From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:45532) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TCBvt-0006fS-O1 for qemu-devel@nongnu.org; Thu, 13 Sep 2012 12:06:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TCBvn-0003vA-TL for qemu-devel@nongnu.org; Thu, 13 Sep 2012 12:06:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:6372) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TCBvn-0003uM-Jv for qemu-devel@nongnu.org; Thu, 13 Sep 2012 12:06:19 -0400 Date: Thu, 13 Sep 2012 19:06:13 +0300 From: Gleb Natapov Message-ID: <20120913160613.GF25041@redhat.com> References: <20120912151549.GT20907@redhat.com> <87y5kfrtne.fsf@codemonkey.ws> <20120913104940.GA20907@redhat.com> <5051DC20.4090204@redhat.com> <20120913132804.GO7767@redhat.com> <87r4q6xbiy.fsf@codemonkey.ws> <20120913142228.GK20907@redhat.com> <87boha7zyx.fsf@codemonkey.ws> <20120913144811.GL20907@redhat.com> <87ehm5or07.fsf@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87ehm5or07.fsf@codemonkey.ws> Subject: Re: [Qemu-devel] Rethinking missed tick catchup List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: qemu-devel@nongnu.org, Jan Kiszka , Michael Roth , Luiz Capitulino , Avi Kivity , Paolo Bonzini , Eric Blake On Thu, Sep 13, 2012 at 10:56:56AM -0500, Anthony Liguori wrote: > Gleb Natapov writes: > > > On Thu, Sep 13, 2012 at 09:35:18AM -0500, Anthony Liguori wrote: > >> Gleb Natapov writes: > >> > >> > On Thu, Sep 13, 2012 at 09:06:29AM -0500, Anthony Liguori wrote: > >> >> "Daniel P. Berrange" writes: > >> >> > >> >> I think it's better for QEMU to talk to qemu-ga. We can tell when a large > >> >> period of time has passed in QEMU because we'll accumulate a large > >> >> number of missed ticks. > >> >> > >> > With RTC configured to use vm clock we will not. > >> > >> Not for host suspend. For stop and live migration, we stop vm_clock. > >> But QEMU isn't aware of host suspend so vm_clock cannot be stopped. > >> > > Hmm, true. What about hooking into suspend and doing vmstop during > > suspend. > > Is suspend the only foreseeable way for this problem to happen? I don't > think it is which is what concerns me about any approach that relies on > "hooking suspend". > With RTC using real time clock setting host time far ahead of what is it will trigger same behaviour I think. > Also, I don't think there is a generic way to "hook suspend". > > >> >> This could happen because of stop, host suspend, live migration to a > >> >> file, etc. > >> >> > >> >> It's much easier for us to call into qemu-ga to do the time correction > >> >> whenever this event occurs than to try and have libvirt figure out when > >> >> it's necessary. > >> > And if guest does not have qemu-ga what is better inject interrupts like > >> > crazy for next 2 minutes or leave guest with incorrect time? > >> > >> Yes, at least that's fixable by the end-user. QEMU consuming 100% CPU > >> for a prolonged period of time isn't fixable. > >> > > You mean yes to "leave guest with incorrect time"? QEMU will still > > consume 100% of cpu for some time calling qemu_timer callback millions > > times. timedrift code is not the right level to fix that. > > Not if we put a cap on how many interrupts we'll try to catch up. > Interrupts ctachup happens at another level. If guest was stopped for 24 hours while RTC was configured to 1kHz qemu_timer will fire callback 88473600 times. Each invocation will try to inject interrupt and fail incrementing coalesced_irq instead. You can cap coalesced_irq but callback will still fire 88473600 times. > As I mentioned previously, if we acrue more than X number of missed > ticks, we should simply declare bankruptcy and reset the counter. > > When that occurs, *if* qemu-ga is present, we should ask qemu-ga to > reset the guest's clock based on reading the hardware clock via a > 'guest-resync-time' command. > > If it isn't, time will be off. Hopefully the guest is running NTP and > can correct itself. Otherwise, at least the admin can manually fix the > time. > > Regards, > > Anthony Liguori > > > > > -- > > Gleb. -- Gleb.