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

[Mike Galbraith]

On Mon, 2016-08-15 at 10:51 +0200, Peter Zijlstra wrote:
> On Wed, Aug 10, 2016 at 08:57:28PM +0200, Mike Galbraith wrote:
> >  
> > +> > 	> > /*
> > +> > 	> >  * 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;
> 
> urgh...
> 
> Valid scenario.. not sure about the solution though. This would mean the
> task has _no_ running time if it forever dodges the tick, which would be
> bad.
> 
> Does something like so cure things too?

Yeah, it's a happy camper.

> ---
>  kernel/sched/cputime.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 9858266fb0b3..2ee83b200504 100644
> --- 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: