tracing/histogram: Document expression arithmetic and constants

JIRA: https://issues.redhat.com/browse/RHEL-67679

commit 2d2f6d4
Author: Kalesh Singh <kaleshsingh@google.com>
Date:   Mon Oct 25 13:08:40 2021 -0700

    tracing/histogram: Document expression arithmetic and constants

    Histogram expressions now support division, and multiplication in
    addition to the already supported subtraction and addition operators.

    Numeric constants can also be used in a hist trigger expressions
    or assigned to a variable and used by refernce in an expression.

    Link: https://lkml.kernel.org/r/20211025200852.3002369-9-kaleshsingh@google.com

    Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
    Reviewed-by: Namhyung Kim <namhyung@kernel.org>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Signed-off-by: Tomas Glozar <tglozar@redhat.com>

Author: lenticularis39

Documentation/trace/histogram.rst

@@ -1762,6 +1762,20 @@ using the same key and variable from yet another event::
 
   # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
 
+Expressions support the use of addition, subtraction, multiplication and
+division operators (+-*/).
+
+Note that division by zero always returns -1.
+
+Numeric constants can also be used directly in an expression::
+
+  # echo 'hist:keys=next_pid:timestamp_secs=common_timestamp/1000000 ...' >> event/trigger
+
+or assigned to a variable and referenced in a subsequent expression::
+
+  # echo 'hist:keys=next_pid:us_per_sec=1000000 ...' >> event/trigger
+  # echo 'hist:keys=next_pid:timestamp_secs=common_timestamp/$us_per_sec ...' >> event/trigger
+
 2.2.2 Synthetic Events
 ----------------------