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 B0C88C0032E for ; Wed, 25 Oct 2023 05:35:05 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qvWWh-0000aY-OZ; Wed, 25 Oct 2023 01:33:55 -0400 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 1qvWWg-0000Zn-1U for qemu-devel@nongnu.org; Wed, 25 Oct 2023 01:33:54 -0400 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 1qvWWR-0001z5-Sr for qemu-devel@nongnu.org; Wed, 25 Oct 2023 01:33:53 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698212017; 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: in-reply-to:in-reply-to:references:references; bh=CrSihu8VKiDdOji9QEUdPRRNz3WMdd/axe6tmn5zv0Y=; b=TNxcEzkkim1YEQskTh8IP5WDll4lQRcSf1ggYJf6OsyMuDQQKbMzmgUFCGKYSOUaOagP+m jA22dZHfsOSnJWkbq0+i65snrLwU52OvSbxiF49jhZGFOvq5QDfhb/MPN9LCrHdtgUlV4u kAdD7QyWGHFGoqR8H8RP29EJzww7Svo= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-490-wv59m29sO26OIrmDHLwY3g-1; Wed, 25 Oct 2023 01:33:23 -0400 X-MC-Unique: wv59m29sO26OIrmDHLwY3g-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id 57FF628237D8; Wed, 25 Oct 2023 05:33:23 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.193.56]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2BCB82026D66; Wed, 25 Oct 2023 05:33:23 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 1D8D821E6A1F; Wed, 25 Oct 2023 07:33:22 +0200 (CEST) From: Markus Armbruster To: Fabiano Rosas Cc: qemu-devel@nongnu.org, berrange@redhat.com, Juan Quintela , Peter Xu , Leonardo Bras , Claudio Fontana , Eric Blake Subject: Re: [PATCH v2 06/29] migration: Add auto-pause capability References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-7-farosas@suse.de> <87r0lkvbfd.fsf@pond.sub.org> <8734xzevoj.fsf@suse.de> Date: Wed, 25 Oct 2023 07:33:22 +0200 In-Reply-To: <8734xzevoj.fsf@suse.de> (Fabiano Rosas's message of "Tue, 24 Oct 2023 15:12:44 -0300") Message-ID: <871qdjmfkt.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.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, 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_H3=0.001, RCVD_IN_MSPIKE_WL=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 Fabiano Rosas writes: > Markus Armbruster writes: > >> Fabiano Rosas writes: >> >>> Add a capability that allows the management layer to delegate to QEMU >>> the decision of whether to pause a VM and perform a non-live >>> migration. Depending on the type of migration being performed, this >>> could bring performance benefits. >>> >>> Note that the capability is enabled by default but at this moment no >>> migration scheme is making use of it. >> >> This sounds as if the capability has no effect unless the "migration >> scheme" (whatever that may be) opts into using it. Am I confused? >> > > What I mean here is that this capability is implemented and functional, > but I'm not retroactively enabling any existing migration code to use > auto-pause. Otherwise people would start seeing their guests being > paused before migraton in scenarios they never used to pause. > > By "migration scheme" I mean types of migration. Or modes of > operation. Or exclusive parameters. Anything that is different enough > from what exists today that we would consider a different type of > migration. Anything that allow us to break backward compatibility > (because it never existed before to begin with). > > E.g. this series introduces the fixed-ram migration. That never existed > before. So from the moment we enable that code to use this capability, > it will always do auto-pause, unless the management layer wants to avoid > it. So the auto-pause's *effective* default depends on the migration scheme: certain new schemes pause by default, everything else doesn't. Is this a good idea? If it is, then we need to document this behavior clearly. Here's another way to design the interface: keep the default behavior consistent (no pause), and provide a way to ask for pause (fails if migration scheme doesn't support it). >>> Signed-off-by: Fabiano Rosas >> >> [...] >> >>> diff --git a/qapi/migration.json b/qapi/migration.json >>> index db3df12d6c..74f12adc0e 100644 >>> --- a/qapi/migration.json >>> +++ b/qapi/migration.json >>> @@ -523,6 +523,10 @@ >>> # and can result in more stable read performance. Requires KVM >>> # with accelerator property "dirty-ring-size" set. (Since 8.1) >>> # >>> +# @auto-pause: If enabled, allows QEMU to decide whether to pause the >>> +# VM before migration for an optimal migration performance. >>> +# Enabled by default. (since 8.1) >> >> If this needs an opt-in to take effect, it should be documented. > > Someting like this perhaps? > > # @auto-pause: If enabled, allows QEMU to decide whether to pause the VM > # before migration for an optimal migration performance. Enabled by > # default. New migration code needs to opt-in at > # migration_should_pause(), otherwise this behaves as if > # disabled. (since 8.2) Remember, this is user-facing documentation. Talking about migration_should_pause() makes no sense there. Instead, you need to document what @auto-pause does: pause when a condition specific to the migration scheme is met, and specify the condition for each migration scheme. The condition could be "never" (auto-pause has no effect), "always", or something in between. A configuration knob that has no effect feels like an interface blemish. >>> +# >>> # Features: >>> # >>> # @unstable: Members @x-colo and @x-ignore-shared are experimental. >>> @@ -539,7 +543,7 @@ >>> { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] }, >>> 'validate-uuid', 'background-snapshot', >>> 'zero-copy-send', 'postcopy-preempt', 'switchover-ack', >>> - 'dirty-limit'] } >>> + 'dirty-limit', 'auto-pause'] } >>> >>> ## >>> # @MigrationCapabilityStatus: