linux-kernel - Re: [PATCH v3 3/7] sched: throttle cfs_rq entities which exceed their local quota

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Date:	Thu, 14 Oct 2010 11:59:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	bharata@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>,
	Paul Turner <pjt@...gle.com>, Nikhil Rao <ncrao@...gle.com>
Subject: Re: [PATCH v3 3/7] sched: throttle cfs_rq entities which exceed
 their local quota

On Thu, 2010-10-14 at 18:50 +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 14 Oct 2010 11:12:22 +0200
> Peter Zijlstra <peterz@...radead.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.


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