Commit 173be9a1 authored Aug 15, 2016 by Peter Zijlstra Committed by Ingo Molnar Aug 18, 2016

sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression



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: 9d7fb042 ("sched/cputime: Guarantee stime + utime == rtime")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 3684b03d

kernel/sched/cputime.c

+10 −5

Original line number	Diff line number	Diff line
		@@ -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:

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