From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler Date: Thu, 27 Sep 2012 11:33:56 +0200 Message-ID: <50641D84.2020807@redhat.com> References: <20120921115942.27611.67488.sendpatchset@codeblue> <20120921120000.27611.71321.sendpatchset@codeblue> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit 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 To: Gleb Natapov Return-path: In-Reply-To: <20120927091112.GG23096@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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). > >> 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. -- error compiling committee.c: too many arguments to function