linux-kernel - Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

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Message-ID: <20100803051659.GB29526@linux.vnet.ibm.com>
Date:	Tue, 3 Aug 2010 10:46:59 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	npiggin@...e.de, kvm@...r.kernel.org, bharata@...ibm.com,
	Balbir Singh <balbir@...ibm.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

On Mon, Aug 02, 2010 at 11:40:23AM +0300, Avi Kivity wrote:
> >>Can you do a directed yield?
> >We don't have that support yet in Linux scheduler.
> 
> If you think it's useful, it would be good to design it into the
> interface, and fall back to ordinary yield if the host doesn't
> support it.
> 
> A big advantage of directed yield vs yield is that you conserve
> resources within a VM; a simple yield will cause the guest to drop
> its share of cpu to other guest.

Hmm .. I see possibility of modifying yield to reclaim its "lost" timeslice when
its scheduled next as well. Basically remember what timeslice we have given
up and add that as its "bonus" when it runs next. That would keep the dynamics
of yield donation/reclaim local to the (physical) cpu and IMHO is less complex
than dealing with directed yield between tasks located across different physical
cpus. That would also address the fairness issue with yield you are pointing at?

- vatsa
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