From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC -v3 PATCH 0/3] directed yield for Pause Loop Exiting Date: Tue, 04 Jan 2011 11:14:33 +0200 Message-ID: <4D22E4F9.4060909@redhat.com> References: <20110103162637.29f23c40@annuminas.surriel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Srivatsa Vaddagiri , Peter Zijlstra , Mike Galbraith , Chris Wright To: Rik van Riel Return-path: In-Reply-To: <20110103162637.29f23c40@annuminas.surriel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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