[PATCH 4.4 063/113] sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@infradead.org>

commit 173be9a14f7b2e901cf77c18b1aafd4d672e9d9e upstream.

Mike reports:

 Roughly 10% of the time, ltp testcase getrusage04 fails:
 getrusage04    0  TINFO  :  Expected timers granularity is 4000 us
 getrusage04    0  TINFO  :  Using 1 as multiply factor for max [us]time increment (1000+4000us)!
 getrusage04    0  TINFO  :  utime:           0us; stime:         179us
 getrusage04    0  TINFO  :  utime:        3751us; stime:           0us
 getrusage04    1  TFAIL  :  getrusage04.c:133: stime increased > 5000us:

And tracked it down to the case where the task simply doesn't get
_any_ [us]time ticks.

Update the code to assume all rtime is utime when we lack information,
thus ensuring a task that elides the tick gets time accounted.

Reported-by: Mike Galbraith <redacted>
Tested-by: Mike Galbraith <redacted>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <redacted>
Cc: Fredrik Markstrom <redacted>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim <redacted>
Cc: Rik van Riel <redacted>
Cc: Stephane Eranian <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Vince Weaver <redacted>
Cc: Wanpeng Li <redacted>
Fixes: 9d7fb0427648 ("sched/cputime: Guarantee stime + utime == rtime")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/sched/cputime.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c

@@ -600,19 +600,25 @@ static void cputime_adjust(struct task_c
 	stime = curr->stime;
 	utime = curr->utime;
 
-	if (utime == 0) {
-		stime = rtime;
+	/*
+	 * If either stime or both stime and utime are 0, assume all runtime is
+	 * userspace. Once a task gets some ticks, the monotonicy code at
+	 * 'update' will ensure things converge to the observed ratio.
+	 */
+	if (stime == 0) {
+		utime = rtime;
 		goto update;
 	}
 
-	if (stime == 0) {
-		utime = rtime;
+	if (utime == 0) {
+		stime = rtime;
 		goto update;
 	}
 
 	stime = scale_stime((__force u64)stime, (__force u64)rtime,
 			    (__force u64)(stime + utime));
 
+update:
 	/*
 	 * Make sure stime doesn't go backwards; this preserves monotonicity
 	 * for utime because rtime is monotonic.

@@ -635,7 +641,6 @@ static void cputime_adjust(struct task_c
 		stime = rtime - utime;
 	}
 
-update:
 	prev->stime = stime;
 	prev->utime = utime;
 out: