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Subject: Re: [PATCH v3 3/7] sched: throttle cfs_rq entities which exceed
 their local quota
From: Peter Zijlstra <peterz@infradead.org>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
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Date: Thu, 14 Oct 2010 11:59:55 +0200
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On Thu, 2010-10-14 at 18:50 +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 14 Oct 2010 11:12:22 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Wed, 2010-10-13 at 15:34 +0900, KAMEZAWA Hiroyuki wrote:
> > > cpu.share and bandwidth control can't be used simultaneously or...
> > > is this fair ? I'm not familiar with scheduler but this allows boost this tg.
> > > Could you add a brief documentaion of a spec/feature. in the next post ?
> > 
> > Like explained, shares control the proportional distribution of time
> > between groups, bandwidth puts a limit on how much time a group can
> > take. It can cause a group to receive less than its fair share, but
> > never more.
> > 
> > There is, however, a problem with all this, and that is that all this
> > explicit idling of tasks can lead to a form of priority inversion.
> > Regular preemptive scheduling already suffers from this, but explicitly
> > idling tasks exacerbates the situation.
> > 
> > You basically get to add the longest induced idle time to all your lock
> > hold times.
> > 
> 
> What is the user-visible difference of the problem between
>   1) limit share to be very small.
>   2) use throttole.
> 
> If share is used, lock-hodler's priority is boosted ?

No, both lead to the same problem, its just that this adds another
dimension to it.. and I'm fairly sure people won't realise this until it
bites them in the ass.