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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/trace/ftrace.rst | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst index 6ce2763a2a3e..7c5e6d6ab5d1 100644 --- a/Documentation/trace/ftrace.rst +++ b/Documentation/trace/ftrace.rst @@ -233,6 +233,12 @@ of ftrace. Here is a list of some of the key files: This interface also allows for commands to be used. See the "Filter commands" section for more details. + As a speed up, since processing strings can't be quite expensive + and requires a check of all functions registered to tracing, instead + an index can be written into this file. A number (starting with "1") + written will instead select the same corresponding at the line position + of the "available_filter_functions" file. + set_ftrace_notrace: This has an effect opposite to that of @@ -2835,6 +2841,38 @@ Produces:: We can see that there's no more lock or preempt tracing. +Selecting function filters via index +------------------------------------ + +Because processing of strings is expensive (the address of the function +needs to be looked up before comparing to the string being passed in), +an index can be used as well to enable functions. This is useful in the +case of setting thousands of specific functions at a time. By passing +in a list of numbers, no string processing will occur. Instead, the function +at the specific location in the internal array (which corresponds to the +functions in the "available_filter_functions" file), is selected. + +:: + + # echo 1 > set_ftrace_filter + +Will select the first function listed in "available_filter_functions" + +:: + + # head -1 available_filter_functions + trace_initcall_finish_cb + + # cat set_ftrace_filter + trace_initcall_finish_cb + + # head -50 available_filter_functions | tail -1 + x86_pmu_commit_txn + + # echo 1 50 > set_ftrace_filter + # cat set_ftrace_filter + trace_initcall_finish_cb + x86_pmu_commit_txn Dynamic ftrace with the function graph tracer --------------------------------------------- |