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

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Date:	Thu, 29 May 2014 21:44:09 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity

On 29 May 2014 16:04, Peter Zijlstra <peterz@...radead.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
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