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 lists1p.gnu.org (lists1p.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 6F3B4CD6E55 for ; Mon, 1 Jun 2026 17:21:11 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wU6Jy-0005YI-8N; Mon, 01 Jun 2026 13:21:02 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wU6Jw-0005UZ-DI for qemu-devel@nongnu.org; Mon, 01 Jun 2026 13:21:00 -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 1wU6Jt-0003Dq-6B for qemu-devel@nongnu.org; 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Mon, 1 Jun 2026 17:20:45 +0000 (UTC) Received: from localhost (unknown [10.2.16.102]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id E54391684; Mon, 1 Jun 2026 17:20:44 +0000 (UTC) Date: Mon, 1 Jun 2026 13:20:43 -0400 From: Stefan Hajnoczi To: Fiona Ebner Cc: "open list:Network Block Dev..." , QEMU Developers , Fam Zheng , Hanna Czenczek , Kevin Wolf , Thomas Lamprecht , Jens Axboe Subject: Re: Excessive IO PSI for iothread when using io_uring since QEMU 10.2 Message-ID: <20260601172043.GB458909@fedora> References: <017dc767-90e3-4983-8417-e541b3fb04f6@proxmox.com> <20260427191343.GD218226@fedora> <317ab384-f8c2-49af-89c4-407bb2f5617c@proxmox.com> <20260428161927.GB278591@fedora> <9b11dec1-f27c-4c69-8dfc-34ef0bdc7e8f@proxmox.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="vzItGSfQCP1+5cK4" Content-Disposition: inline In-Reply-To: <9b11dec1-f27c-4c69-8dfc-34ef0bdc7e8f@proxmox.com> X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 8 X-Spam_score: 0.8 X-Spam_bar: / X-Spam_report: (0.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, 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, RCVD_IN_SBL_CSS=3.335, 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 --vzItGSfQCP1+5cK4 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Apr 29, 2026 at 10:00:34AM +0200, Fiona Ebner wrote: > Am 28.04.26 um 6:18 PM schrieb Stefan Hajnoczi: > > On Tue, Apr 28, 2026 at 02:10:02PM +0200, Fiona Ebner wrote: > >> Hi Stefan, > >> > >> Am 27.04.26 um 9:12 PM schrieb Stefan Hajnoczi: > >>> On Fri, Apr 24, 2026 at 12:25:41PM +0200, Fiona Ebner wrote: > >>>> Dear maintainers, > >>>> > >>>> since QEMU 10.2, if io_uring is enabled, it will be used for the eve= nt > >>>> loop of iothreads and this causes an IO pressure stall value of near= ly > >>>> 100 when idle. > >>>> > >>>> The issue was also reported on the kernel mailing list [0]. The > >>>> suggestion from Jens Axboe was to just turn off the iowait accounting > >>>> completely. But since (for block/file-posix.c), there is actual IO > >>>> submitted via the same ring, I wasn't sure if that is the right appr= oach. > >>>> > >>>> So the idea was to keep track of whether the event loop is otherwise > >>>> idle and only use the IORING_ENTER_NO_IOWAIT flag in that case [1]. > >>>> > >>>> However, doing so would only help for block/file-posix.c, which subm= its > >>>> IO via luring_co_submit() -> fdmon_io_uring_add_sqe(). For example, = for > >>>> block/rbd.c, only a poll SQE for the AioHandler node's fd is used. W= hen > >>>> submitting that poll SQE in the iothread, we would need to be able to > >>>> know if IO for RBD is currently in-flight or not to be able to decide > >>>> whether to use the IORING_ENTER_NO_IOWAIT flag or not. Is there a go= od > >>>> way to do this (in a general way)? > >>>> > >>>> Or should the flag really always be used (if supported by the kernel= )? > >>>> Is there a way to tell io_uring/kernel that we are an event loop and= our > >>>> waiting should only be accounted for when there is actual IO in-flig= ht? > >>>> > >>>> Happy to hear your opinions and suggestions! > >>>> > >>>> [0]: > >>>> https://lore.kernel.org/io-uring/14bc6266-5bc9-4454-9518-d1016bfe417= b@proxmox.com/T/ > >>> > >>> Hi Fiona, > >>> Jens replied yesterday confirmed your suspicion that the number of > >>> inflight requests is not being tracked correctly. > >>> > >>> Is there still a problem after fixing the kernel's inflight counting?= If > >>> not, then no QEMU change is necessary and that seems like the cleanest > >>> solution anyway. The kernel should know whether there is I/O in flight > >>> and so it doesn't seem right that userspace needs to hint this. > >> > >> > >> unfortunately, yes. Even with the kernel fix [2], the real problem with > >> poll SQEs described above remains. I'm still seeing high IO pressure > >> stall values when using QEMU. In add_poll_add_sqe(), QEMU submits poll > >> SQEs for the AioHandler node fd, and that does count as pending IO. A > >> small reproducer modeling this [3]. > >=20 > > Does the kernel account POLL_ADD SQEs as blocking I/O activity? >=20 > Apparently yes. See the C program below [3]. >=20 > > That behavior is inconsistent if select(2)/poll(2)/epoll_wait(2) > > syscalls do not count as blocking I/O activity. The kernel io_uring code > > should account them correctly and not rely on a userspace hint. >=20 > @Jens Axboe: should there be a separate internal counter for > poll/timeout SQEs and have them not count towards IO wait by default? Hi Fiona, Any update on this issue? Was it resolved in io_uring or is a QEMU patch still needed? 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