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

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Message-Id: <20101014190828.f835c5e2.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 14 Oct 2010 19:08:28 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, 14 Oct 2010 11:59:55 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> 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.
> 
Hmm, them, existing problem but this add a new pitfall.

What's your recomendation to make progess on this work ?

I think 1st step will be..
- explain the problem of priority inversion in cgroup+cfs documenation with
  !!CAUTION!!

I'm sorry I'm not sure there have been trials for fixing priority inversion
in the linux scheduler development.

Explaining my motivation, a user of this feature on my customer is virtual machine
rental service. So, some fuctionality as
"When vcpu holds spinlock in kernel, please don't sleep.." will be nice.
Is there patch already ?

Thanks,
-Kame










 

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