linux-kernel - Re: [RFC PATCH 2/3] sched: add yield

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Date:	Fri, 10 Dec 2010 14:09:06 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Avi Kiviti <avi@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 2/3] sched: add yield_to function

On Thu, Dec 09, 2010 at 11:34:46PM -0500, Rik van Riel wrote:
> On 12/03/2010 09:06 AM, Srivatsa Vaddagiri wrote:
> >On Fri, Dec 03, 2010 at 03:03:30PM +0100, Peter Zijlstra wrote:
> >>No, because they do receive service (they spend some time spinning
> >>before being interrupted), so the respective vruntimes will increase, at
> >>some point they'll pass B0 and it'll get scheduled.
> >
> >Is that sufficient to ensure that B0 receives its fair share (1/3 cpu in this
> >case)?
> 
> I have a rough idea for a simpler way to ensure
> fairness.
> 
> At yield_to time, we could track in the runqueue
> structure that a task received CPU time (and on
> the other runqueue that a task donated CPU time).
> 
> The balancer can count time-given-to CPUs as
> busier, and donated-time CPUs as less busy,
> moving tasks away in the unlikely event that
> the same task gets keeping CPU time given to
> it.

I think just capping donation (either on send side or receive side) may be more 
simpler here than to mess with load balancer logic.

> Conversely, it can move other tasks onto CPUs
> that have tasks on them that cannot make progress
> right now and are just donating their CPU time.
> 
> Most of the time the time-given and time-received
> should balance out and there should be little to
> no influence on the load balancer. This code would
> just be there to deal with exceptional circumstances,
> to avoid the theoretical worst case people have
> described.

- vatsa
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