On Tue, 2020-04-28 at 22:56 +0100, Valentin Schneider wrote: > On 28/04/20 06:02, Scott Wood wrote: > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index dfde7f0ce3db..e7437e4e40b4 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -9394,6 +9400,10 @@ static int should_we_balance(struct lb_env *env) > > struct sched_group *sg = env->sd->groups; > > int cpu, balance_cpu = -1; > > > > + /* Run the realtime task now; load balance later. */ > > + if (rq_has_runnable_rt_task(env->dst_rq)) > > + return 0; > > + > > I have a feeling this isn't very nice to CFS tasks, since we would now > "waste" load-balance attempts if they happen to coincide with an RT task > being runnable. > > On your 72 CPUs machine, the system-wide balance happens (at best) every > 72ms if you have idle time, every ~2300ms otherwise (every balance > CPU gets to try to balance however, so it's not as horrible as I'm making > it sound). This is totally worst-case scenario territory, and you'd hope > newidle_balance() could help here and there (as it isn't gated by any > balance interval). > > Still, even for a single rq, postponing a system-wide balance for a > full balance interval (i.e. ~2 secs worst case here) just because we had a > single RT task running when we tried to balance seems a bit much. > > It may be possible to hack something to detect those cases and reset the > interval to "now" when e.g. dequeuing the last RT task (& after having > previously aborted a load-balance due to RT/DL/foobar). Yeah, some way to retry at an appropriate time after aborting a rebalance would be good. > > + > > +/* Is there a task of a high priority class? */ > > +static inline bool rq_has_runnable_rt_task(struct rq *rq) > > +{ > > + return unlikely(rq->nr_running != rq->cfs.h_nr_running); > > Seeing as that can be RT, DL or stopper, that name is somewhat misleading. rq_has_runnable_rt_dl_task()? Or is there some term that unambiguously encompasses both? -Scott