linux-kernel - Re: [RFC -v3 PATCH 2/3] sched: add yield

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Message-ID: <4D235E9D.1040603@redhat.com>
Date:	Tue, 04 Jan 2011 12:53:33 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Hillf Danton <dhillf@...il.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Avi Kiviti <avi@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [RFC -v3 PATCH 2/3] sched: add yield_to function

On 01/04/2011 12:08 PM, Peter Zijlstra wrote:
> On Wed, 2011-01-05 at 00:51 +0800, Hillf Danton wrote:
>> Where is the yield_to callback in the patch for RT schedule class?
>> If @p is RT, what could you do?
>
> RT guests are a pipe dream, you first need to get the hypervisor (kvm in
> this case) to be RT, which it isn't. Then you either need to very
> statically set-up the host and the guest scheduling constraints (not
> possible with RR/FIFO) or have a complete paravirt RT scheduler which
> communicates its requirements to the host.

There's a limited use case.

One host can have a few RT guests, say a host with 8 CPUs
can have up to 6 or 7 RT VCPUs.  Those guests get top
priority.

The host can then have some other, low priority, guests
that scavenge remaining CPU time.

In this case, no yield_to is required for RT guests,
because they do not do overcommit.

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