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

Avi Kivity
2011-01-04T09:14:33Z

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.

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