linux-kernel - Re: [PATCH 4/5] sched: don't consider upper se in sched

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Date:	Mon, 01 Apr 2013 12:36:52 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 4/5] sched: don't consider upper se in sched_slice()

Hi Joonsoo,

On 04/01/2013 09:38 AM, Joonsoo Kim wrote:
> Hello, Preeti.
> 

>>
>> Ideally the children's cpu share must add upto the parent's share.
>>
> 
> I don't think so.
> 
> We should schedule out the parent tg if 5ms is over. As we do so, we can
> fairly distribute time slice to every tg within short term. If we add
> the children's cpu share upto the parent's, the parent tg may have
> large time slice, so it cannot be preempted easily. There may be a latency
> problem if there are many tgs.

In the case where the #running < sched_nr_latency, the children's
sched_slices add up to the parent's.

A rq with two tgs,each with 3 tasks.

Each of these tasks have a sched slice of
[(sysctl_sched_latency / 3) / 2] as of the present implementation.

The sum of the above sched_slice of all tasks of a tg will lead to the
sched_slice of its parent: sysctl_sched_latency / 2

This breaks when the nr_running on each tg > sched_nr_latency. However I
don't know if this is a good thing or a bad thing.

Regards
Preeti U Murthy

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