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

[Peter Zijlstra]

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.