tracing/filters: Document cpumask filtering

Cpumask, scalar and CPU fields can now be filtered by a user-provided
cpumask, document the syntax.

Link: https://lkml.kernel.org/r/[e-mail]

Cc: Masami Hiramatsu <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Juri Lelli <[e-mail]>
Cc: Daniel Bristot de Oliveira <[e-mail]>
Cc: Marcelo Tosatti <[e-mail]>
Cc: Leonardo Bras <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Signed-off-by: Valentin Schneider <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

Documentation/trace/events.rst

···
 The ".function" postfix can only be attached to values of size long, and can only
 be compared with "==" or "!=".
 
+Cpumask fields or scalar fields that encode a CPU number can be filtered using
+a user-provided cpumask in cpulist format. The format is as follows::
+
+  CPUS{$cpulist}
+
+Operators available to cpumask filtering are:
+
+& (intersection), ==, !=
+
+For example, this will filter events that have their .target_cpu field present
+in the given cpumask::
+
+  target_cpu & CPUS{17-42}
+
 5.2 Setting filters
 -------------------