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

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Date:	Thu, 2 Dec 2010 14:41:22 -0800
From:	Chris Wright <chrisw@...s-sol.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kiviti <avi@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

* Rik van Riel (riel@...hat.com) 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.

Seems like simply increasing the spin window help in that case?  Or is
it just too contended a lock (I think they use mcs locks, so I can see a
single wrong sleep causing real contention problems).

> 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.
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