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

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Date:	Fri, 03 Dec 2010 12:33:29 -0500
From:	Rik van Riel <riel@...hat.com>
To:	vatsa@...ux.vnet.ibm.com
CC:	Mike Galbraith <efault@....de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Avi Kiviti <avi@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 12:29 PM, Srivatsa Vaddagiri wrote:
> On Fri, Dec 03, 2010 at 12:09:01PM -0500, Rik van Riel wrote:
>> I don't see how that is going to help get the lock
>> released, when the VCPU holding the lock is on another
>> CPU.
>
> Even the directed yield() is not guaranteed to get the lock released, given its
> shooting in the dark?

True, that's a fair point.

> Anyway, the intention of yield() proposed was not to get lock released
> immediately (which will happen eventually), but rather to avoid inefficiency
> associated with (long) spinning and at the same time make sure we are not
> leaking our bandwidth to other guests because of a naive yield ..

A KVM guest can run on the host alongside short-lived
processes, though.  How can we ensure that a VCPU that
donates time gets it back again later, when the task
time was donated to may no longer exist?

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