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Documentation: Update ftrace-design.txt

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Correct grammos.  Spell out words.  Add missing words.
Consistent use of "mcount()" function name.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4B2C0D78.6060707@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Randy Dunlap 2009-12-18 15:17:12 -08:00 • committed by Ingo Molnar
parent b41df645c8
commit 7e25f44cbf
1 changed files with 7 additions and 7 deletions
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14
Documentation/trace/ftrace-design.txt
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@ -53,14 +53,14 @@ size of the mcount call that is embedded in the function).
For example, if the function foo() calls bar(), when the bar() function calls For example, if the function foo() calls bar(), when the bar() function calls
mcount(), the arguments mcount() will pass to the tracer are: mcount(), the arguments mcount() will pass to the tracer are:
"frompc" - the address bar() will use to return to foo() "frompc" - the address bar() will use to return to foo()
"selfpc" - the address bar() (with _mcount() size adjustment) "selfpc" - the address bar() (with mcount() size adjustment)
Also keep in mind that this mcount function will be called *a lot*, so Also keep in mind that this mcount function will be called *a lot*, so
optimizing for the default case of no tracer will help the smooth running of optimizing for the default case of no tracer will help the smooth running of
your system when tracing is disabled. So the start of the mcount function is your system when tracing is disabled. So the start of the mcount function is
typically the bare min with checking things before returning. That also means typically the bare minimum with checking things before returning. That also
the code flow should usually kept linear (i.e. no branching in the nop case). means the code flow should usually be kept linear (i.e. no branching in the nop
This is of course an optimization and not a hard requirement. case). This is of course an optimization and not a hard requirement.
Here is some pseudo code that should help (these functions should actually be Here is some pseudo code that should help (these functions should actually be
implemented in assembly): implemented in assembly):
@ -131,10 +131,10 @@ some functions to save (hijack) and restore the return address.
The mcount function should check the function pointers ftrace_graph_return The mcount function should check the function pointers ftrace_graph_return
(compare to ftrace_stub) and ftrace_graph_entry (compare to (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 ftrace_graph_entry_stub). If either of those is not set to the relevant stub
function, call the arch-specific function ftrace_graph_caller which in turn function, call the arch-specific function ftrace_graph_caller which in turn
calls the arch-specific function prepare_ftrace_return. Neither of these calls the arch-specific function prepare_ftrace_return. Neither of these
function names are strictly required, but you should use them anyways to stay function names is strictly required, but you should use them anyway to stay
consistent across the architecture ports -- easier to compare & contrast consistent across the architecture ports -- easier to compare & contrast
things. things.
@ -144,7 +144,7 @@ but the first argument should be a pointer to the "frompc". Typically this is
located on the stack. This allows the function to hijack the return address located on the stack. This allows the function to hijack the return address
temporarily to have it point to the arch-specific function return_to_handler. temporarily to have it point to the arch-specific function return_to_handler.
That function will simply call the common ftrace_return_to_handler function and That function will simply call the common ftrace_return_to_handler function and
that will return the original return address with which, you can return to the that will return the original return address with which you can return to the
original call site. original call site.
Here is the updated mcount pseudo code: Here is the updated mcount pseudo code: