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Mon, 28 Sep 2020 11:30:55 +0200 (CEST) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v7 00/13] monitor: Optionally run handlers in coroutines References: <20200909151149.490589-1-kwolf@redhat.com> <20200910132439.GE45048@stefanha-x1.localdomain> <20200925171541.GK5731@linux.fritz.box> Date: Mon, 28 Sep 2020 11:30:55 +0200 In-Reply-To: <20200925171541.GK5731@linux.fritz.box> (Kevin Wolf's message of "Fri, 25 Sep 2020 19:15:41 +0200") Message-ID: <87zh5ajlio.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/28 03:29:35 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -36 X-Spam_score: -3.7 X-Spam_bar: --- X-Spam_report: (-3.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.576, 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_H5=-1, RCVD_IN_MSPIKE_WL=-0.01, 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: qemu-block@nongnu.org, armbru@redhat.com, dgilbert@redhat.com, qemu-devel@nongnu.org, marcandre.lureau@gmail.com, Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > Am 10.09.2020 um 15:24 hat Stefan Hajnoczi geschrieben: >> On Wed, Sep 09, 2020 at 05:11:36PM +0200, Kevin Wolf wrote: >> > Some QMP command handlers can block the main loop for a relatively lon= g >> > time, for example because they perform some I/O. This is quite nasty. >> > Allowing such handlers to run in a coroutine where they can yield (and >> > therefore release the BQL) while waiting for an event such as I/O >> > completion solves the problem. >> >=20 >> > This series adds the infrastructure to allow this and switches >> > block_resize to run in a coroutine as a first example. >> >=20 >> > This is an alternative solution to Marc-Andr=C3=A9's "monitor: add >> > asynchronous command type" series. >>=20 >> Please clarify the following in the QAPI documentation: >> * Is the QMP monitor suspended while the command is pending? > > Suspended as in monitor_suspend()? No. A suspended monitor doesn't read monitor input. We suspend * a QMP monitor while the request queue is full * an HMP monitor while it executes a command * a multiplexed HMP monitor while the "mux-focus" is elsewhere * an HMP monitor when it executes command "quit", forever * an HMP monitor while it executes command "migrate" without -d Let me explain the first item in a bit more detail. Before OOB, a QMP monitor was also suspended while it executed a command. To make OOB work, we moved the QMP monitors to an I/O thread and added a request queue, drained by the main loop. QMP monitors continue to read commands, execute right away if OOB, else queue, suspend when queue gets full, resume when it gets non-full. The "run command in coroutine context" feature does not affect any of this. qapi-code-gen.txt does not talk about monitor suspension at all. It's instead discussed in qmp-spec.txt section 2.3.1 Out-of-band execution. Stefan, what would you like us to clarify, and where? >> * Are QMP events reported while the command is pending? > > Hm, I don't know to be honest. But I think so. Yes, events should be reported while a command is being executed. Sending events takes locks. Their critical sections are all short-lived. Another possible delay is the underlying character device failing the send with EAGAIN. That's all. Fine print: qapi_event_emit() takes @monitor_lock. It sends to each QMP monitor with qmp_send_response(), which uses monitor_puts(), which takes the monitor's @mon_lock. The "run command in coroutine context" feature does not affect any of this. > Does it matter, though? I don't think events have a defined order > compared to command results, and the client can't respond to the event > anyway until the current command has completed. Stefan, what would you like us to clarify, and where?