From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E0A3BC352AA for ; Tue, 1 Oct 2019 10:07:44 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id B6D0721920 for ; Tue, 1 Oct 2019 10:07:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B6D0721920 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:39966 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iFF4J-0004pN-O9 for qemu-devel@archiver.kernel.org; Tue, 01 Oct 2019 06:07:43 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:42306) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iFF2F-0003Qo-6D for qemu-devel@nongnu.org; Tue, 01 Oct 2019 06:05:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iFF2A-00075t-TV for qemu-devel@nongnu.org; Tue, 01 Oct 2019 06:05:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47150) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iFF2A-00074q-Ji for qemu-devel@nongnu.org; Tue, 01 Oct 2019 06:05:30 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D07764ACA7; Tue, 1 Oct 2019 10:05:27 +0000 (UTC) Received: from work-vm (unknown [10.36.118.45]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3E1675D9C9; Tue, 1 Oct 2019 10:05:26 +0000 (UTC) Date: Tue, 1 Oct 2019 11:05:23 +0100 From: "Dr. David Alan Gilbert" To: Felipe Franciosi , armbru@redhat.com, berrange@redhat.com Subject: Re: Thoughts on VM fence infrastructure Message-ID: <20191001100510.GD2781@work-vm> References: <20190930142954.GA2801@work-vm> <20190930160316.GH2759@work-vm> <417D4B96-2641-4DA8-B00B-3302E211E939@nutanix.com> <20190930171109.GL2759@work-vm> <20190930175914.GM2759@work-vm> <20191001082345.GA2781@work-vm> <2248E813-102F-4E60-AF9B-A5A2F21C1687@nutanix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2248E813-102F-4E60-AF9B-A5A2F21C1687@nutanix.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 01 Oct 2019 10:05:27 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Rafael David Tinoco , Aditya Ramesh , qemu-devel Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Felipe Franciosi (felipe@nutanix.com) wrote: > > > > On Oct 1, 2019, at 9:23 AM, Dr. David Alan Gilbert wrote: > > > > * Felipe Franciosi (felipe@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 6:59 PM, Dr. David Alan Gilbert wrote: > >>> > >>> * Felipe Franciosi (felipe@nutanix.com) wrote: > >>>> > >>>> > >>>>> On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert wrote: > >>>>> > >>>>> * Felipe Franciosi (felipe@nutanix.com) wrote: > >>>>>> > >>>>>> > >>>>>>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert wrote: > >>>>>>> > >>>>>>> * Felipe Franciosi (felipe@nutanix.com) wrote: > >>>>>>>> Hi David, > >>>>>>>> > >>>>>>>>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert wrote: > >>>>>>>>> > >>>>>>>>> * Felipe Franciosi (felipe@nutanix.com) wrote: > >>>>>>>>>> Heyall, > >>>>>>>>>> > >>>>>>>>>> We have a use case where a host should self-fence (and all VMs should > >>>>>>>>>> die) if it doesn't hear back from a heartbeat within a certain time > >>>>>>>>>> period. Lots of ideas were floated around where libvirt could take > >>>>>>>>>> care of killing VMs or a separate service could do it. The concern > >>>>>>>>>> with those is that various failures could lead to _those_ services > >>>>>>>>>> being unavailable and the fencing wouldn't be enforced as it should. > >>>>>>>>>> > >>>>>>>>>> Ultimately, it feels like Qemu should be responsible for this > >>>>>>>>>> heartbeat and exit (or execute a custom callback) on timeout. > >>>>>>>>> > >>>>>>>>> It doesn't feel doing it inside qemu would be any safer; something > >>>>>>>>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > >>>>>>>> > >>>>>>>> The argument above is that we would have to rely on this external > >>>>>>>> service being functional. Consider the case where the host is > >>>>>>>> dysfunctional, with this service perhaps crashed and a corrupt > >>>>>>>> filesystem preventing it from restarting. The VMs would never die. > >>>>>>> > >>>>>>> Yeh that could fail. > >>>>>>> > >>>>>>>> It feels like a Qemu timer-driven heartbeat check and calls abort() / > >>>>>>>> exit() would be more reliable. Thoughts? > >>>>>>> > >>>>>>> OK, yes; perhaps using a timer_create and telling it to send a fatal > >>>>>>> signal is pretty solid; it would take the kernel to do that once it's > >>>>>>> set. > >>>>>> > >>>>>> I'm confused about why the kernel needs to be involved. If this is a > >>>>>> timer off the Qemu main loop, it can just check on the heartbeat > >>>>>> condition (which should be customisable) and call abort() if that's > >>>>>> not satisfied. If you agree on that I'd like to talk about how that > >>>>>> check could be made customisable. > >>>>> > >>>>> There are times when the main loop can get blocked even though the CPU > >>>>> threads can be running and can in some configurations perform IO > >>>>> even without the main loop (I think!). > >>>> > >>>> Ah, that's a very good point. Indeed, you can perform IO in those > >>>> cases specially when using vhost devices. > >>>> > >>>>> By setting a timer in the kernel that sends a signal to qemu, the kernel > >>>>> will send that signal however broken qemu is. > >>>> > >>>> Got you now. That's probably better. Do you reckon a signal is > >>>> preferable over SIGEV_THREAD? > >>> > >>> Not sure; probably the safest is getting the kernel to SIGKILL it - but > >>> that's a complete nightmare to debug - your process just goes *pop* > >>> with no apparent reason why. > >>> I've not used SIGEV_THREAD - it looks promising though. > >> > >> I'm worried that SIGEV_THREAD could be a bit heavyweight (if it fires > >> up a new thread each time). On the other hand, as you said, SIGKILL > >> makes it harder to debug. > >> > >> Also, asking the kernel to defer the SIGKILL (ie. updating the timer) > >> needs to come from Qemu itself (eg. a timer in the main loop, > >> something we already ruled unsuitable, or a qmp command which > >> constitutes an external dependency that we also ruled undesirable). > > > > OK, there's two reasons I think this isn't that bad/is good: > > a) It's an external dependency - but if it fails the result is the > > system fails, rather than the system keeps on running; so I think > > that's the balance you were after; it's the opposite from > > the external watchdog. > > Right. I like where you are coming from. And I think a mix of these > may be the best way forwards. I'll elaborate on it below. > > > > > b) You need some external system anyway to tell QEMU when it's > > OK - what's your definitino of a failed system? > > The feature is targeted at providing a self-fencing mechanism for > Qemu. If a host is unreachable for whatever reason (eg. sshd down, ovs > died, oomkiller took services out, physical network failure), it > should guarantee that VMs won't be running after a certain amount of > time. To your point, if this external software doesn't come in and > touch the file, that's because it can't reach the host or it wants the > host to self-fence. The qualifying Qemus should therefore be > considered dead after a "deadline" period (since the last time the > control file was touched). > > > > >> What if, when self-fencing is enabled, Qemu kicks off a new thread > >> from the start which does nothing but periodically wake up, verify the > >> heartbeat condition and log()+abort() if required? (Then we wouldn't > >> need the kernel timer.) > > > > I'd make that thread bump the kernel timer along. > > I think combining the thread's logic with the kernel timer makes the > whole thing a lot more solid. See below. > > > > >>> > >>>> I'm still wondering how to make this customisable so that different > >>>> types of heartbeat could be implemented (preferably without creating > >>>> external dependencies per discussion above). Thoughts welcome. > >>> > >>> Yes, you need something to enable it, and some safe way to retrigger > >>> the timer. A qmp command marked as 'oob' might be the right way - > >>> another qm command can't block it. > >> > >> This qmp approach is slightly different than the external dependency > >> that itself kills Qemu; if it doesn't run, then Qemu dies because the > >> kernel timer is not updated. But this is also a heavyweight approach. > >> We are talking about a service that needs to frequently connect to all > >> running VMs on a host to reset the timer. > >> > >> But it does allow for the customisable heartbeat: the logic behind > >> what triggers the command is completely flexible. > >> > >> Thinking about this idea of a separate Qemu thread, one thing that > >> came to mind is this: > >> > >> qemu -fence heartbeat=/path/to/file,deadline=60[,recheck=5] > >> > >> Qemu could fire up a thread that stat()s (every > >> seconds or on a default interval) and log()+abort() the whole process > >> if the last modification time of the file is older than . If > >> goes away (ie. stat() gives ENOENT), then it either fences > >> immediately or ignores it, not sure which is more sensible. > >> > >> Thoughts? > > > > As above; I'm OK with using a file with that; but I'd make that thread > > bump the kernel timer along; if that thread gets stuck somehow the > > kernel still nukes your process. > > > Awesome. So check this out: > > qemu -fence heartbeat=/path/to/file,deadline=60[,recheck=5][,harddeadline=61] > > We can default to and enforce that: > - is a multiple of . > - is bigger than > > When expires, we can log() + abort(), but if > expires, we can rest assured the kernel will come around and SIGKILL > Qemu. If there's demand for it, this can later be enhanced by adding > more parameters which set the fence thread scheduling priority, &c. > > If that sounds ok I'll send an RFC as soon as I get a chance and we > can take it from there. So I think I'm OK with that; but I've copied in Markus and Daniel who normally have ideas on how the command line/libvirt interface should look like. Dave > F. > > > > > Dave > > > >> F. > >> > >>> > >>> Dave > >>> > >>> > >>>> F. > >>>> > >>>>> > >>>>>> > >>>>>>> > >>>>>>> IMHO the safer way is to kick the host off the network by reprogramming > >>>>>>> switches; so even if the qemu is actually alive it can't get anywhere. > >>>>>>> > >>>>>>> Dave > >>>>>> > >>>>>> Naturally some off-host STONITH is preferable, but that's not always > >>>>>> available. A self-fencing mechanism right at the heart of the emulator > >>>>>> can do the job without external hardware dependencies. > >>>>> > >>>>> Dave > >>>>> > >>>>>> Cheers, > >>>>>> Felipe > >>>>>> > >>>>>>> > >>>>>>> > >>>>>>>> Felipe > >>>>>>>> > >>>>>>>>> > >>>>>>>>>> Does something already exist for this purpose which could be used? > >>>>>>>>>> Would a generic Qemu-fencing infrastructure be something of interest? > >>>>>>>>> Dave > >>>>>>>>> > >>>>>>>>> > >>>>>>>>>> Cheers, > >>>>>>>>>> F. > >>>>>>>>>> > >>>>>>>>> -- > >>>>>>>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >>>>>>>> > >>>>>>> -- > >>>>>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >>>>>> > >>>>> -- > >>>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >>>> > >>> -- > >>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >> > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK