From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753365Ab2I0KAN (ORCPT ); Thu, 27 Sep 2012 06:00:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:20936 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751468Ab2I0KAK (ORCPT ); Thu, 27 Sep 2012 06:00:10 -0400 Date: Thu, 27 Sep 2012 11:58:24 +0200 From: Gleb Natapov To: Avi Kivity 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 Message-ID: <20120927095824.GJ23096@redhat.com> References: <505C654B.2050106@redhat.com> <505CA2EB.7050403@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50641D84.2020807@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 27, 2012 at 11:33:56AM +0200, Avi Kivity wrote: > On 09/27/2012 11:11 AM, Gleb Natapov wrote: > >> > >> User return notifier is per-cpu, not per-task. There is a new task_work > >> () that does what you want. With these > >> technicalities out of the way, I think it's the wrong idea. If a vcpu > >> thread is in userspace, that doesn't mean it's preempted, there's no > >> point in boosting it if it's already running. > >> > > Ah, so you want to set bit in kvm->preempted_vcpus if task is _not_ > > TASK_RUNNING in sched_out (you wrote opposite in your email)? If a task > > is in userspace it is definitely not preempted. > > No, as I originally wrote. If it's TASK_RUNNING when it saw sched_out, > then it is preempted (i.e. runnable), not sleeping on some waitqueue, > voluntarily (HLT) or involuntarily (page fault). > Of course, I got it all backwards. Need more coffee. > > > >> 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? -- Gleb.