linux-kernel - Re: [PATCH v9 08/10] sched: replace capacity

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Message-ID: <CAKfTPtBtdnHUYjbrUiqED7Mr2d4HGB1YcyARED6Zx+rskZRsBw@mail.gmail.com>
Date:	Mon, 24 Nov 2014 15:41:28 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v9 08/10] sched: replace capacity_factor by usage

On 21 November 2014 at 13:37, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Mon, Nov 03, 2014 at 04:54:45PM +0000, Vincent Guittot wrote:

[snip]

>>                  */
>>                 if (prefer_sibling && sds->local &&
>> -                   sds->local_stat.group_has_free_capacity)
>> -                       sgs->group_capacity_factor = min(sgs->group_capacity_factor, 1U);
>> +                   group_has_capacity(env, &sds->local_stat) &&
>> +                   (sgs->sum_nr_running > 1)) {
>> +                       sgs->group_no_capacity = 1;
>> +                       sgs->group_type = group_overloaded;
>> +               }
>
> I'm still a bit confused about SD_PREFER_SIBLING. What is the flag
> supposed to do and why?

The goal is to spread tasks across the group even if the the latter is
not overloaded. for SMT level, the goal is to have 1 task per core
before 1 task per HW thread

>
> It looks like a weak load balancing bias attempting to consolidate tasks
> on domains with spare capacity. It does so by marking non-local groups
> as overloaded regardless of their actual load if the local group has
> spare capacity. Correct?
>
> In patch 9 this behaviour is enabled for SMT level domains, which
> implies that tasks will be consolidated in MC groups, that is we prefer
> multiple tasks on sibling cpus (hw threads). I must be missing something
> essential. I was convinced that we wanted to avoid using sibling cpus on
> SMT systems as much as possible?
>
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