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Wed, 13 May 2020 11:26:33 +0000 (UTC) Date: Wed, 13 May 2020 12:26:30 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Kevin Wolf Subject: Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu Message-ID: <20200513112630.GJ1215285@redhat.com> References: <20200511114947.GJ1135885@redhat.com> <20200511120718.GD2811@work-vm> <20200511121714.GL1135885@redhat.com> <20200511154645.GI2811@work-vm> <20200512113206.62836e44@luklap> <20200512094337.GK1191162@redhat.com> <20200513103245.GD6202@linux.fritz.box> <20200513105359.GF3225@work-vm> <20200513111320.GE6202@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200513111320.GE6202@linux.fritz.box> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=berrange@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/13 03:05:18 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -10 X-Spam_score: -1.1 X-Spam_bar: - X-Spam_report: (-1.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FROM_EXCESS_BASE64=0.979, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Lukas Straub , qemu-block , Juan Quintela , qemu-devel , Max Reitz , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Paolo Bonzini , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, May 13, 2020 at 01:13:20PM +0200, Kevin Wolf wrote: > Am 13.05.2020 um 12:53 hat Dr. David Alan Gilbert geschrieben: > > * Kevin Wolf (kwolf@redhat.com) wrote: > > > Am 12.05.2020 um 11:43 hat Daniel P. Berrangé geschrieben: > > > > On Tue, May 12, 2020 at 11:32:06AM +0200, Lukas Straub wrote: > > > > > On Mon, 11 May 2020 16:46:45 +0100 > > > > > "Dr. David Alan Gilbert" wrote: > > > > > > > > > > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > > > > > ... > > > > > > > That way if QEMU does get stuck, you can start by tearing down the > > > > > > > least distruptive channel. eg try tearing down the migration connection > > > > > > > first (which shouldn't negatively impact the guest), and only if that > > > > > > > doesn't work then, move on to tear down the NBD connection (which risks > > > > > > > data loss) > > > > > > > > > > > > I wonder if a different way would be to make all network connections > > > > > > register with yank, but then make yank take a list of connections to > > > > > > shutdown(2). > > > > > > > > > > Good Idea. We could name the connections (/yank callbacks) in the > > > > > form "nbd:", "chardev:" and "migration" > > > > > (and add "netdev:...", etc. in the future). Then make yank take a > > > > > list of connection names as you suggest and silently ignore connections > > > > > that don't exist. And maybe even add a 'query-yank' oob command returning > > > > > a list of registered connections so the management application can do > > > > > pattern matching if it wants. > > > > > > I'm generally not a big fan of silently ignoring things. Is there a > > > specific requirement to do it in this case, or can management > > > applications be expected to know which connections exist? > > > > > > > Yes, that would make the yank command much more flexible in how it can > > > > be used. > > > > > > > > As an alternative to using formatted strings like this, it could be > > > > modelled more explicitly in QAPI > > > > > > > > { 'struct': 'YankChannels', > > > > 'data': { 'chardev': [ 'string' ], > > > > 'nbd': ['string'], > > > > 'migration': bool } } > > > > > > > > In this example, 'chardev' would accept a list of chardev IDs which > > > > have it enabled, 'nbd' would accept a list of block node IDs which > > > > have it enabled, and migration is a singleton on/off. > > > > > > Of course, it also means that the yank code needs to know about every > > > single object that supports the operation, whereas if you only have > > > strings, the objects could keep registering their connection with a > > > generic function like yank_register_function() in this version. > > > > > > I'm not sure if the additional complexity is worth the benefits. > > > > I tend to agree; although we do have to ensure we either use an existing > > naming scheme (e.g. QOM object names?) or make sure we've got a well > > defined list of prefixes. > > Not everything that has a network connection is a QOM object (in fact, > neither migration nor chardev nor nbd are QOM objects). > > I guess it would be nice to have a single namespace for everything in > QEMU, but the reality is that we have a few separate ones. As long as we > consistently add a prefix that identifies the namespace in question, I > think that would work. > > This means that if we're using node-name to identify the NBD connection, > the namespace should be 'block' rather than 'nbd'. > > One more thing to consider is, what if a single object has multiple > connections? In the case of node-names, we have a limited set of allowed > characters, so we can use one of the remaining characters as a separator > and then suffix a counter. In other places, the identifier isn't > restricted, so suffixing doesn't work. Maybe prefixing does, but it > would have to be there from the beginning then. > > And another thing: Do we really want to document this as limited to > network connections? Another common cause of hangs is when you have > image files on an NFS mount and the connection goes away. Of course, in > the end this is still networking, but inside of QEMU it looks like > accessing any other file. I'm not sure that we'll allow yanking access > to image files anytime soon, but it might not hurt to keep it at the > back of our mind as a potential option we might want the design to > allow. Are you referring to the in-kernel NFS client hangs here ? AFAIK, it is impossible to do anything to get out of those hangs from userspace, because the thread is stuck in an uninterruptable sleep in kernel space. If using the in-QEMU NFS client, then there is a network connection that can be yanked just like the NBD client. Regards, Daniel -- |: 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 :|