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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-46729791f3csm8609991cf.65.2024.12.05.07.04.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Dec 2024 07:04:01 -0800 (PST) Date: Thu, 5 Dec 2024 10:03:58 -0500 From: Peter Xu To: Fabiano Rosas Cc: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , 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: <87bjxqi7ya.fsf@suse.de> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@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: , 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 10:18:53AM -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. 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