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 56DE6FD2D83 for ; Tue, 10 Mar 2026 13:32:59 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vzxCQ-0003gW-Db; Tue, 10 Mar 2026 09:32:40 -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 1vzxCE-0003Zk-Ic for qemu-devel@nongnu.org; Tue, 10 Mar 2026 09:32:27 -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 1vzxCC-0008Ez-2n for qemu-devel@nongnu.org; Tue, 10 Mar 2026 09:32:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1773149542; 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=ovxQUbtmY5HawT+7RnCmX9AF/zgMuZDZQGN6eRSEjLQ=; b=N6aNad9FhO2YCgBzKGGGlMYillJTTEC1wlSZniixK7cvRHiFh0QaMBYNAVNDnEp3plGvM+ sF3onqIyxNs/5pNeLpTolGn5HHCpNqiUgXSvPqanlVNUE4eA5YapC1FRs2dfNb1ddNskTU 1JN/NNPJST+9dDBd80OcakymjO+flmA= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-512-j22-o2RrNKG_W8mToZ6llQ-1; Tue, 10 Mar 2026 09:32:20 -0400 X-MC-Unique: j22-o2RrNKG_W8mToZ6llQ-1 X-Mimecast-MFC-AGG-ID: j22-o2RrNKG_W8mToZ6llQ_1773149539 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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 665D118002E0; Tue, 10 Mar 2026 13:32:19 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.12]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 89CFE3000223; Tue, 10 Mar 2026 13:32:18 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 20F1D21E681B; Tue, 10 Mar 2026 14:32:16 +0100 (CET) From: Markus Armbruster To: Kevin Wolf Cc: qemu-block@nongnu.org, hreitz@redhat.com, xeor@yandex-team.ru, vsementsov@yandex-team.ru, pkrempa@redhat.com, qemu-devel@nongnu.org Subject: Re: [PATCH v2] block: Never drop BLOCK_IO_ERROR with action=stop for rate limiting In-Reply-To: <20260304122800.51923-1-kwolf@redhat.com> (Kevin Wolf's message of "Wed, 4 Mar 2026 13:28:00 +0100") References: <20260304122800.51923-1-kwolf@redhat.com> Date: Tue, 10 Mar 2026 14:32:16 +0100 Message-ID: <87y0jzresf.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 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: -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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org This is now commit 544ddbb6373d61292a0e2dc269809cd6bd5edec6. I'm not objecting. Just a few remarks. I dropped qemu-stable@ from cc. Kevin Wolf writes: > Commit 2155d2dd introduced rate limiting for BLOCK_IO_ERROR to emit an > event only once a second. This makes sense for cases in which the guest > keeps running and can submit more requests that would possibly also fail > because there is a problem with the backend. > > However, if the error policy is configured so that the VM is stopped on > errors, this is both unnecessary because stopping the VM means that the > guest can't issue more requests and in fact harmful because stopping the > VM is an important state change that management tools need to keep track > of even if it happens more than once in a given second. If an event is > dropped, the management tool would see a VM randomly going to paused > state without an associated error, so it has a hard time deciding how to > handle the situation. > > This patch disables rate limiting for action=stop by not relying on the > event type alone any more in monitor_qapi_event_queue_no_reenter(), but > checking action for BLOCK_IO_ERROR, too. If the error is reported to the > guest or ignored, the rate limiting stays in place. > > Fixes: 2155d2dd7f73 ('block-backend: per-device throttling of BLOCK_IO_ERROR reports') > Signed-off-by: Kevin Wolf > --- > qapi/block-core.json | 2 +- > monitor/monitor.c | 21 ++++++++++++++++++++- > 2 files changed, 21 insertions(+), 2 deletions(-) > > diff --git a/qapi/block-core.json b/qapi/block-core.json > index b66bf316e2f..da0b36a3751 100644 > --- a/qapi/block-core.json > +++ b/qapi/block-core.json > @@ -5794,7 +5794,7 @@ > # .. note:: If action is "stop", a `STOP` event will eventually follow > # the `BLOCK_IO_ERROR` event. > # > -# .. note:: This event is rate-limited. > +# .. note:: This event is rate-limited, except if action is "stop". > # > # Since: 0.13 > # > diff --git a/monitor/monitor.c b/monitor/monitor.c > index 1273eb72605..37fa674cfe6 100644 > --- a/monitor/monitor.c > +++ b/monitor/monitor.c > @@ -367,14 +367,33 @@ monitor_qapi_event_queue_no_reenter(QAPIEvent event, QDict *qdict) > { > MonitorQAPIEventConf *evconf; > MonitorQAPIEventState *evstate; > + bool throttled; > > assert(event < QAPI_EVENT__MAX); > evconf = &monitor_qapi_event_conf[event]; > trace_monitor_protocol_event_queue(event, qdict, evconf->rate); > + throttled = evconf->rate; > + > + /* > + * Rate limit BLOCK_IO_ERROR only for action != "stop". > + * > + * If the VM is stopped after an I/O error, this is important information > + * for the management tool to keep track of the state of QEMU and we can't > + * merge any events. At the same time, stopping the VM means that the guest > + * can't send additional requests and the number of events is already > + * limited, so we can do without rate limiting. > + */ > + if (event == QAPI_EVENT_BLOCK_IO_ERROR) { > + QDict *data = qobject_to(QDict, qdict_get(qdict, "data")); > + const char *action = qdict_get_str(data, "action"); > + if (!strcmp(action, "stop")) { > + throttled = false; > + } > + } Having event-specific logic in the general event emission function is ugly. Before the patch, the "throttle this event?" logic is coded in one place: table monitor_qapi_event_conf[]. The table maps from event kind (enum QAPIEvent) to the minimum time between two events. Non-zero specifies a rate limit, zero makes it unlimited. Aside: as far as I can tell, we've only ever used one MonitorQAPIEventConf: { .rate = 1000 * SCALE_MS }. Could be dumbed down to bool. This is insufficient for QAPIEvent QAPI_EVENT_BLOCK_IO_ERROR, where the desired rate depends on event data, not just the QAPIEvent. The patch gives us the desired rate, but it splits the logic between the map and the map's user. I think the cleaner solution is to make the map more capable: have it maps from the entire event, not just its kind. An obvious way to do that would be a table of function pointers bool (*)(QAPIEvent, QDict *) Null means unlimited. The table entry for BLOCK_IO_ERROR returns false for unlimited when "action" is "stop", else true. The table entries for the other rate-limited events simply return true. Thoughts? > > QEMU_LOCK_GUARD(&monitor_lock); > > - if (!evconf->rate) { > + if (!throttled) { > /* Unthrottled event */ > monitor_qapi_event_emit(event, qdict); > } else {