From 0792538e4b78afc28313ae6eeca72d3b3886e8fb Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Wed, 28 Nov 2012 17:00:57 +0100
Subject: [PATCH] --- yaml --- r: 339092 b: refs/heads/master c:
 fa09205783d11cc05122ad6e4ce06074624b2c0c h: refs/heads/master v: v3

---
 [refs]                       |  2 +-
 trunk/kernel/sched/cputime.c | 18 ++++++++++++++++--
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/[refs] b/[refs]
index addac7e2bb73..c15dcde13985 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: d37f761dbd276790f70dcf73a287fde2c3464482
+refs/heads/master: fa09205783d11cc05122ad6e4ce06074624b2c0c
diff --git a/trunk/kernel/sched/cputime.c b/trunk/kernel/sched/cputime.c
index 220fdc4db770..b7f731768625 100644
--- a/trunk/kernel/sched/cputime.c
+++ b/trunk/kernel/sched/cputime.c
@@ -516,6 +516,10 @@ static cputime_t scale_utime(cputime_t utime, cputime_t rtime, cputime_t total)
 	return (__force cputime_t) temp;
 }
 
+/*
+ * Adjust tick based cputime random precision against scheduler
+ * runtime accounting.
+ */
 static void cputime_adjust(struct task_cputime *curr,
 			   struct cputime *prev,
 			   cputime_t *ut, cputime_t *st)
@@ -524,8 +528,16 @@ static void cputime_adjust(struct task_cputime *curr,
 
 	utime = curr->utime;
 	total = utime + curr->stime;
+
 	/*
-	 * Use CFS's precise accounting:
+	 * Tick based cputime accounting depend on random scheduling
+	 * timeslices of a task to be interrupted or not by the timer.
+	 * Depending on these circumstances, the number of these interrupts
+	 * may be over or under-optimistic, matching the real user and system
+	 * cputime with a variable precision.
+	 *
+	 * Fix this by scaling these tick based values against the total
+	 * runtime accounted by the CFS scheduler.
 	 */
 	rtime = nsecs_to_cputime(curr->sum_exec_runtime);
 
@@ -535,7 +547,9 @@ static void cputime_adjust(struct task_cputime *curr,
 		utime = rtime;
 
 	/*
-	 * Compare with previous values, to keep monotonicity:
+	 * If the tick based count grows faster than the scheduler one,
+	 * the result of the scaling may go backward.
+	 * Let's enforce monotonicity.
 	 */
 	prev->utime = max(prev->utime, utime);
 	prev->stime = max(prev->stime, rtime - prev->utime);