linux-kernel - Re: [RFCv3 PATCH 42/48] sched: Introduce energy awareness into find_busiest

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Message-ID: <5511A741.4080203@arm.com>
Date:	Tue, 24 Mar 2015 18:04:49 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFCv3 PATCH 42/48] sched: Introduce energy awareness into find_busiest_queue

On 24/03/15 15:21, Peter Zijlstra wrote:
> On Wed, Feb 04, 2015 at 06:31:19PM +0000, Morten Rasmussen wrote:
>> +++ b/kernel/sched/fair.c
>> @@ -7216,6 +7216,37 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>>   	unsigned long busiest_load = 0, busiest_capacity = 1;
>>   	int i;
>>
>> +	if (env->use_ea) {
>> +		struct rq *costliest = NULL;
>> +		unsigned long costliest_usage = 1024, costliest_energy = 1;
>> +
>> +		for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
>> +			unsigned long usage = get_cpu_usage(i);
>> +			struct rq *rq = cpu_rq(i);
>> +			struct sched_domain *sd = rcu_dereference(rq->sd);
>> +			struct energy_env eenv = {
>> +				.sg_top = sd->groups,
>> +				.usage_delta    = 0,
>> +				.src_cpu        = -1,
>> +				.dst_cpu        = -1,
>> +			};
>> +			unsigned long energy = sched_group_energy(&eenv);
>> +
>> +			/*
>> +			 * We're looking for the minimal cpu efficiency
>> +			 * min(u_i / e_i), crosswise multiplication leads to
>> +			 * u_i * e_j < u_j * e_i with j as previous minimum.
>> +			 */
>> +			if (usage * costliest_energy < costliest_usage * energy) {
>> +				costliest_usage = usage;
>> +				costliest_energy = energy;
>> +				costliest = rq;
>> +			}
>> +		}
>> +
>> +		return costliest;
>> +	}
>> +
>>   	for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
>>   		unsigned long capacity, wl;
>>   		enum fbq_type rt;
>
> So I've thought about parametrizing the whole load balance thing to
> avoid things like this.
>
> Irrespective of whether we balance on pure load or another metric we
> have the same structure, only different units plugged in.
>
> I've not really spend too much time on it to see what it would look
> like, but I think it would be a good avenue to investigate to avoid
> patches like this.

Yes, although I tried to keep the EAS specific code in lb small, w/o 
such an abstraction the code becomes very quickly pretty ugly.

So far I see the following parameters for such a 'unit':

conv. CFS vs. EAS:

(weighted) load vs. (cpu) usage
(cpu) capacity vs. (group) energy
(per task) imbalance vs. (per task) energy diff

For me this 'unit' is very close to the existing struct lb_env. Not sure 
yet what to do with s[dg]_lb_stats here? So far they obviously only 
contain data necessary for conv. CFS.

But I assume some of the if/else conditions in the lb code path 
(load_balance, fbg, update_s[dg]_lb_stats, fbq, detach_tasks, acive lb) 
will stay 'unit' specific.

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