mcount() function should check the function pointer
> + ftrace_trace_function to see if it is set to ftrace_stub. If
> + it is, there is nothing for you to do -- return. If it isn't,
> + then call it the same way the _mcount() function normally calls
> + __mcount_internal() -- the first argument is the "frompc" while
> + the second argument is the "selfpc" (adjusted to remove the size
> + of the embedded _mcount() call).
> + For example, if the function foo() calls bar(), when the bar()
> + function calls _mcount(), the arguments to the tracer are:
> + "frompc" - the address bar() will use to return to foo()
> + "selfpc" - the address bar() (with _mcount() size adjustment)
> config HAVE_FUNCTION_GRAPH_TRACER
> + help
> + Update your _mcount() function to check the function pointers
> + ftrace_graph_return (compare to ftrace_stub) and ftrace_graph_entry
> + (compare to ftrace_graph_entry_stub). If either of those are not
> + set to the relevant stub function, call the arch-specific function
> + ftrace_graph_caller which in turn calls the arch-specific function
> + prepare_ftrace_return.
> + The arguments to prepare_ftrace_return() are slightly different.
> + The first argument will instead be a pointer to the "frompc" --
> + typically located in the stack. This allows the function to
> + hijack the return address temporarily to have it point to the
> + arch-specific function return_to_handler(). For exact details of
> + how to implement prepare_ftrace_return(), consult an existing port.
> + It is fairly straight forward C code.
> + The return_to_handler() function is simple as well. Have it call
> + the common ftrace_return_to_handler() function. The return value
> + is the original return address, so restore this to the right place
> + and return.
This does not belong in the Kconfig. That's the last place a developer
will look for implementing something. I would recommend adding a
> config HAVE_FUNCTION_TRACE_MCOUNT_TEST
> @@ -25,11 +62,17 @@ config HAVE_FUNCTION_TRACE_MCOUNT_TEST
> variable at the mcount call site. Otherwise, this variable
> is tested by the called function.
> + It is of course optional for arches, but does improve performance
> + a bit as it prevents constant state saving and call overhead when
> + the function tracer is actually disabled.
> config HAVE_DYNAMIC_FTRACE
> config HAVE_FTRACE_MCOUNT_RECORD
> + help
> + See scripts/recordmcount.pl for more info.
> config HAVE_HW_BRANCH_TRACER
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