Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758075Ab0LBTpt (ORCPT ); Thu, 2 Dec 2010 14:45:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36430 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758042Ab0LBTpq (ORCPT ); Thu, 2 Dec 2010 14:45:46 -0500 Date: Thu, 2 Dec 2010 14:41:29 -0500 From: Rik van Riel To: kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Avi Kiviti , Srivatsa Vaddagiri , Peter Zijlstra , Ingo Molnar , Anthony Liguori Subject: [RFC PATCH 0/3] directed yield for Pause Loop Exiting Message-ID: <20101202144129.4357fe00@annuminas.surriel.com> Organization: Red Hat, Inc. Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1428 Lines: 33 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. Scheduler people, please flame me with anything I may have done wrong, so I can do it right for a next version :) -- All rights reversed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/