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 622DDC0032E for ; Wed, 25 Oct 2023 17:32:53 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qvhjg-0001oO-Gh; Wed, 25 Oct 2023 13:32:04 -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 1qvhjd-0001o2-Ou for qemu-devel@nongnu.org; Wed, 25 Oct 2023 13:32:01 -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 1qvhjb-00043Z-Mw for qemu-devel@nongnu.org; Wed, 25 Oct 2023 13:32:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698255118; 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=f4wK1uiJOOXomvn6RpcNqq5w8/Vdx1evptxgBfj4uxg=; b=gH4PvQJ6LdbskW5O8rZ6QsKERldaluFhStCywUo9AHrwXw98qQlu+lqtv2LUVYcwoAyjbN jy20SrolOG849WhZMwaHbv/lPtgnqvk9DtN1Gy/rN2X4hMQFYyxzDHauRoX4sA/ukf7COZ aGTGYQNAfV3uk3ZEUNkhHxnFaPRLk7s= 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-470-7HAVtI1FM6-Xj9ZnRMr6gA-1; Wed, 25 Oct 2023 13:31:56 -0400 X-MC-Unique: 7HAVtI1FM6-Xj9ZnRMr6gA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (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 73FAC383D8B3; Wed, 25 Oct 2023 17:31:56 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.154]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2F5B61C060B0; Wed, 25 Oct 2023 17:31:55 +0000 (UTC) Date: Wed, 25 Oct 2023 18:31:53 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Fabiano Rosas , qemu-devel@nongnu.org, armbru@redhat.com, Juan Quintela , Leonardo Bras , Claudio Fontana , Eric Blake Subject: Re: [PATCH v2 06/29] migration: Add auto-pause capability Message-ID: References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-7-farosas@suse.de> <87y1fqerev.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/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.7 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: -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: , 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 Wed, Oct 25, 2023 at 01:20:52PM -0400, Peter Xu wrote: > On Wed, Oct 25, 2023 at 04:40:52PM +0100, Daniel P. Berrangé wrote: > > On Wed, Oct 25, 2023 at 11:36:27AM -0400, Peter Xu wrote: > > > On Wed, Oct 25, 2023 at 04:25:23PM +0100, Daniel P. Berrangé wrote: > > > > > Libvirt will still use fixed-ram for live snapshot purpose, especially for > > > > > Windows? Then auto-pause may still be useful to identify that from what > > > > > Fabiano wants to achieve here (which is in reality, non-live)? > > > > > > > > > > IIRC of previous discussion that was the major point that libvirt can still > > > > > leverage fixed-ram for a live case - since Windows lacks efficient live > > > > > snapshot (background-snapshot feature). > > > > > > > > Libvirt will use fixed-ram for all APIs it has that involve saving to > > > > disk, with CPUs both running and paused. > > > > > > There are still two scenarios. How should we identify them, then? For > > > sure we can always make it live, but QEMU needs that information to make it > > > efficient for non-live. > > > > > > Considering when there's no auto-pause, then Libvirt will still need to > > > know the scenario first then to decide whether pausing VM before migration > > > or do nothing, am I right? > > > > libvirt will issue a 'stop' before invoking 'migrate' if it > > needs to. QEMU should be able to optimize that scenario if > > it sees CPUs already stopped when migrate is started ? > > > > > If so, can Libvirt replace that "pause VM" operation with setting > > > auto-pause=on here? Again, the benefit is QEMU can benefit from it. > > > > > > I think when pausing Libvirt can still receive an event, then it can > > > cooperate with state changes? Meanwhile auto-pause=on will be set by > > > Libvirt too, so Libvirt will even have that expectation that QMP migrate > > > later on will pause the VM. > > > > > > > > > > > > From that POV it sounds like auto-pause is a good knob for that. > > > > > > > > From libvirt's POV auto-pause will create extra work for integration > > > > for no gain. > > > > > > Yes, I agree for Libvirt there's no gain, as the gain is on QEMU's side. > > > Could you elaborate what is the complexity for Libvirt to support it? > > > > It increases the code paths because we will have to support > > and test different behaviour wrt CPU state for fixed-ram > > vs non-fixed ram usage. > > To me if the user scenario is different, it makes sense to have a flag > showing what the user wants to do. > > Guessing that from "whether VM is running or not" could work in many cases > but not all. > > It means at least for dirty tracking, we only have one option to make it > fully transparent, starting dirty tracking when VM starts during such > migration. The complexity moves from Libvirt into migration / kvm from > this aspect. Even with auto-pause we can't skip dirty tracking, as we don't guarantee the app won't run 'cont' at some point. We could have an explicit capability 'dirty-tracking' which an app could set as an explicit "promise" that it won't ever need to (re)start CPUs while migration is running. If dirty-tracking==no, then any attempt to run 'cont' should return an hard error while migration is running. > Meanwhile we lose some other potential optimizations for good, early > releasing of resources will never be possible anymore because they need to > be prepared to be reused very soon, even if we know they will never. But > maybe that's not a major concern. What resources can we release early, without harming our ability to restart the current QEMU on failure ? > No strong opinion from my side. I'll leave it to Fabiano. I didn't see > any further optimization yet with the new cap in this series. I think the > trick is current extra overheads are just not high enough for us to > care.. even if we know some work is pure overhead. Then indeed we can also > postpone the optimizations until justified worthwhile. 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 :|