From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andre Przywara Subject: Re: 2.6.35-rc1 regression with pvclock and smp guests Date: Wed, 28 Jul 2010 09:55:02 +0200 Message-ID: <4C4FE256.7000309@amd.com> References: <4C483F67.1010007@amd.com> <4C4BF96B.7010005@redhat.com> <4C4D4B8B.80006@amd.com> <4C4EAEFC.20207@redhat.com> <4C4EC7D1.6030708@amd.com> <4C4ECBC7.1070405@redhat.com> <4C4ECF2E.4070103@amd.com> <4C4ED257.40002@redhat.com> <4C4EE3B3.2090900@amd <4C4EE61F.3070908@redhat.com> <4C4EF361.4010303@amd.com> <4C4F54F5.6080400@amd.com> <4C4F9D55.1090804@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Cc: Avi Kivity , "glommer@redhat.com" , KVM list To: Zachary Amsden Return-path: Received: from va3ehsobe001.messaging.microsoft.com ([216.32.180.11]:16122 "EHLO VA3EHSOBE001.bigfish.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751173Ab0G1Hzj (ORCPT ); Wed, 28 Jul 2010 03:55:39 -0400 In-Reply-To: <4C4F9D55.1090804@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Zachary Amsden wrote: > On 07/27/2010 11:51 AM, Andre Przywara wrote: >> Andre Przywara wrote: >>> Avi Kivity wrote: >>>> On 07/27/2010 04:48 PM, Andre Przywara wrote: >>>>>> Wierd. Maybe the clock goes crazy. >>>>>> >>>>>> Let's see if it jumps forward alot: >>>>>> >>>>>> } while (unlikely(last != ret)); >>>>>> + >>>>>> + { >>>>>> + static u64 last_report; >>>>>> + if (ret > last_report + 10000) { >>>>>> + last_report = ret; >>>>>> + printk("kvmclock: %llx\n", ret); >>>>>> + } >>>>>> + >>>>>> + } >>>>>> >>>>>> return ret; >>>>>> } >>>>>> >>>>>> Worth updating the 'return last' to update ret and goto the new >>>>>> code, so we don't miss that path. >>>>> Did that. There is _a lot_ of output (about 350 lines per second >>>>> via the 115k serial console), both with smp=1 and smp=2. >>>>> The majority is differing about 2,000,000 (ticks?), but a handful >>>>> of them are in the range of 20 million. >>>> nanoseconds. So 2-20ms. Consistent with 350 lines/sec. >>>> >>>>> No difference between smp=2 and smp=1. >>>>> I also get some "BUG: recent printk recursion!" and I don't see any >>>>> kernel boot progress beyond outputting the BogoMIPS value. >>>> Right, printk() wants the time too. >>>> >>>>> BTW: I found two message from your earlier debug statement: >>>>> [ 0.000000] kvm-clock: cpu 0, msr 0:1ac0401, boot clock >>>>> [ 0.000000] kvm-clock: cpu 0, msr 0:1e15401, primary cpu clock >>>> Those are from kvmclock initialization, not from the older patch. >>>> >>>> I'm completely confused, everything seems to be in order. >>>> >>>> Let's see. if you s/return last/return ret/ in the original, does >>>> this help things along? this makes pvclock drop the computation and >>>> should be exactly the same as before the patch. >>> Yes, this works, both smp version boot. I see a short very short >>> break after the line in question, but then it proceeds well. >>> Thanks for your help, now I got a much better insight into the issue. >>> I will see if I can find something more. >> Did some more investigations, some observations: >> - The cmpxchg does not seem to be a problem, I didn't see the loop >> iterated more than once. >> - Turning off printk-timestamps makes the bug go away. But I guess it >> is just hiding or deferring it, and it's no real workaround anyway. >> - I instrumented the "if (ret < last) return last;" statement, when >> the kernel hangs I get only printks from there, although it has hit >> before: >> ---------- >> [ 0.820000] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled >> [ 0.820000] returning last instead (cnt=19001) >> [ 0.820000] returning last instead (cnt=20001) >> The last line repeats forever with the same timestamp, the counter >> (counting the number of "return last;") increments about 3500 >> times/second. >> >> I will see if I find something more... >> > gcc --version? That would be 4.3.3 I compiled the guest kernel with 4.4.4 also, that made no difference. Regards, Andre. -- Andre Przywara AMD-Operating System Research Center (OSRC), Dresden, Germany Tel: +49 351 448-3567-12