ftrace: Allow enabling of filters via index of available_filter_functions
Enabling of large number of functions by echoing in a large subset of the
functions in available_filter_functions can take a very long time. The
process requires testing all functions registered by the function tracer
(which is in the 10s of thousands), and doing a kallsyms lookup to convert
the ip address into a name, then comparing that name with the string passed
in.
When a function causes the function tracer to crash the system, a binary
bisect of the available_filter_functions can be done to find the culprit.
But this requires passing in half of the functions in
available_filter_functions over and over again, which makes it basically a
O(n^2) operation. With 40,000 functions, that ends up bing 1,600,000,000
opertions! And enabling this can take over 20 minutes.
As a quick speed up, if a number is passed into one of the filter files,
instead of doing a search, it just enables the function at the corresponding
line of the available_filter_functions file. That is:
# echo 50 > set_ftrace_filter
# cat set_ftrace_filter
x86_pmu_commit_txn
# head -50 available_filter_functions | tail -1
x86_pmu_commit_txn
This allows setting of half the available_filter_functions to take place in
less than a second!
# time seq 20000 > set_ftrace_filter
real 0m0.042s
user 0m0.005s
sys 0m0.015s
# wc -l set_ftrace_filter
20000 set_ftrace_filter
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 6ce2763..7c5e6d6 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -233,6 +233,12 @@
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 @@
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
---------------------------------------------