sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

Peter Zijlstra · Ingo Molnar · commit 173be9a14f7b · 2016-08-18T10:48:46.000+02:00
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 <umgwanakikbuti@gmail.com> Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Fredrik Markstrom <fredrik.markstrom@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Radim <rkrcmar@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Stephane Eranian <eranian@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vince Weaver <vincent.weaver@maine.edu> Cc: Wanpeng Li <wanpeng.li@hotmail.com> Cc: stable@vger.kernel.org # 4.3+ Fixes: 9d7fb04 ("sched/cputime: Guarantee stime + utime == rtime") Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
@@ -614,19 +614,25 @@ static void cputime_adjust(struct task_cputime *curr,
 	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.
@@ -649,7 +655,6 @@ static void cputime_adjust(struct task_cputime *curr,
 		stime = rtime - utime;
 	}
 
-update:
 	prev->stime = stime;
 	prev->utime = utime;
 out:
