Re: [RFCv3 PATCH 43/48] sched: Introduce energy awareness into detach_tasks

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: "pang.xunlei@zte.com.cn" <pang.xunlei@zte.com.cn>
Cc: Juri Lelli <Juri.Lelli@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-kernel-owner@vger.kernel.org" 
	<linux-kernel-owner@vger.kernel.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	Morten Rasmussen <Morten.Rasmussen@arm.com>,
	"mturquette@linaro.org" <mturquette@linaro.org>,
	"nico@linaro.org" <nico@linaro.org>,
	Peter Boonstoppel <pboonstoppel@nvidia.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"preeti@linux.vnet.ibm.com" <preeti@linux.vnet.ibm.com>,
	"rjw@rjwysocki.net" <rjw@rjwysocki.net>,
	Sai Gurrappadi <sgurrappadi@nvidia.com>,
	"vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
	"yuyang.du@intel.com" <yuyang.du@intel.com>
Subject: Re: [RFCv3 PATCH 43/48] sched: Introduce energy awareness into detach_tasks
Date: Thu, 30 Apr 2015 21:35:04 +0100	[thread overview]
Message-ID: <554291F8.1070700@arm.com> (raw)
In-Reply-To: <OFDCE15EEF.2F536D7F-ON48257E37.002565ED-48257E37.0027A8B9@zte.com.cn>

On 30/04/15 08:12, pang.xunlei@zte.com.cn wrote:
> linux-kernel-owner@vger.kernel.org wrote 2015-03-27 PM 11:03:24:
>> Re: [RFCv3 PATCH 43/48] sched: Introduce energy awareness into
> detach_tasks

[...]

>> > Hi Morten, Dietmar,
>> >
>> > Wouldn't the above energy_diff() use the 'old' value of dst_cpu's util?
>> > Tasks are detached/dequeued in this loop so they have their util
>> > contrib. removed from src_cpu but their contrib. hasn't been added to
>> > dst_cpu yet (happens in attach_tasks).
>>
>> You're absolutely right Sai. Thanks for pointing this out! I guess I
> rather
>> have to accumulate the usage of tasks I've detached and add this to the
>> eenv::usage_delta of the energy_diff() call for the next task.
>>
>> Something like this (only slightly tested):
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 8d4cc72f4778..d0d0e965fd0c 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6097,6 +6097,7 @@ static int detach_tasks(struct lb_env *env)
>>         struct task_struct *p;
>>         unsigned long load = 0;
>>         int detached = 0;
>> +       int usage_delta = 0;
>>  
>>         lockdep_assert_held(&env->src_rq->lock);
>>  
>> @@ -6122,16 +6123,19 @@ static int detach_tasks(struct lb_env *env)
>>                         goto next;
>>  
>>                 if (env->use_ea) {
>> +                       int util = task_utilization(p);
>>                         struct energy_env eenv = {
>>                                 .src_cpu = env->src_cpu,
>>                                 .dst_cpu = env->dst_cpu,
>> -                               .usage_delta = task_utilization(p),
>> +                               .usage_delta = usage_delta + util,
>>                         };
>>                         int e_diff = energy_diff(&eenv);
> 
> If any or total utilization of tasks detached exceeds the orig capacity,
> of src_cpu, should we bail out in case of performance plus avoiding
> reaching the tipping point easily(because in such cases, src_cpu tends
> to be ended up overutilized)?

Yes, correct. We have to limit the dst cpu from pulling  more than its
remaining capacity worth of usage. I already integrated a check that the
dst cpu is not over-utilized by taking the usage_delta.

> Also like what I just replied to "[RFCv4 PATCH 31/48]", when doing
> energy_diff() it should be allowed to exceed cur capacity if not
> reaching its original capacity(is capacity_of(cpu) better?).

True.

[...]