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:
