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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 90550C83F01 for ; Wed, 30 Aug 2023 07:55:20 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qbG2a-0002Kp-1P; Wed, 30 Aug 2023 03:55:04 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qbG2Q-0002KN-NT for qemu-devel@nongnu.org; Wed, 30 Aug 2023 03:54:56 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qbG2L-0005Lf-CS for qemu-devel@nongnu.org; Wed, 30 Aug 2023 03:54:53 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1693382088; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=EZy+KSLH6nfPYgmgbDDKWlNHIyj9++xoyYi6hDFezRY=; b=C+oJWoArrfTGZA8W7O08RqwO/45loJmzqLJfmZmY5cxleNWv4m5qWP1wdy2AAGzS959i6T w6v4E7OuSfVvZzwFcVaynPpeYWzhRAdZ57r+9csNeviV3b6D5tjv/ao4BSG0bmIcJ/bwcG zRyX3jvRodufam1OL30UFbD7pIVHcsQ= Received: from mimecast-mx02.redhat.com (66.187.233.73 [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-479-cDeXtSHBOxO_CLRymVXlRA-1; Wed, 30 Aug 2023 03:54:44 -0400 X-MC-Unique: cDeXtSHBOxO_CLRymVXlRA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 0DE9E3C100B0; Wed, 30 Aug 2023 07:54:44 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.52]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0AFD72166B25; Wed, 30 Aug 2023 07:54:41 +0000 (UTC) Date: Wed, 30 Aug 2023 08:54:40 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, Leonardo Bras , farosas@suse.de, kwolf@redhat.com, Peter Xu , Paolo Bonzini , Vladimir Sementsov-Ogievskiy , Juan Quintela , qemu-block@nongnu.org, Coiby Xu , Fam Zheng , Hanna Reitz Subject: Re: [PATCH v2 4/4] io: follow coroutine AioContext in qio_channel_yield() Message-ID: References: <20230829160623.277411-1-stefanha@redhat.com> <20230829160623.277411-5-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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_H4=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Aug 29, 2023 at 02:32:46PM -0500, Eric Blake wrote: > On Tue, Aug 29, 2023 at 12:06:22PM -0400, Stefan Hajnoczi wrote: > > The ongoing QEMU multi-queue block layer effort makes it possible for multiple > > threads to process I/O in parallel. The nbd block driver is not compatible with > > the multi-queue block layer yet because QIOChannel cannot be used easily from > > coroutines running in multiple threads. This series changes the QIOChannel API > > to make that possible. > > > > In the current API, calling qio_channel_attach_aio_context() sets the > > AioContext where qio_channel_yield() installs an fd handler prior to yielding: > > > > qio_channel_attach_aio_context(ioc, my_ctx); > > ... > > qio_channel_yield(ioc); // my_ctx is used here > > ... > > qio_channel_detach_aio_context(ioc); > > > > This API design has limitations: reading and writing must be done in the same > > AioContext and moving between AioContexts involves a cumbersome sequence of API > > calls that is not suitable for doing on a per-request basis. > > > > There is no fundamental reason why a QIOChannel needs to run within the > > same AioContext every time qio_channel_yield() is called. QIOChannel > > only uses the AioContext while inside qio_channel_yield(). The rest of > > the time, QIOChannel is independent of any AioContext. > > > > In the new API, qio_channel_yield() queries the AioContext from the current > > coroutine using qemu_coroutine_get_aio_context(). There is no need to > > explicitly attach/detach AioContexts anymore and > > qio_channel_attach_aio_context() and qio_channel_detach_aio_context() are gone. > > One coroutine can read from the QIOChannel while another coroutine writes from > > a different AioContext. > > > > This API change allows the nbd block driver to use QIOChannel from any thread. > > It's important to keep in mind that the block driver already synchronizes > > QIOChannel access and ensures that two coroutines never read simultaneously or > > write simultaneously. > > > > This patch updates all users of qio_channel_attach_aio_context() to the > > new API. Most conversions are simple, but vhost-user-server requires a > > new qemu_coroutine_yield() call to quiesce the vu_client_trip() > > coroutine when not attached to any AioContext. > > > > While the API is has become simpler, there is one wart: QIOChannel has a > > special case for the iohandler AioContext (used for handlers that must not run > > in nested event loops). I didn't find an elegant way preserve that behavior, so > > I added a new API called qio_channel_set_follow_coroutine_ctx(ioc, true|false) > > for opting in to the new AioContext model. By default QIOChannel uses the > > iohandler AioHandler. Code that formerly called > > qio_channel_attach_aio_context() now calls > > qio_channel_set_follow_coroutine_ctx(ioc, true) once after the QIOChannel is > > created. > > > > Signed-off-by: Stefan Hajnoczi > > --- > > +++ b/include/io/channel-util.h > > @@ -49,4 +49,27 @@ > > QIOChannel *qio_channel_new_fd(int fd, > > Error **errp); > > > > +/** > > + * qio_channel_yield: > > Wrong function name... > > > + * @read_fd: the file descriptor for the read handler > > + * @read_ctx: the AioContext for the read handler > > + * @io_read: the read handler > > + * @write_fd: the file descriptor for the write handler > > + * @write_ctx: the AioContext for the write handler > > + * @io_write: the write handler > > + * @opaque: the opaque argument to the read and write handler > > + * > > + * Set the read and write handlers when @read_ctx and @write_ctx are non-NULL, > > + * respectively. To leave a handler in its current state, pass a NULL > > + * AioContext. To clear a handler, pass a non-NULL AioContext and a NULL > > + * handler. > > + */ > > +void qio_channel_util_set_aio_fd_handler(int read_fd, > > ...should be this. > > > + AioContext *read_ctx, > > + IOHandler *io_read, > > + int write_fd, > > + AioContext *write_ctx, > > + IOHandler *io_write, > > + void *opaque); > > + > > #endif /* QIO_CHANNEL_UTIL_H */ > > diff --git a/include/io/channel.h b/include/io/channel.h > > index 229bf36910..5f9dbaab65 100644 > > --- a/include/io/channel.h > > > +++ b/io/channel.c > > > > > -static void qio_channel_set_aio_fd_handlers(QIOChannel *ioc) > > +static void coroutine_fn > > +qio_channel_set_fd_handlers(QIOChannel *ioc, GIOCondition condition) > > { > > - IOHandler *rd_handler = NULL, *wr_handler = NULL; > > + AioContext *ctx = ioc->follow_coroutine_ctx ? > > + qemu_coroutine_get_aio_context(qemu_coroutine_self()) : > > + iohandler_get_aio_context(); > > + AioContext *read_ctx = NULL; > > + IOHandler *io_read = NULL; > > + AioContext *write_ctx = NULL; > > + IOHandler *io_write = NULL; > > + > > + if (condition == G_IO_IN) { > > + ioc->read_coroutine = qemu_coroutine_self(); > > + ioc->read_ctx = ctx; > > + read_ctx = ctx; > > + io_read = qio_channel_restart_read; > > + > > + /* > > + * Thread safety: if the other coroutine is set and its AioContext > > + * match ours, then there is mutual exclusion between read and write > > matches > > > + * because they share a single thread and it's safe to set both read > > + * and write fd handlers here. If the AioContext does not match ours, > > + * then both threads may run in parallel but there is no shared state > > + * to worry about. > > + */ > > + if (ioc->write_coroutine && ioc->write_ctx == ctx) { > > + write_ctx = ctx; > > + io_write = qio_channel_restart_write; > > + } > > + } else if (condition == G_IO_OUT) { > > + ioc->write_coroutine = qemu_coroutine_self(); > > + ioc->write_ctx = ctx; > > + write_ctx = ctx; > > + io_write = qio_channel_restart_write; > > + if (ioc->read_coroutine && ioc->read_ctx == ctx) { > > + read_ctx = ctx; > > + io_read = qio_channel_restart_read; > > + } > > + } else { > > + abort(); > > + } > > + > > + qio_channel_set_aio_fd_handler(ioc, read_ctx, io_read, > > + write_ctx, io_write, ioc); > > +} > > + > > With those minor fixes, > > Reviewed-by: Eric Blake Acked-by: Daniel P. Berrangé Eric, feel free to merge the two io subsystem changes at the same time as the NBD changes. With 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 :|