Re: [PATCH 3/4] sched: drop group_capacity to 1 only if local group has extra capacity

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-10-15 at 09:13 -0700, Nikhil Rao wrote:

> >> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> >> index 0dd1021..da0c688 100644
> >> --- a/kernel/sched_fair.c
> >> +++ b/kernel/sched_fair.c
> >> @@ -2030,6 +2030,7 @@ struct sd_lb_stats {
> >>       unsigned long this_load;
> >>       unsigned long this_load_per_task;
> >>       unsigned long this_nr_running;
> >> +     unsigned long this_group_capacity;
> >>
> >>       /* Statistics of the busiest group */
> >>       unsigned long max_load;
> >> @@ -2546,15 +2547,18 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
> >>               /*
> >>                * In case the child domain prefers tasks go to siblings
> >>                * first, lower the sg capacity to one so that we'll try
> >> -              * and move all the excess tasks away.
> >> +              * and move all the excess tasks away. We lower capacity only
> >> +              * if the local group can handle the extra capacity.
> >>                */
> >> -             if (prefer_sibling)
> >> +             if (prefer_sibling && !local_group &&
> >> +                 sds->this_nr_running < sds->this_group_capacity)
> >>                       sgs.group_capacity = min(sgs.group_capacity, 1UL);
> >>
> >>               if (local_group) {
> >>                       sds->this_load = sgs.avg_load;
> >>                       sds->this = sg;
> >>                       sds->this_nr_running = sgs.sum_nr_running;
> >> +                     sds->this_group_capacity = sgs.group_capacity;
> >>                       sds->this_load_per_task = sgs.sum_weighted_load;
> >>               } else if (update_sd_pick_busiest(sd, sds, sg, &sgs, this_cpu)) {
> >>                       sds->max_load = sgs.avg_load;

OK, but then you assume that local_group will always be the first group
served, nor is there any purpose for adding sds->this_group_capacity,
you could keep that local to this function.

For regular balancing local_group will be the first, since we only
ascend the domain tree on the local groups. But its not true for no_hz
balancing afaikt.