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SubjectRe: [PATCH 2/2] ftrace: document basic ftracer/ftracer graph needs


On Wed, 10 Jun 2009, Mike Frysinger wrote:
> 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
> bool
> + 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)

This is too much for Kconfig.

>
> config HAVE_FUNCTION_GRAPH_TRACER
> bool
> + 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 is also too much for Kconfig.

>
> config HAVE_FUNCTION_TRACE_MCOUNT_TEST
> bool
> @@ -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.
> +

The above would be fine, although I would reword it a little:

This is optional for archs, but will improve performance, as it
prevents constant state saving and call overhead when the function tracer
tracer is active, but has been disabled.

I put in 'is active' because with dynamic function tracing it has no more
overhead when disabled. We can enable function tracing, but soft disable
it with a check of a bit.


> config HAVE_DYNAMIC_FTRACE
> bool
>
> config HAVE_FTRACE_MCOUNT_RECORD
> bool
> + help
> + See scripts/recordmcount.pl for more info.

This is fine.

-- Steve

>
> config HAVE_HW_BRANCH_TRACER
> bool
> --
> 1.6.3.1
>
>


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