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dkim=none (message not signed) header.d=none;openvz.org; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AS8PR08MB6069.eurprd08.prod.outlook.com (2603:10a6:20b:29c::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4042.18; Fri, 16 Apr 2021 08:21:38 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133%8]) with mapi id 15.20.4042.019; Fri, 16 Apr 2021 08:21:38 +0000 Subject: Re: [PATCH v3 33/33] block/nbd: drop connection_co From: Vladimir Sementsov-Ogievskiy To: qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, eblake@redhat.com, mreitz@redhat.com, kwolf@redhat.com, rvkagan@yandex-team.ru, den@openvz.org References: <20210416080911.83197-1-vsementsov@virtuozzo.com> <20210416080911.83197-34-vsementsov@virtuozzo.com> Message-ID: <60b083d2-62ae-425d-ece9-31dbd4ff9692@virtuozzo.com> Date: Fri, 16 Apr 2021 11:21:36 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR05-DB8-obe.outbound.protection.outlook.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 16.04.2021 11:14, Vladimir Sementsov-Ogievskiy wrote: > 16.04.2021 11:09, Vladimir Sementsov-Ogievskiy wrote: >> OK, that's a big rewrite of the logic. >> >> Pre-patch we have an always running coroutine - connection_co. It does >> reply receiving and reconnecting. And it leads to a lot of difficult >> and unobvious code around drained sections and context switch. We also >> abuse bs->in_flight counter which is increased for connection_co and >> temporary decreased in points where we want to allow drained section to >> begin. One of these place is in another file: in nbd_read_eof() in >> nbd/client.c. >> >> We also cancel reconnect and requests waiting for reconnect on drained >> begin which is not correct. >> >> Let's finally drop this always running coroutine and go another way: >> >> 1. reconnect_attempt() goes to nbd_co_send_request and called under >>     send_mutex >> >> 2. We do receive headers in request coroutine. But we also should >>     dispatch replies for another pending requests. So, >>     nbd_connection_entry() is turned into nbd_receive_replies(), which >>     does reply dispatching until it receive another request headers, and >>     returns when it receive the requested header. >> >> 3. All old staff around drained sections and context switch is dropped. >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy > > Please consider this last patch as RFC for now: > > 1. It is complicated, and doesn't have good documentation. Please look through and ask everything that is not obvious, I'll explain. Don't waste your time trying to understand what is not clean. > > 2. I also failed to image, how to split the patch into smaller simple patches.. Ideas are welcome. > > 3. It actually reverts what was done in > > commit 8c517de24a8a1dcbeb54e7e12b5b0fda42a90ace > Author: Vladimir Sementsov-Ogievskiy > Date:   Thu Sep 3 22:02:58 2020 +0300 > >    block/nbd: fix drain dead-lock because of nbd reconnect-delay > > and I didn't check yet, does this dead-lock still here or not. Even if it still here I believe that nbd driver is a wrong place to workaround this bug, but I should check it first at least. > 4. As Roman said, there is a problem in new architecture: when guest is idle, we will not detect disconnect immediately but only on the next request from the guest. That may be considered as a degradation. Still, let's implement a kind of keep-alive on top of this series, some ideas: - add an idle-timeout, and do simple NBD request by timeout, which will result in some expected error reply from the server. - or add an idle coroutine, which will do endless "read" when there no requests. It will be a kind of old connection_co, but it will have only one function and will be extremely simple. And we may just cancel it on drained-begin and restart on drained-end. -- Best regards, Vladimir