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

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Message-ID: <20130402022556.GD16699@lge.com>
Date:	Tue, 2 Apr 2013 11:25:56 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.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()

Hello, Preeti.

On Mon, Apr 01, 2013 at 12:36:52PM +0530, Preeti U Murthy wrote:
> 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.

Ah.. Now I get your point. Yes, you are right and it may be good thing.
With that property, all tasks in the system can be scheduled at least once
in sysctl_sched_latency. sysctl_sched_latency is system-wide configuration,
so my patch may be wrong. With my patch, all tasks in the system cannot be
scheduled at least once in sysctl_sched_latency. Instead, it schedule
all tasks in cfs_rq at least once in sysctl_sched_latency if there is
no other tgs.

I think that it is real problem that sysctl_sched_min_granularity is not
guaranteed for each task.
Instead of this patch, how about considering low bound?

if (slice < sysctl_sched_min_granularity)
	slice = sysctl_sched_min_granularity;

Thanks.

> 
> Regards
> Preeti U Murthy
> 
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