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SubjectRe: [patch] sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression
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On Wed, 2016-08-10 at 14:30 +0200, Peter Zijlstra wrote:
> On Wed, Aug 10, 2016 at 01:14:29PM +0200, Mike Galbraith wrote:

> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -608,11 +608,13 @@ static void cputime_adjust(struct task_c
> >
> > if (utime == 0) {
> > stime = rtime;
> > + utime = prev->utime;
> > goto update;
> > }
> >
> > if (stime == 0) {
> > utime = rtime;
> > + stime = prev->stime;
> > goto update;
> > }
>
> This cannot be right; it violates that utime+stime==rtime. Let me try
> and figure out what actually happens.

How about this instead.

sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

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:

If ->sum_exec_runtime has moved beyond the rtime of ->prev_cputime, but
no time has as yet been accounted to the task, bail.

Fixes: 9d7fb0427648 ("sched/cputime: Guarantee stime + utime == rtime")
Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: stable@vger.kernel.org # 4.3+
---
kernel/sched/cputime.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -606,6 +606,13 @@ static void cputime_adjust(struct task_c
stime = curr->stime;
utime = curr->utime;

+ /*
+ * sum_exec_runtime has moved, but nothing has yet been
+ * accounted to the task, there's nothing to update.
+ */
+ if (utime + stime == 0)
+ goto out;
+
if (utime == 0) {
stime = rtime;
goto update;
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