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

From: Avi Kivity
Date: Tue Jan 04 2011 - 04:14:52 EST

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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