Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity

On 29 May 2014 16:04, Peter Zijlstra peterz@infradead.org wrote:

On Fri, May 23, 2014 at 05:53:04PM +0200, Vincent Guittot wrote:

...

@@ -7282,6 +7289,12 @@ static inline int nohz_kick_needed(struct rq *rq)

          if (nr_busy > 1)
                  goto need_kick_unlock;

•         if ((rq->cfs.h_nr_running >= 1)
  

•          && ((rq->cpu_power * sd->imbalance_pct) <
  

•                                 (rq->cpu_power_orig * 100)))
  

•                 goto need_kick_unlock;
  

• }
  
  sd = rcu_dereference(per_cpu(sd_asym, cpu));
  

So what happens when a cpu is consistently low on power (say due to a pinned RT task) the balancer would quickly adjust the load level, but this would endlessly kick things into action, even though we're balanced just fine.

If there is more than 1 running task or more than 1 busy CPU, we will kick the ilb because of the former conditions. Then, if there is only 1 task and no other busy cpu, we should trig the ILB. Nevertheless, I can add a test to check that there is an idle cpu in the sched_domain