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 BB6EEE7716D for ; Thu, 5 Dec 2024 15:41:32 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tJDyS-0001Zx-2N; Thu, 05 Dec 2024 10:41:04 -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 1tJDyQ-0001ZR-0s for qemu-devel@nongnu.org; Thu, 05 Dec 2024 10:41:02 -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 1tJDyM-0007jS-R8 for qemu-devel@nongnu.org; Thu, 05 Dec 2024 10:41:01 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1733413257; 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=RlHVopj+6C7Ukf7azLzcpIVy+CQ1pHF0HNfvZzmV78o=; b=Vk+DawSY/fnXtSP5AzBX0YnovyTrQmveXuIlBfz7qVit6olwtkobwkHyW0s4tk5AZopnYu UM8wmFxPmVB3+U0t+w9IlQM1FL8lM1UrxNDLFW1ly07D6FOsB4qakNTlphb+LBPw1ytPGf ReULBxVdHAyKTPIv/xQlAuWyHWVqF8g= Received: from mx-prod-mc-04.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-344-mGuddDbgP1isyahZ9jiSiA-1; Thu, 05 Dec 2024 10:40:54 -0500 X-MC-Unique: mGuddDbgP1isyahZ9jiSiA-1 X-Mimecast-MFC-AGG-ID: mGuddDbgP1isyahZ9jiSiA 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-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8C85C1955F2F; Thu, 5 Dec 2024 15:40:53 +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 90E3330001A1; Thu, 5 Dec 2024 15:40:51 +0000 (UTC) Date: Thu, 5 Dec 2024 15:40:47 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Fabiano Rosas Cc: Peter Xu , 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> 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 10:18:53AM -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. Yes, in general there should never be a for "yank", *if* QMEU is implemented correctly. yank is there in case something unexpected happens. IOW, even if we think migration is perfect today, yank is still worth having there as a safety net. > 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? We've not wired up any API for this libvirt. I can be issued via libvirt's QMP passthrough API if desired though. 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 :|