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Fri, 2 Oct 2020 10:01:13 +0200 (CEST) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v7 09/13] qmp: Move dispatcher to a coroutine References: <20200909151149.490589-1-kwolf@redhat.com> <20200909151149.490589-10-kwolf@redhat.com> Date: Fri, 02 Oct 2020 10:01:13 +0200 In-Reply-To: <20200909151149.490589-10-kwolf@redhat.com> (Kevin Wolf's message of "Wed, 9 Sep 2020 17:11:45 +0200") Message-ID: <878scpoy46.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 10.5.11.14 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 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 = 2020/10/02 01:13:31 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: stefanha@redhat.com, marcandre.lureau@gmail.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, dgilbert@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Additional nitpick detail on Kevin's request. Kevin Wolf writes: > This moves the QMP dispatcher to a coroutine and runs all QMP command > handlers that declare 'coroutine': true in coroutine context so they > can avoid blocking the main loop while doing I/O or waiting for other > events. > > For commands that are not declared safe to run in a coroutine, the > dispatcher drops out of coroutine context by calling the QMP command > handler from a bottom half. > > Signed-off-by: Kevin Wolf > Reviewed-by: Markus Armbruster [...] > diff --git a/monitor/monitor.c b/monitor/monitor.c > index 629aa073ee..ac2722bf91 100644 > --- a/monitor/monitor.c > +++ b/monitor/monitor.c > @@ -55,8 +55,32 @@ typedef struct { > /* Shared monitor I/O thread */ > IOThread *mon_iothread; > > -/* Bottom half to dispatch the requests received from I/O thread */ > -QEMUBH *qmp_dispatcher_bh; > +/* Coroutine to dispatch the requests received from I/O thread */ > +Coroutine *qmp_dispatcher_co; > + > +/* Set to true when the dispatcher coroutine should terminate */ > +bool qmp_dispatcher_co_shutdown; > + > +/* > + * qmp_dispatcher_co_busy is used for synchronisation between the > + * monitor thread and the main thread to ensure that the dispatcher > + * coroutine never gets scheduled a second time when it's already > + * scheduled (scheduling the same coroutine twice is forbidden). > + * > + * It is true if the coroutine is active and processing requests. > + * Additional requests may then be pushed onto mon->qmp_requests, > + * and @qmp_dispatcher_co_shutdown may be set without further ado. > + * @qmp_dispatcher_co_busy must not be woken up in this case. > + * > + * If false, you also have to set @qmp_dispatcher_co_busy to true and > + * wake up @qmp_dispatcher_co after pushing the new requests. > + * > + * The coroutine will automatically change this variable back to false > + * before it yields. Nobody else may set the variable to false. > + * > + * Access must be atomic for thread safety. > + */ > +bool qmp_dispatcher_co_busy; > > /* > * Protects mon_list, monitor_qapi_event_state, coroutine_mon, > @@ -623,9 +647,24 @@ void monitor_cleanup(void) > } > qemu_mutex_unlock(&monitor_lock); > > - /* QEMUBHs needs to be deleted before destroying the I/O thread */ > - qemu_bh_delete(qmp_dispatcher_bh); > - qmp_dispatcher_bh = NULL; > + /* > + * The dispatcher needs to stop before destroying the I/O thread. > + * > + * We need to poll both qemu_aio_context and iohandler_ctx to make > + * sure that the dispatcher coroutine keeps making progress and > + * eventually terminates. qemu_aio_context is automatically > + * polled by calling AIO_WAIT_WHILE on it, but we must poll > + * iohandler_ctx manually. > + */ > + qmp_dispatcher_co_shutdown = true; > + if (!atomic_xchg(&qmp_dispatcher_co_busy, true)) { > + aio_co_wake(qmp_dispatcher_co); > + } > + > + AIO_WAIT_WHILE(qemu_get_aio_context(), > + (aio_poll(iohandler_get_aio_context(), false), > + atomic_mb_read(&qmp_dispatcher_co_busy))); > + > if (mon_iothread) { > iothread_destroy(mon_iothread); > mon_iothread = NULL; > @@ -649,9 +688,9 @@ void monitor_init_globals_core(void) > * have commands assuming that context. It would be nice to get > * rid of those assumptions. > */ > - qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(), > - monitor_qmp_bh_dispatcher, > - NULL); > + qmp_dispatcher_co = qemu_coroutine_create(monitor_qmp_dispatcher_co, NULL); Rather long line, caused by rather long identifiers. Not your fault; you imitated the existing pattern static variable qmp_dispatcher_bh / extern function monitor_qmp_bh_dispatcher(). But the prefix monitor_qmp_ is awfully long, and not just here. Let's leave this for another day. > + atomic_mb_set(&qmp_dispatcher_co_busy, true); > + aio_co_schedule(iohandler_get_aio_context(), qmp_dispatcher_co); > } > > int monitor_init(MonitorOptions *opts, bool allow_hmp, Error **errp) [...] > diff --git a/util/aio-posix.c b/util/aio-posix.c > index f7f13ebfc2..30bb21d699 100644 > --- a/util/aio-posix.c > +++ b/util/aio-posix.c > @@ -15,6 +15,7 @@ > > #include "qemu/osdep.h" > #include "block/block.h" > +#include "qemu/main-loop.h" > #include "qemu/rcu.h" > #include "qemu/rcu_queue.h" > #include "qemu/sockets.h" > @@ -558,8 +559,13 @@ bool aio_poll(AioContext *ctx, bool blocking) > * There cannot be two concurrent aio_poll calls for the same AioContext (or > * an aio_poll concurrent with a GSource prepare/check/dispatch callback). > * We rely on this below to avoid slow locked accesses to ctx->notify_me. > + * > + * aio_poll() may only be called in the AioContext's thread. iohandler_ctx > + * is special in that it runs in the main thread, but that thread's context > + * is qemu_aio_context. Wrapping the comment around column 70 or so would make it easier to read. Up to you. > */ > - assert(in_aio_context_home_thread(ctx)); > + assert(in_aio_context_home_thread(ctx == iohandler_get_aio_context() ? > + qemu_get_aio_context() : ctx)); > > qemu_lockcnt_inc(&ctx->list_lock);