Documentation/osnoise: Add osnoise/options documentation

Add the documentation about the osnoise/options file, along
with an explanation about the OSNOISE_WORKLOAD option.

Link: https://lkml.kernel.org/r/777af8f3d87beedd304805f98eff6c8291d64226.1668692096.git.bristot@kernel.org

Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Documentation/trace/osnoise-tracer.rst

@@ -109,6 +109,11 @@ The tracer has a set of options inside the osnoise directory, they are:
  - tracing_threshold: the minimum delta between two time() reads to be
    considered as noise, in us. When set to 0, the default value will
    be used, which is currently 5 us.
+ - osnoise/options: a set of on/off options that can be enabled by
+   writing the option name to the file or disabled by writing the option
+   name preceded with the 'NO_' prefix. For example, writing
+   NO_OSNOISE_WORKLOAD disables the OSNOISE_WORKLOAD option. The
+   special DEAFAULTS option resets all options to the default value.
 
 Additional Tracing
 ------------------
@@ -150,3 +155,10 @@ tracepoints is smaller than eight us reported in the sample_threshold.
 The reason roots in the overhead of the entry and exit code that happens
 before and after any interference execution. This justifies the dual
 approach: measuring thread and tracing.
+
+Running osnoise tracer without workload
+---------------------------------------
+
+By enabling the osnoise tracer with the NO_OSNOISE_WORKLOAD option set,
+the osnoise: tracepoints serve to measure the execution time of
+any type of Linux task, free from the interference of other tasks.