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

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Message-ID: <20150815091523.GB10304@worktop.programming.kicks-ass.net>
Date:	Sat, 15 Aug 2015 11:15:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, vincent.guittot@...aro.org,
	daniel.lezcano@...aro.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	yuyang.du@...el.com, mturquette@...libre.com, rjw@...ysocki.net,
	Juri Lelli <Juri.Lelli@....com>, sgurrappadi@...dia.com,
	pang.xunlei@....com.cn, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [RFCv5 PATCH 34/46] sched: Enable idle balance to pull single
 task towards cpu with higher capacity

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
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