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 02:58:33 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	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>,
	Nikhil Rao <ncrao@...gle.com>
Subject: Re: [PATCH v3 3/7] sched: throttle cfs_rq entities which exceed their
 local quota

On Thu, Oct 14, 2010 at 2:12 AM, 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.
>

This is a concern (especially for exit starvation, since the task
needs to be scheduled for reaping); the way throttling is enacted
should help to mitigate the risk / starvation here.  When a group
exceeds its bandwidth we don't actively force it off the cpu, we only
set TIF_RESCHED; this the "enforcement" of throttling until we drop
back down through put_prev_task().

This should mean we won't extend a semaphore wait time unless someone
explicitly issues something a cond_resched() within it [at which point
they are choosing a potentially arbitrary latency delay anyway,
although this does expand it relative to the target
sched_latency_period].

>
>
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