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 B558DC636CC for ; Wed, 15 Feb 2023 12:07:19 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pSGYu-0000vf-7h; Wed, 15 Feb 2023 07:07:00 -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 1pSGYr-0000uz-NV for qemu-devel@nongnu.org; Wed, 15 Feb 2023 07:06:58 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pSGYo-0001lr-E7 for qemu-devel@nongnu.org; Wed, 15 Feb 2023 07:06:57 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676462807; 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=kidTgwvwjgmftsLG+RhYWVVdNJh0MM9sobOT2WarSSk=; b=F67RqhllDDvH4wbsH/tyuu8/8BjP2eHfQMn9F0WkRrC62GjZeT0rBBlJrAuU2uVFXdZii8 5MmHTad7+7yciAY3ivqwA39xaOyy+c/W9Vy9f3VpqjgqHccKcHk+VKw/e3QWwwaELNDICb 0H7j7tMt8KXLZbdH700sTPjJ9fVm68Q= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-321-DiI52SWzMiijGxvbgaNCLQ-1; Wed, 15 Feb 2023 07:06:45 -0500 X-MC-Unique: DiI52SWzMiijGxvbgaNCLQ-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5DC328027FD; Wed, 15 Feb 2023 12:06:45 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.13]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EA33E492B15; Wed, 15 Feb 2023 12:06:44 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id EA03F21E6A1F; Wed, 15 Feb 2023 13:06:43 +0100 (CET) From: Markus Armbruster To: Juan Quintela Cc: qemu-devel@nongnu.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , "Dr. David Alan Gilbert" , Eduardo Habkost , Eric Blake , Yanan Wang , Marcel Apfelbaum Subject: Re: [PATCH v5 1/3] multifd: Create property multifd-sync-after-each-section References: <20230213091548.76444-1-quintela@redhat.com> <20230213091548.76444-2-quintela@redhat.com> <87mt5goctt.fsf@pond.sub.org> <87zg9g5dk7.fsf@secure.mitica> Date: Wed, 15 Feb 2023 13:06:43 +0100 In-Reply-To: <87zg9g5dk7.fsf@secure.mitica> (Juan Quintela's message of "Tue, 14 Feb 2023 19:04:08 +0100") Message-ID: <87mt5fb0a4.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 3.1 on 10.11.54.10 Received-SPF: pass client-ip=170.10.129.124; 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_H2=-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 Juan Quintela writes: > Markus Armbruster wrote: >> Juan Quintela writes: >> >>> We used to synchronize all channels at the end of each RAM section >>> sent. That is not needed, so preparing to only synchronize once every >>> full round in latests patches. >>> >>> Notice that we initialize the property as true. We will change the >>> default when we introduce the new mechanism. >>> >>> Signed-off-by: Juan Quintela >>> Reviewed-by: Dr. David Alan Gilbert >>> Signed-off-by: Juan Quintela >>> >>> --- >>> >>> Rename each-iteration to after-each-section >>> >>> Signed-off-by: Juan Quintela >>> --- >>> qapi/migration.json | 10 +++++++++- >>> migration/migration.h | 1 + >>> hw/core/machine.c | 1 + >>> migration/migration.c | 15 +++++++++++++-- >>> 4 files changed, 24 insertions(+), 3 deletions(-) >>> >>> diff --git a/qapi/migration.json b/qapi/migration.json >>> index c84fa10e86..2907241b9c 100644 >>> --- a/qapi/migration.json >>> +++ b/qapi/migration.json >>> @@ -478,6 +478,13 @@ >>> # should not affect the correctness of postcopy migration. >>> # (since 7.1) >>> # >>> +# @multifd-sync-after-each-section: Synchronize channels after each >>> +# section is sent. >> >> What does it mean to synchronize channels? >> >> When would I want to, and why? >> >>> +# We used to do >>> +# that in the past, but it is >>> +# suboptimal. >> >> This isn't particularly helpful, I'm afraid. >> >>> +# Default value is true until all code is in. >> >> As far as I can tell, it's actually *unused* for now, and a later patch >> will put it to use ... > > We (well, libvert preffers) want capabilities to be false by default. > When I introduce a new capability/parameter: > - Patch1: I introduce the capability/parameter, it does nothing yet. > - Patch2: I conditionalize the old code on this capability. > Default value is true (old code). > - Patch3: I introduce the new code to implement the feature. > At this point I change the default. > > Depending on complexity, Patch2 and 3 can be a series, but you get the > idea O:-) I'm fine with this approach, as long as commit messages and comments reflect reality :) >>> +# (since 8.0) > > Retry. What about: > > # @multifd-sync-after-each-section: flush each channel after each > # section sent. This assures that > # we can't mix pages from one > # iteration through the dirty bitmap > # with pages for the following > # iteration. We really only need to > # do this flush after we have go > # trhough all the dirty bitmap. For s/trhough/through/ > # historical reasons, we do that after > # each section. This is suboptimal > # (we flush too many times). > # Default value is true until the code > # to implement it is in tree. > # (since 8.0) > > > Better? Yes, except the comment suggests value false does something, which isn't true, yet. Possible solutions: 1. Accept only configurations that work as advertized: Patch1: add code to reject value false with a suitable "not implemented" error message. Since the behavior is temporary within a single series, documenting this feels optional. Patch2: no change. Patch3: drop the code rejecting false. 2. Document configurations that don't yet work as advertized: Patch1: doc comment states the capability is not yet implemented. Patch2: no change. Patch3: drop the comment. No need to mess with documenting temporary default true in either case. >>> +bool migrate_multifd_sync_after_each_section(void) >>> +{ >>> + MigrationState *s = migrate_get_current(); >>> + >>> + return true; >>> + // We will change this when code gets in. >>> + return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION]; >> >> ... here. >> >> No warning about unreachable code? Checking... nope, gcc seems to not >> to care. > > Yeap. Gcc thinks this is ok. > In others try's I have done: > > return true || s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION]; > > If you preffer I can change to this, not strong opinions. Matter of taste, you pick what you like best. I'd simply start with return true; /* TODO implement */ and replace it with the real expression when its callers are ready for it.