Patch "sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-cputime-fix-no_hz_full-getrusage-monotonicity-regression.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Zijlstra <[email protected]>
Date: Mon, 15 Aug 2016 18:38:42 +0200
Subject: sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

From: Peter Zijlstra <[email protected]>

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.

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

@@ -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:

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-mm-disable-preemption-during-cr3-read-write.patch queue-4.4/uprobes-x86-fix-rip-relative-handling-of-evex-encoded-instructions.patch queue-4.4/sched-nohz-fix-affine-unpinned-timers-mess.patch queue-4.4/sched-cputime-fix-no_hz_full-getrusage-monotonicity-regression.patch