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 7B390E7716D for ; Thu, 5 Dec 2024 15:44:41 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tJE1b-0002ow-V4; Thu, 05 Dec 2024 10:44:21 -0500 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 1tJE1a-0002ob-Jp for qemu-devel@nongnu.org; Thu, 05 Dec 2024 10:44:18 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tJE1X-0008OF-Ko for qemu-devel@nongnu.org; Thu, 05 Dec 2024 10:44:17 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1733413454; 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=hs4vPF/qzzT10UlmBLZPyKF09YvrkoBZOQDu+RBM+dM=; b=bOP5X1C+pRLWVi8Kk2Slts0bOxB5dB4X5cTX89bH6wSFzgYNWY12LrXs+PEybFMxBtfYKB E3tBgAsUqyg8uPcwdHoMaqdjSEOtTkuGD7ugfc2YRi1gV4+fIVIUkCbckd/o0w3jdRPZJl 8vfBRFYAjCCZ6BsbCx4E5CVf8nJwHpE= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-456-65ddeVOFM6ObzNAf_LZM_w-1; Thu, 05 Dec 2024 10:44:13 -0500 X-MC-Unique: 65ddeVOFM6ObzNAf_LZM_w-1 X-Mimecast-MFC-AGG-ID: 65ddeVOFM6ObzNAf_LZM_w Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 151AF1955E8E; Thu, 5 Dec 2024 15:44:12 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.137]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 707833000197; Thu, 5 Dec 2024 15:44:10 +0000 (UTC) Date: Thu, 5 Dec 2024 15:44:05 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Fabiano Rosas , qemu-devel@nongnu.org Subject: Re: [PATCH 2/6] migration: Kick postcopy threads on cancel Message-ID: References: <20241202220137.32584-1-farosas@suse.de> <20241202220137.32584-3-farosas@suse.de> <87r06ni84z.fsf@suse.de> <87bjxqi7ya.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.13 (2024-03-09) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -50 X-Spam_score: -5.1 X-Spam_bar: ----- X-Spam_report: (-5.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.996, 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, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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 Thu, Dec 05, 2024 at 10:03:58AM -0500, Peter Xu wrote: > On Thu, Dec 05, 2024 at 10:18:53AM -0300, Fabiano Rosas wrote: > > Daniel P. Berrangé writes: > > > > > On Wed, Dec 04, 2024 at 03:51:27PM -0500, Peter Xu wrote: > > >> On Wed, Dec 04, 2024 at 08:02:31PM +0000, Daniel P. Berrangé wrote: > > >> > I would say the difference is like a graceful shutdown vs pulling the > > >> > power plug in a bare metal machine > > >> > > > >> > 'cancel' is intended to be graceful. It should leave you with a functional > > >> > QEMU (or refuse to run if unsafe). > > >> > > > >> > 'yank' is intended to be forceful, letting you get out of bad situations > > >> > that would otherwise require you to kill the entire QEMU process, but > > >> > still with possible associated risk data loss to the QEMU backends. > > >> > > > >> > They have overlap, but are none the less different. > > >> > > >> The question is more about whether yank should be used at all for > > >> migration only, not about the rest instances. > > >> > > >> My answer is yank should never be used for migration, because > > >> "migrate_cancel" also unplugs the power plug.. It's not anything more > > >> enforced. It's only doing less always. > > >> > > >> E.g. migration_yank_iochannel() is exactly what we do with > > >> qmp_migrate_cancel() in the first place, only that migrate_cancel only does > > >> it on the main channel (on both qemufiles even if ioc is one), however it > > >> should be suffice, and behave the same way, as strong as "yank". > > > > > > I recall at the time the yank stuff was introduced, one of the scenarios > > > they were concerned about was related to locks held by QEMU code. eg that > > > there are scenarios where migrate_cancel may not be processed promptly > > > enough due to being stalled on mutexes held by other concurrently running > > > threads. Now I would expect any such long duration stalls on migration > > > mutexes to be bugs, but the intent of yank is to give a recovery mechanism > > > that can workaround such bugs. The yank QMP command only interacts with > > > its own local mutexes. > > > > Ok, so that could only mean a thread stuck in recv() while holding the > > BQL. I don't think we have any other locks which would stop > > migrate_cancel from making progress or other stall situations that could > > be helped by a shutdown(). Note that most of locks around qemu_file were > > a late addition. I don't think that scenario is possible today. I'll > > have to do some tests. > > And if that is a real difference, I'd think whether we should simply make > migrate_cancel be oob-capable too.. IOW, I still think it'll be good to > stick with always one API to cancel a migration, no matter which it is. If > we want to move over to yank then I think we should move all migrate_cancel > operations into yank and deprecate "migrate_cancel', but that sounds > overkill. Well migrate_cancel ought to be safer than yank. eg migrate_cancel (sh|c)ould refuse to run if issued during post-copy phase. Or even in precopy, if in the final vmstate copy & switchover phase we shouldn't need to cancel. yank meanwhile will always run, no matter what, because by design, it has no interaction with the migration code beyond knowing that a socket exists. I don't think we should combine them. They have alot of common, but there are subtle differences that are relevant to the scenarios in which thye are intended to be used. > There's only one thing that might not be oob-compatible there so far, which > is bdrv_activate_all(). But I plan to remove it very soon (so that disks > will be activated in the migration thread instead, just like failure cases). > > > > > On that note, how is yank supposed to be accessed? I don't see support > > in libvirt. Is there a way to hook into QMP after the fact somehow? > > > > -- > Peter Xu > 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 :|