Re: [RFCv5 PATCH 34/46] sched: Enable idle balance to pull single task towards cpu with higher capacity

Peter Zijlstra
2015-08-15T19:51:36Z

On Tue, Jul 07, 2015 at 07:24:17PM +0100, Morten Rasmussen wrote:
> +++ b/kernel/sched/fair.c
> @@ -7569,6 +7569,13 @@ static int need_active_balance(struct lb_env *env)
> return 1;
> }
>
> + if ((capacity_of(env->src_cpu) < capacity_of(env->dst_cpu)) &&
> + env->src_rq->cfs.h_nr_running == 1 &&
> + cpu_overutilized(env->src_cpu) &&
> + !cpu_overutilized(env->dst_cpu)) {
> + return 1;
> + }
> +
> return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
> }

Doesn't this allow for a nice game of ping-pong? Where if a task runs on
CPU X and generates interrupts there, its capacity will lower and we'll
migrate it over to CPU Y because that isn't receiving interrupts.

Now the task is running on Y, will generate interrupts there, and sees X
as a more attractive destination.

goto 1