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=-2.5 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 48C86C32792 for ; Mon, 30 Sep 2019 18:00:40 +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 D11C6224EC for ; Mon, 30 Sep 2019 18:00:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D11C6224EC 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]:55846 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iEzyQ-0003Bw-0I for qemu-devel@archiver.kernel.org; Mon, 30 Sep 2019 14:00:38 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:54621) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iEzxC-0002KJ-2l for qemu-devel@nongnu.org; Mon, 30 Sep 2019 13:59:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iEzx9-0003dE-E5 for qemu-devel@nongnu.org; Mon, 30 Sep 2019 13:59:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56588) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iEzx9-0003co-1m for qemu-devel@nongnu.org; Mon, 30 Sep 2019 13:59:19 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 247F610C0933; Mon, 30 Sep 2019 17:59:18 +0000 (UTC) Received: from work-vm (ovpn-117-232.ams2.redhat.com [10.36.117.232]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 34A2060BF1; Mon, 30 Sep 2019 17:59:17 +0000 (UTC) Date: Mon, 30 Sep 2019 18:59:14 +0100 From: "Dr. David Alan Gilbert" To: Felipe Franciosi Subject: Re: Thoughts on VM fence infrastructure Message-ID: <20190930175914.GM2759@work-vm> References: <42837590-2563-412B-ADED-57B8A10A8E68@nutanix.com> <20190930142954.GA2801@work-vm> <20190930160316.GH2759@work-vm> <417D4B96-2641-4DA8-B00B-3302E211E939@nutanix.com> <20190930171109.GL2759@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.66]); Mon, 30 Sep 2019 17:59:18 +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: 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 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 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. 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