linux-kernel - [PATCH] Correct documentation for /proc/schedstat

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Message-Id: <20190724185029.26822-1-indigo@bitglue.com>
Date:   Wed, 24 Jul 2019 11:50:27 -0700
From:   Phil Frost <indigo@...glue.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Ingo Molnar <mingo@...e.hu>, trivial@...nel.org,
        Phil Frost <indigo@...glue.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] Correct documentation for /proc/schedstat

Commit 425e0968a25fa3f111f9919964cac079738140b5 ("sched: move code into
kernel/sched_stats.h") appears to have inadvertently changed the unit of
time from jiffies to nanoseconds as part of the implementation of CFS.

Signed-off-by: Phil Frost <indigo@...glue.com>
---
 Documentation/scheduler/sched-stats.txt | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/scheduler/sched-stats.txt b/Documentation/scheduler/sched-stats.txt
index 8259b34a66ae..b6c1807a01b3 100644
--- a/Documentation/scheduler/sched-stats.txt
+++ b/Documentation/scheduler/sched-stats.txt
@@ -19,6 +19,11 @@ are no architectures which need more than three domain levels. The first
 field in the domain stats is a bit map indicating which cpus are affected
 by that domain.
 
+2.6.23 introduced the CFS scheduler, and also an inadvertent
+backwards-incompatible change to the statistics. Although the schedstat version
+is 14 in either case, in 2.6.23 and later, counters accumulate time in
+nanoseconds. Prior to that, jiffies.
+
 These fields are counters, and only increment.  Programs which make use
 of these will need to start with a baseline observation and then calculate
 the change in the counters at each subsequent observation.  A perl script
@@ -48,9 +53,10 @@ Next two are try_to_wake_up() statistics:
      6) # of times try_to_wake_up() was called to wake up the local cpu
 
 Next three are statistics describing scheduling latency:
-     7) sum of all time spent running by tasks on this processor (in jiffies)
+     7) sum of all time spent running by tasks on this processor (in
+        nanoseconds, or jiffies prior to 2.6.23)
      8) sum of all time spent waiting to run by tasks on this processor (in
-        jiffies)
+        nanoseconds, or jiffies prior to 2.6.23)
      9) # of timeslices run on this cpu
 
 
-- 
2.20.1 (Apple Git-117)