linux-kernel - [PATCH v2 5/5] tracing/histogram: Document expression arithmetic and constants

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Date:   Tue, 19 Oct 2021 18:31:42 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     surenb@...gle.com, hridya@...gle.com, namhyung@...nel.org,
        kernel-team@...roid.com, Kalesh Singh <kaleshsingh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: [PATCH v2 5/5] 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.

Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
Reviewed-by: Namhyung Kim <namhyung@...nel.org>
---
Changes in v2:
  - Add Namhyung's Reviewed-by

 Documentation/trace/histogram.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst
index 533415644c54..e12699abaee8 100644
--- a/Documentation/trace/histogram.rst
+++ b/Documentation/trace/histogram.rst
@@ -1763,6 +1763,20 @@ 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
 ----------------------
 
-- 
2.33.0.1079.g6e70778dc9-goog