From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dong Liu Subject: Re: cpu stall and hyperthread Date: Fri, 06 Jul 2012 13:32:24 -0400 Message-ID: <4FF72128.2070804@gmail.com> References: <4FEBCDDE.60503@gmail.com> <4FF68396.2010904@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit To: linux-rt-users@vger.kernel.org Return-path: Received: from mail-ey0-f174.google.com ([209.85.215.174]:58411 "EHLO mail-ey0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757110Ab2GFRc3 (ORCPT ); Fri, 6 Jul 2012 13:32:29 -0400 Received: by eaak11 with SMTP id k11so3741332eaa.19 for ; Fri, 06 Jul 2012 10:32:28 -0700 (PDT) In-Reply-To: <4FF68396.2010904@gmail.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: I think I narrowed down this problem a little bit. When I set RCU_BOOST, it was quite easy to generate cpu stall, but the system is not really hung. So I turned off the cpu stall checking by doing echo "1" > /sys/module/rcutree/parameters/rcu_cpu_stall_suppress at boot time. Then I tried to start VMs. Sometime VM started fine, sometime VM hung, I need to destroied. When the starting of VM hung, the WCHAN of qemu-kvm was wait_rcu_gp. And when I did "virsh destroy id", the VM also hung, qemu-kvm was stuck at rcu_barrier. It really has something to do with KVM. Dong On 7/6/12 2:20 AM, Dong Liu wrote: > I can quite reliably trigger this cpu stall error now. Just try to start > several KVM guests. > > It seems the kvm progress uses too much CPU? > > Since ps -efl showed that the priority of the kvm kernel thread was -20. > So I tried, > > CONFIG_RCU_BOOST=y > CONFIG_RCU_BOOST_PRIO=50 > CONFIG_RCU_BOOST_DELAY=500 > > But I still got the same error :( > > Are there any way I can use to narrow down this error? > > Is it possible to non-preemptible RCU in PREEMPT_RT kernel? I know it > will reduce the system's RT performance, but we can make sure the reason > of this cpu stall error is the preemptible RCU > > Thanks, > > Dong > > On 6/27/12 11:22 PM, Dong Liu wrote: >> Hi, >> >> I'm running 3.4.3-rt11 and 3.2.18-rt29 on a system with Intel I7 860, I >> noticed that with hyperthread enabled, I'm getting kernel error >> >> rcu_preempt detected stalls on CPUs/tasks: { 3} (detected by 5, t=60002 >> jiffies) >> >> But with hyperthread disabled, no such error. >> >> Is preempt_rt not supposed to be used with hyperthread? >> >> Thanks, >> >> Dong >> >> P.S. >