Re: [patch] sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

[Peter Zijlstra]

On Wed, Aug 10, 2016 at 02:30:33PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 10, 2016 at 01:14:29PM +0200, Mike Galbraith wrote:
> > Hi Peter,
> > 
> > While running ltp, the fates decided it was time for me to encounter
> > the roughly 1 out of every 10 call failure below.  As much as I run
> > ltp, I'm a bit surprised that I (or anyone else) haven't met this
> > before, but then the fates are known to be a tad fickle.
> > 
> > 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:
> > 
> > When applying the full rtime to either stime or utime, do not overwrite
> > the previously tallied value.
> > 
> > Fixes: 9d7fb0427648 ("sched/cputime: Guarantee stime + utime == rtime")
> > Signed-off-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 4.3+
> > ---
> >  kernel/sched/cputime.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > --- 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.

Any idea where your [us]time are coming from? Do you end up in the
vtime_accounting_enabled() path or not?