linux-kernel - Re: [RFC -v3 PATCH 0/3] directed yield for Pause Loop Exiting

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Message-ID: <4D22E4F9.4060909@redhat.com>
Date:	Tue, 04 Jan 2011 11:14:33 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rik van Riel <riel@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [RFC -v3 PATCH 0/3] directed yield for Pause Loop Exiting

On 01/03/2011 11:26 PM, Rik van Riel wrote:
> When running SMP virtual machines, it is possible for one VCPU to be
> spinning on a spinlock, while the VCPU that holds the spinlock is not
> currently running, because the host scheduler preempted it to run
> something else.
>
> Both Intel and AMD CPUs have a feature that detects when a virtual
> CPU is spinning on a lock and will trap to the host.
>
> The current KVM code sleeps for a bit whenever that happens, which
> results in eg. a 64 VCPU Windows guest taking forever and a bit to
> boot up.  This is because the VCPU holding the lock is actually
> running and not sleeping, so the pause is counter-productive.
>
> In other workloads a pause can also be counter-productive, with
> spinlock detection resulting in one guest giving up its CPU time
> to the others.  Instead of spinning, it ends up simply not running
> much at all.
>
> This patch series aims to fix that, by having a VCPU that spins
> give the remainder of its timeslice to another VCPU in the same
> guest before yielding the CPU - one that is runnable but got
> preempted, hopefully the lock holder.
>

Looks great.

Assuming there are no objections, Mike, can you get 2/3 into a 
fast-forward-only branch of tip.git?  I'll then merge it into kvm.git 
and apply 1/3 and 2/3.

Since the merge window is about to open I'd like to merge this for 
2.6.39 so it gets to stew a while in tip.git, kvm.git, and linux-next.

-- 
error compiling committee.c: too many arguments to function

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