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[79.176.44.197]) by smtp.gmail.com with ESMTPSA id i5sm1608583wrw.45.2020.12.03.04.03.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Dec 2020 04:03:02 -0800 (PST) Date: Thu, 3 Dec 2020 07:02:58 -0500 From: "Michael S. Tsirkin" To: "Dr. David Alan Gilbert" Subject: Re: [PATCH v2 01/27] migration: Network Failover can't work with a paused guest Message-ID: <20201203070119-mutt-send-email-mst@kernel.org> References: <87mtywlbvq.fsf@secure.mitica> <20201202105515.GD2360260@redhat.com> <20201202061641-mutt-send-email-mst@kernel.org> <20201202112639.GE2360260@redhat.com> <20201202063656-mutt-send-email-mst@kernel.org> <20201202120121.GF2360260@redhat.com> <20201203061907-mutt-send-email-mst@kernel.org> <20201203113253.GD2952498@redhat.com> <20201203063452-mutt-send-email-mst@kernel.org> <20201203114341.GB2919@work-vm> MIME-Version: 1.0 In-Reply-To: <20201203114341.GB2919@work-vm> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.495, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Eduardo Habkost , Juan Quintela , Jason Wang , qemu-devel@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Dec 03, 2020 at 11:43:41AM +0000, Dr. David Alan Gilbert wrote: > * Michael S. Tsirkin (mst@redhat.com) wrote: > > On Thu, Dec 03, 2020 at 11:32:53AM +0000, Daniel P. Berrangé wrote: > > > On Thu, Dec 03, 2020 at 06:21:47AM -0500, Michael S. Tsirkin wrote: > > > > On Wed, Dec 02, 2020 at 12:01:21PM +0000, Daniel P. Berrangé wrote: > > > > > On Wed, Dec 02, 2020 at 06:37:46AM -0500, Michael S. Tsirkin wrote: > > > > > > On Wed, Dec 02, 2020 at 11:26:39AM +0000, Daniel P. Berrangé wrote: > > > > > > > On Wed, Dec 02, 2020 at 06:19:29AM -0500, Michael S. Tsirkin wrote: > > > > > > > > On Wed, Dec 02, 2020 at 10:55:15AM +0000, Daniel P. Berrangé wrote: > > > > > > > > > On Wed, Dec 02, 2020 at 11:51:05AM +0100, Juan Quintela wrote: > > > > > > > > > > "Michael S. Tsirkin" wrote: > > > > > > > > > > > On Wed, Dec 02, 2020 at 05:31:53AM -0500, Michael S. Tsirkin wrote: > > > > > > > > > > >> On Wed, Dec 02, 2020 at 10:27:18AM +0000, Daniel P. Berrangé wrote: > > > > > > > > > > >> > On Wed, Dec 02, 2020 at 05:13:18AM -0500, Michael S. Tsirkin wrote: > > > > > > > > > > >> > > On Wed, Nov 18, 2020 at 09:37:22AM +0100, Juan Quintela wrote: > > > > > > > > > > >> > > > If we have a paused guest, it can't unplug the network VF device, so > > > > > > > > > > >> > > > we wait there forever. Just change the code to give one error on that > > > > > > > > > > >> > > > case. > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > Signed-off-by: Juan Quintela > > > > > > > > > > >> > > > > > > > > > > > > >> > > It's certainly possible but it's management that created > > > > > > > > > > >> > > this situation after all - why do we bother to enforce > > > > > > > > > > >> > > a policy? It is possible that management will unpause immediately > > > > > > > > > > >> > > afterwards and everything will proceed smoothly. > > > > > > > > > > >> > > > > > > > > > > > > >> > > Yes migration will not happen until guest is > > > > > > > > > > >> > > unpaused but the same it true of e.g. a guest that is stuck > > > > > > > > > > >> > > because of a bug. > > > > > > > > > > >> > > > > > > > > > > > >> > That's pretty different behaviour from how migration normally handles > > > > > > > > > > >> > a paused guest, which is that it is guaranteed to complete the migration > > > > > > > > > > >> > in as short a time as network bandwidth allows. > > > > > > > > > > >> > > > > > > > > > > > >> > Just ignoring the situation I think will lead to surprise apps / admins, > > > > > > > > > > >> > because the person/entity invoking the migration is not likely to have > > > > > > > > > > >> > checked wether this particular guest uses net failover or not before > > > > > > > > > > >> > invoking - they'll just be expecting a paused migration to run fast and > > > > > > > > > > >> > be guaranteed to complete. > > > > > > > > > > >> > > > > > > > > > > > >> > Regards, > > > > > > > > > > >> > Daniel > > > > > > > > > > >> > > > > > > > > > > >> Okay I guess. But then shouldn't we handle the reverse situation too: > > > > > > > > > > >> pausing guest after migration started but before device was > > > > > > > > > > >> unplugged? > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > Thinking of which, I have no idea how we'd handle it - fail > > > > > > > > > > > pausing guest until migration is cancelled? > > > > > > > > > > > > > > > > > > > > > > All this seems heavy handed to me ... > > > > > > > > > > > > > > > > > > > > This is the minimal fix that I can think of. > > > > > > > > > > > > > > > > > > > > Further solution would be: > > > > > > > > > > - Add a new migration parameter: migrate-paused > > > > > > > > > > - change libvirt to use the new parameter if it exist > > > > > > > > > > - in qemu, when we do start migration (but after we wait for the unplug > > > > > > > > > > device) paused the guest before starting migration and resume it after > > > > > > > > > > migration finish. > > > > > > > > > > > > > > > > > > It would also have to handle issuing of paused after migration has > > > > > > > > > been started - delay the pause request until the nuplug is complete > > > > > > > > > is one answer. > > > > > > > > > > > > > > > > Hmm my worry would be that pausing is one way to give cpu > > > > > > > > resources back to host. It's problematic if guest can delay > > > > > > > > that indefinitely. > > > > > > > > > > > > > > hmm, yes, that is awkward. Perhaps we should just report an explicit > > > > > > > error then. > > > > > > > > > > > > Report an error in response to which command? Do you mean > > > > > > fail migration? > > > > > > > > > > If mgt attempt to pause an existing migration that hasn't finished > > > > > the PCI unplug stage, then fail the pause request. > > > > > > > > Pause guest not migration ... > > > > Might be tricky ... > > > > > > > > Let me ask this, why not just produce a warning > > > > that migration wan't finish until guest actually runs? > > > > User will then know and unpause the guest when he wants > > > > migration to succeed ... > > > > > > A warning is going to be essentally invisible if the pause command > > > succeeeds. > > > > > > Regards, > > > Daniel > > > > I mean the situation here isn't earth shattering, an admin > > created it. Maybe he will unpause shortly > > and all will be well ... > > > > How about we make it possible for admin to detect that the > > reason for migration not making progress is that it is > > waiting for unplug? And maybe that guest is paused too? > > We already know that from the state of the VM. You don't know that migration is waiting for the guest action, no. This is what we care about here right? > > I just don't see how we can detect all cases and I am not > > sure it is worth it to try and detect only some of them, > > making users think they can rely on command failure to > > detect them. > > Another way to solve this would be to remove the unplugging from the > migration layer and leave it as a problem for the management layer to do > the unplug. > > Dave > > > > > > -- > > > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > > > |: https://libvirt.org -o- https://fstop138.berrange.com :| > > > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| > > > -- > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK