diff options
Diffstat (limited to 'Documentation/trace/ftrace-design.txt')
-rw-r--r-- | Documentation/trace/ftrace-design.txt | 14 |
1 files changed, 7 insertions, 7 deletions
diff --git a/Documentation/trace/ftrace-design.txt b/Documentation/trace/ftrace-design.txt index 641a1ef2a7ff..239f14b2b55a 100644 --- a/Documentation/trace/ftrace-design.txt +++ b/Documentation/trace/ftrace-design.txt | |||
@@ -53,14 +53,14 @@ size of the mcount call that is embedded in the function). | |||
53 | For example, if the function foo() calls bar(), when the bar() function calls | 53 | For example, if the function foo() calls bar(), when the bar() function calls |
54 | mcount(), the arguments mcount() will pass to the tracer are: | 54 | mcount(), the arguments mcount() will pass to the tracer are: |
55 | "frompc" - the address bar() will use to return to foo() | 55 | "frompc" - the address bar() will use to return to foo() |
56 | "selfpc" - the address bar() (with _mcount() size adjustment) | 56 | "selfpc" - the address bar() (with mcount() size adjustment) |
57 | 57 | ||
58 | Also keep in mind that this mcount function will be called *a lot*, so | 58 | Also keep in mind that this mcount function will be called *a lot*, so |
59 | optimizing for the default case of no tracer will help the smooth running of | 59 | optimizing for the default case of no tracer will help the smooth running of |
60 | your system when tracing is disabled. So the start of the mcount function is | 60 | your system when tracing is disabled. So the start of the mcount function is |
61 | typically the bare min with checking things before returning. That also means | 61 | typically the bare minimum with checking things before returning. That also |
62 | the code flow should usually kept linear (i.e. no branching in the nop case). | 62 | means the code flow should usually be kept linear (i.e. no branching in the nop |
63 | This is of course an optimization and not a hard requirement. | 63 | case). This is of course an optimization and not a hard requirement. |
64 | 64 | ||
65 | Here is some pseudo code that should help (these functions should actually be | 65 | Here is some pseudo code that should help (these functions should actually be |
66 | implemented in assembly): | 66 | implemented in assembly): |
@@ -131,10 +131,10 @@ some functions to save (hijack) and restore the return address. | |||
131 | 131 | ||
132 | The mcount function should check the function pointers ftrace_graph_return | 132 | The mcount function should check the function pointers ftrace_graph_return |
133 | (compare to ftrace_stub) and ftrace_graph_entry (compare to | 133 | (compare to ftrace_stub) and ftrace_graph_entry (compare to |
134 | ftrace_graph_entry_stub). If either of those are not set to the relevant stub | 134 | ftrace_graph_entry_stub). If either of those is not set to the relevant stub |
135 | function, call the arch-specific function ftrace_graph_caller which in turn | 135 | function, call the arch-specific function ftrace_graph_caller which in turn |
136 | calls the arch-specific function prepare_ftrace_return. Neither of these | 136 | calls the arch-specific function prepare_ftrace_return. Neither of these |
137 | function names are strictly required, but you should use them anyways to stay | 137 | function names is strictly required, but you should use them anyway to stay |
138 | consistent across the architecture ports -- easier to compare & contrast | 138 | consistent across the architecture ports -- easier to compare & contrast |
139 | things. | 139 | things. |
140 | 140 | ||
@@ -144,7 +144,7 @@ but the first argument should be a pointer to the "frompc". Typically this is | |||
144 | located on the stack. This allows the function to hijack the return address | 144 | located on the stack. This allows the function to hijack the return address |
145 | temporarily to have it point to the arch-specific function return_to_handler. | 145 | temporarily to have it point to the arch-specific function return_to_handler. |
146 | That function will simply call the common ftrace_return_to_handler function and | 146 | That function will simply call the common ftrace_return_to_handler function and |
147 | that will return the original return address with which, you can return to the | 147 | that will return the original return address with which you can return to the |
148 | original call site. | 148 | original call site. |
149 | 149 | ||
150 | Here is the updated mcount pseudo code: | 150 | Here is the updated mcount pseudo code: |