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

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Message-ID: <4D01ADE6.3010508@redhat.com>
Date:	Thu, 09 Dec 2010 23:34:46 -0500
From:	Rik van Riel <riel@...hat.com>
To:	vatsa@...ux.vnet.ibm.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 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.

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.

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