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 D7359EB105B for ; Tue, 10 Mar 2026 14:22:32 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vzxxs-0006JY-9p; Tue, 10 Mar 2026 10:21:40 -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 1vzxxp-0006Iz-D8 for qemu-devel@nongnu.org; Tue, 10 Mar 2026 10:21:37 -0400 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 1vzxxm-0008Mn-0L for qemu-devel@nongnu.org; Tue, 10 Mar 2026 10:21:36 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1773152493; 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=DPDNQB22OH1icWGTv0aradCc9IgBGx5DXINS//ejKek=; b=QbP9RseLJgWm026qPyU10MzwiQMAssXWGgxqtCak0UbyxN5cXT54RrW9NDu2N/xIgEzKgI gPSmTOdXqe2CkKGomLSLtocgnkuFdRB09H5fbVyz9A125e/cpbfQchv1DU5BiAEZtnoFmA 5ngTFkqeHNwL/zA4zaXsmcRKDvFxoh0= Received: from mx-prod-mc-06.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-361-a7TdJ_XjOJqlpaILKeAqRA-1; Tue, 10 Mar 2026 10:21:28 -0400 X-MC-Unique: a7TdJ_XjOJqlpaILKeAqRA-1 X-Mimecast-MFC-AGG-ID: a7TdJ_XjOJqlpaILKeAqRA_1773152486 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0235018005B2; Tue, 10 Mar 2026 14:21:26 +0000 (UTC) Received: from redhat.com (unknown [10.45.224.112]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 41EBD19560A6; Tue, 10 Mar 2026 14:21:23 +0000 (UTC) Date: Tue, 10 Mar 2026 15:21:20 +0100 From: Kevin Wolf To: Markus Armbruster 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 Message-ID: References: <20260304122800.51923-1-kwolf@redhat.com> <87y0jzresf.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87y0jzresf.fsf@pond.sub.org> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@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_H4=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 Am 10.03.2026 um 14:32 hat Markus Armbruster geschrieben: > 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? Like you, I often prefer data to code. However, if the data becomes just a table of function pointers to trivial functions, wouldn't it be better to just have it as code in the first place? It could be a helper function with the same signature as you proposed above and it would simply be a switch statement returning true for a few event types, looking at the QDict for BLOCK_IO_ERROR and returning false for default. Seems a lot simpler than an explicit table of function pointers. Kevin