tracing: Add documentation for trace commands mod, traceon/traceoff

	to be traced. Echoing names of functions into this file

	will limit the trace to only those functions.

	This interface also allows for commands to be used. See the

	"Filter commands" section for more details.

  set_ftrace_notrace:

	This has an effect opposite to that of

 echo > set_graph_function

Filter commands

---------------

A few commands are supported by the set_ftrace_filter interface.

Trace commands have the following format:

<function>:<command>:<parameter>

The following commands are supported:

- mod

  This command enables function filtering per module. The

  parameter defines the module. For example, if only the write*

  functions in the ext3 module are desired, run:

   echo 'write*:mod:ext3' > set_ftrace_filter

  This command interacts with the filter in the same way as

  filtering based on function names. Thus, adding more functions

  in a different module is accomplished by appending (>>) to the

  filter file. Remove specific module functions by prepending

  '!':

   echo '!writeback*:mod:ext3' >> set_ftrace_filter

- traceon/traceoff

  These commands turn tracing on and off when the specified

  functions are hit. The parameter determines how many times the

  tracing system is turned on and off. If unspecified, there is

  no limit. For example, to disable tracing when a schedule bug

  is hit the first 5 times, run:

   echo '__schedule_bug:traceoff:5' > set_ftrace_filter

  These commands are cumulative whether or not they are appended

  to set_ftrace_filter. To remove a command, prepend it by '!'

  and drop the parameter:

   echo '!__schedule_bug:traceoff' > set_ftrace_filter

trace_pipe

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