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 X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 17569C54E8B for ; Tue, 12 May 2020 09:06:53 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id DAF8B20575 for ; Tue, 12 May 2020 09:06:52 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="NRjrtdMC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DAF8B20575 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:49716 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jYQsG-0005ee-3M for qemu-devel@archiver.kernel.org; Tue, 12 May 2020 05:06:52 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:38392) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jYQrf-0005A7-D7 for qemu-devel@nongnu.org; Tue, 12 May 2020 05:06:15 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:57641 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jYQre-0007VK-BF for qemu-devel@nongnu.org; Tue, 12 May 2020 05:06:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1589274373; h=from:from: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=vOKYF3rGd2Wmnlaw6h0LtWDu4P3YqNZB4tXHE19qUVM=; b=NRjrtdMC7jo8Xjk8unA1q0oCKK155gjjRe4LEAiC+c5nb7aUHnQ1SISDib29s5XpLA0Wtx JTbeLQGNxcEz4f4lxWtfDz+WHJJcaZzmkOjkQlc2H+p2fvxXZ79RrlgVKABfDb/ychaNbJ fQC0HmnFtI2RW4DlWNP5RguYjGXYO4E= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-416-SzyF5R17N82X3no7Dr_MbQ-1; Tue, 12 May 2020 05:06:09 -0400 X-MC-Unique: SzyF5R17N82X3no7Dr_MbQ-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8AECC83DB38; Tue, 12 May 2020 09:06:08 +0000 (UTC) Received: from work-vm (ovpn-114-253.ams2.redhat.com [10.36.114.253]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 65ADD5C1B5; Tue, 12 May 2020 09:06:03 +0000 (UTC) Date: Tue, 12 May 2020 10:06:00 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu Message-ID: <20200512090600.GA2802@work-vm> References: <20200511114947.GJ1135885@redhat.com> <20200511201218.76f30e0b@luklap> <20200512090352.GG1191162@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200512090352.GG1191162@redhat.com> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/12 02:02:04 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] 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_H2=-0.001, SPF_PASS=-0.001, T_HK_NAME_DR=0.01, 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: , Cc: Kevin Wolf , Lukas Straub , qemu-block , Juan Quintela , qemu-devel , Max Reitz , Paolo Bonzini , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrangé (berrange@redhat.com) wrote: > On Mon, May 11, 2020 at 08:12:18PM +0200, Lukas Straub wrote: > > On Mon, 11 May 2020 12:49:47 +0100 > > Daniel P. Berrangé wrote: > > > > > On Mon, May 11, 2020 at 01:14:34PM +0200, Lukas Straub wrote: > > > > Hello Everyone, > > > > In many cases, if qemu has a network connection (qmp, migration, chardev, etc.) > > > > to some other server and that server dies or hangs, qemu hangs too. > > > > > > If qemu as a whole hangs due to a stalled network connection, that is a > > > bug in QEMU that we should be fixing IMHO. QEMU should be doing non-blocking > > > I/O in general, such that if the network connection or remote server stalls, > > > we simply stop sending I/O - we shouldn't ever hang the QEMU process or main > > > loop. > > > > > > There are places in QEMU code which are not well behaved in this respect, > > > but many are, and others are getting fixed where found to be important. > > > > > > Arguably any place in QEMU code which can result in a hang of QEMU in the > > > event of a stalled network should be considered a security flaw, because > > > the network is untrusted in general. > > > > The fact that out-of-band qmp commands exist at all shows that we have to make tradeoffs of developer time vs. doing things right. Sure, the migration code can be rewritten to use non-blocking i/o and finegrained locks. But as a hobbyist I don't have time to fix this. > > > > > > These patches introduce the new 'yank' out-of-band qmp command to recover from > > > > these kinds of hangs. The different subsystems register callbacks which get > > > > executed with the yank command. For example the callback can shutdown() a > > > > socket. This is intended for the colo use-case, but it can be used for other > > > > things too of course. > > > > > > IIUC, invoking the "yank" command unconditionally kills every single > > > network connection in QEMU that has registered with the "yank" subsystem. > > > IMHO this is way too big of a hammer, even if we accept there are bugs in > > > QEMU not handling stalled networking well. > > > > > > eg if a chardev hangs QEMU, and we tear down everything, killing the NBD > > > connection used for the guest disk, we needlessly break I/O. > > > > Yeah, these patches are intended to solve the problems with the colo > > use-case where all external connections (migration, chardevs, nbd) > > are just for replication. In other use-cases you'd enable the yank > > feature only on the non-essential connections. > > That is a pretty inflexible design for other use cases though, > as "non-essential" is not a black & white list in general. There > are varying levels of importance to the different channels. We > can afford to loose migration without any user visible effects. > If that doesn't solve it, a serial device chardev, or VNC connection > can be dropped at the inconvenience of loosing interactive console > which is end user visible impact, so may only be want to be yanked > if the migration yank didn't fix it. In the case of COLO that's not the case though - here we explicitly want to kill the migration to be able to ensure that we can recover - and we're under time pressure to get the other member of the pair running again. Dave > 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 :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK