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 DDD591062871 for ; Wed, 11 Mar 2026 12:02:39 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1w0IGo-0002Wu-Df; Wed, 11 Mar 2026 08:02:34 -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 1w0IGa-0002Hu-Fy for qemu-devel@nongnu.org; Wed, 11 Mar 2026 08:02:24 -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 1w0IGY-0005dz-6Q for qemu-devel@nongnu.org; Wed, 11 Mar 2026 08:02:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1773230536; 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=IoYZSMUXLiKFO9q3cK1qRXf0gZW5P67gqip9HXFZRJ4=; b=WRCGEac6cEAKYY5YJsSx5T0HBsQg8t8qN3PFGhGZwLktuCA9BgS3HuwVUbl6HipgWevlDQ 1VVDT0KQE3qxlUcP+b/E4JDD5KgItzSsW1mL319oJwkrwz8hsncA0oXMsMAdwXkmbbTzjv +MloXQlJLAMicCHpp2mtRO6ARfbhBg4= Received: from mx-prod-mc-03.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-674-i43VzrgzNz6S52z33yy1bQ-1; Wed, 11 Mar 2026 08:02:13 -0400 X-MC-Unique: i43VzrgzNz6S52z33yy1bQ-1 X-Mimecast-MFC-AGG-ID: i43VzrgzNz6S52z33yy1bQ_1773230532 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 512391955F39; Wed, 11 Mar 2026 12:02:12 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.160]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B5770195410D; Wed, 11 Mar 2026 12:02:09 +0000 (UTC) Date: Wed, 11 Mar 2026 12:02:05 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Tejus GK , "qemu-devel@nongnu.org" , Fabiano Rosas , Eric Blake , Markus Armbruster Subject: Re: [PATCH v2 1/1] io: make zerocopy fallback accounting more accurate Message-ID: References: <20260309090907.956330-1-tejus.gk@nutanix.com> <0DF1A5F6-E20D-4A3F-9285-9205E87DE641@nutanix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 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: -3 X-Spam_score: -0.4 X-Spam_bar: / X-Spam_report: (-0.4 / 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.819, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.903, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development 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 Mon, Mar 09, 2026 at 02:21:49PM -0400, Peter Xu wrote: > On Mon, Mar 09, 2026 at 05:51:29PM +0000, Daniel P. Berrangé wrote: > > On Mon, Mar 09, 2026 at 05:42:08PM +0000, Tejus GK wrote: > > > > > > > > > > On 9 Mar 2026, at 10:47 PM, Daniel P. Berrangé wrote: > > > > > > > > !-------------------------------------------------------------------| > > > > CAUTION: External Email > > > > > > > > |-------------------------------------------------------------------! > > > > > > > > On Mon, Mar 09, 2026 at 12:59:44PM -0400, Peter Xu wrote: > > > >> On Mon, Mar 09, 2026 at 04:48:37PM +0000, Daniel P. Berrangé wrote: > > > >>>> @@ -881,8 +881,8 @@ static int qio_channel_socket_flush_internal(QIOChannel *ioc, > > > >>>> sioc->zero_copy_sent += serr->ee_data - serr->ee_info + 1; > > > >>>> > > > >>>> /* If any sendmsg() succeeded using zero copy, mark zerocopy success */ > > > >>>> - if (serr->ee_code != SO_EE_CODE_ZEROCOPY_COPIED) { > > > >>>> - sioc->new_zero_copy_sent_success = true; > > > >>>> + if (serr->ee_code == SO_EE_CODE_ZEROCOPY_COPIED) { > > > >>>> + sioc->zero_copy_fallback++; > > > >>> > > > >>> ...this is counting the number of MSG_ERRQUEUE items, which is not > > > >>> the same as the number of IO requests. That's why we only used it > > > >>> as a boolean marker originally, rather than making it a counter. > > > >> > > > >> Would the logic still work and better than before? Say, it's a counter of > > > >> "messages" rather than "IOs" then. > > > > > > > > IIUC it is a counter of processing notifications which is not directly > > > > correlated to any action by QEMU - neither bytes nor syscalls. > > > > > > Please correct me if I'm wrong about this, isn’t each notification an information > > > about what happened to an individual IO? > > > > If userspace hasn't read a queued notification yet, the kernel will > > merge new notifications with the existing queued one. > > > > The line above your change > > > > serr->ee_data - serr->ee_info + 1; > > > > records how many notifications were merged, so we now how many > > syscalls were processed. > > > > If ee_code is SO_EE_CODE_ZEROCOPY_COPIED though it means at least > > one syscall resulted in a copy, but that doesn't imply that *all* > > syscalls resulted in a copy. > > > > AFAICT, it could be 1 out of a 1000 syscalls resulted in a copy, > > or it could be 1000 out of 1000 resulted in a copy. We don't know. > > > > IIUC the kernel's merging of notifications appears lossy wrt this > > information. It could be partially mitigated by doing a flush for > > notifications really really frequently but that feels like it would > > have its own downsides > > IMHO what this change does is removing the false negatives. > > Before this patch, if QEMU reports fallback=0, it doesn't mean all the > MSG_ZEROCOPY requests were all fulfilled by zerocopy. It's because we > justify it with one boolean over "a period of time" between two flushes, we > set the boolean to TRUE as long as there is _one_ successful report of > MSG_ZEROCOPY. So even if every flush reports TRUE it only means "there is > at least one MSG_ZEROCOPY request that didn't fallback". It has no > implication of whether a fallback happened. > > Hence, before this v2 patch, there can be false negative reported by QEMU, > assuming there's no fallback (reflected in stats) but it actually happened. > > After this patch, if QEMU reports fallback=0, it guarantees that _all_ > MSG_ZEROCOPY requests are fulfilled with zerocopy. It's because we monitor > all messages and accumulate any fallback cases. Even if the messages can > be merged, when "fallback" shows anything non-zero would imply some > fallback happened. Here, the counter value doesn't really matter much > IMHO, as long as it becomes non-zero. AFAICT, the v1 of this patch was sufficient to address the original bug and maintain the current intended semantics of the migration counter. This v2 is mixing a bug fix with functional change in behaviour and I don't think the latter is justified. With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|