From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755273Ab2I0KPc (ORCPT ); Thu, 27 Sep 2012 06:15:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56080 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751645Ab2I0KPb (ORCPT ); Thu, 27 Sep 2012 06:15:31 -0400 Message-ID: <5064272F.8070805@redhat.com> Date: Thu, 27 Sep 2012 12:15:11 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Gleb Natapov CC: Raghavendra K T , Peter Zijlstra , Rik van Riel , "H. Peter Anvin" , Ingo Molnar , Marcelo Tosatti , Srikar , "Nikunj A. Dadhania" , KVM , Jiannan Ouyang , chegu vinod , "Andrew M. Theurer" , LKML , Srivatsa Vaddagiri Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler References: <50607F1F.2040704@redhat.com> <5060851E.1030404@redhat.com> <506166B4.4010207@linux.vnet.ibm.com> <5061713D.5060406@redhat.com> <20120927074405.GE23096@redhat.com> <50641569.9060305@redhat.com> <20120927091112.GG23096@redhat.com> <50641D84.2020807@redhat.com> <20120927095824.GJ23096@redhat.com> <506424CA.600@redhat.com> <20120927100806.GK23096@redhat.com> In-Reply-To: <20120927100806.GK23096@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/27/2012 12:08 PM, Gleb Natapov wrote: > On Thu, Sep 27, 2012 at 12:04:58PM +0200, Avi Kivity wrote: >> On 09/27/2012 11:58 AM, Gleb Natapov wrote: >> > >> >> > >> >> >> btw, we can have secondary effects. A vcpu can be waiting for a lock in >> >> >> the host kernel, or for a host page fault. There's no point in boosting >> >> >> anything for that. Or a vcpu in userspace can be waiting for a lock >> >> >> that is held by another thread, which has been preempted. >> >> > Do you mean userspace spinlock? Because otherwise task that's waits on >> >> > a kernel lock will sleep in the kernel. >> >> >> >> I meant a kernel mutex. >> >> >> >> vcpu 0: take guest spinlock >> >> vcpu 0: vmexit >> >> vcpu 0: spin_lock(some_lock) >> >> vcpu 1: take same guest spinlock >> >> vcpu 1: PLE vmexit >> >> vcpu 1: wtf? >> >> >> >> Waiting on a host kernel spinlock is not too bad because we expect to be >> >> out shortly. Waiting on a host kernel mutex can be a lot worse. >> >> >> > We can't do much about it without PV spinlock since there is not >> > information about what vcpu holds which guest spinlock, no? >> >> It doesn't help. If the lock holder is waiting for another lock in the >> host kernel, boosting it doesn't help even if we know who it is. We >> need to boost the real lock holder, but we have no idea who it is (and >> even if we did, we often can't do anything about it). >> > Without PV lock we will boost random preempted vcpu instead of going to > sleep in the situation you described. True. In theory boosting a random vcpu shouldn't have any negative effects though. Right now the problem is that the boosting itself is expensive. -- error compiling committee.c: too many arguments to function