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 9A387EDF17C for ; Fri, 13 Feb 2026 16:05:13 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vqvex-0000n3-3V; Fri, 13 Feb 2026 11:04:47 -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 1vqveu-0000m4-AP for qemu-devel@nongnu.org; Fri, 13 Feb 2026 11:04:44 -0500 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 1vqves-0003yT-NC for qemu-devel@nongnu.org; Fri, 13 Feb 2026 11:04:44 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1770998681; 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=0F5O5OwCVjT3jCHBwLprBwZ/F0o/ChVCcim7L8oUkcs=; b=AMMsw6/9UeOplp1Pog6RzwlyNh0rIlMHkAk0CxBZJIFb2lilxxl+aPSQEoeAp/qKzYbzbK s7+tzsOozgkMCzn4zSWanwDYssJuHVphhNIe1rh9U12gdq28HensgcXlcXlsKCqJjJ8+Us DYjWnVlDra3MRkRDyEgDIn5CiZjQvbw= 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-388-LE-EWCOuMsCjZyQuUtXG1w-1; Fri, 13 Feb 2026 11:04:38 -0500 X-MC-Unique: LE-EWCOuMsCjZyQuUtXG1w-1 X-Mimecast-MFC-AGG-ID: LE-EWCOuMsCjZyQuUtXG1w_1770998677 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 8A8911800366; Fri, 13 Feb 2026 16:04:36 +0000 (UTC) Received: from redhat.com (unknown [10.44.32.253]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E48E319560B9; Fri, 13 Feb 2026 16:04:33 +0000 (UTC) Date: Fri, 13 Feb 2026 17:04:31 +0100 From: Kevin Wolf To: Jens Axboe Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, fam@euphon.net, stefanha@redhat.com, f.ebner@proxmox.com Subject: Re: [PATCH 1/2] fdmon-io_uring: notify main loop when SQEs are queued Message-ID: References: <20260213143225.161043-1-axboe@kernel.dk> <20260213143225.161043-2-axboe@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260213143225.161043-2-axboe@kernel.dk> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@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, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable 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 13.02.2026 um 15:26 hat Jens Axboe geschrieben: > When a vCPU thread handles MMIO (holding BQL), aio_co_enter() runs the > block I/O coroutine inline on the vCPU thread because > qemu_get_current_aio_context() returns the main AioContext when BQL is > held. The coroutine calls luring_co_submit() which queues an SQE via > fdmon_io_uring_add_sqe(), but the actual io_uring_submit() only happens > in gsource_prepare() on the main loop thread. Ouch! Yes, looks like we completely missed I/O submitted in vCPU threads in the recent changes (or I guess worker threads in theory, but I don't think there any that actually make use of aio_add_sqe()). > Since the coroutine ran inline (not via aio_co_schedule()), no BH is > scheduled and aio_notify() is never called. The main loop remains asleep > in ppoll() with up to a 499ms timeout, leaving the SQE unsubmitted until > the next timer fires. > > Fix this by calling aio_notify() after queuing the SQE. This wakes the > main loop via the eventfd so it can run gsource_prepare() and submit the > pending SQE promptly. > > This is a generic fix that benefits all devices using aio=io_uring. > Without it, AHCI/SATA devices see MUCH worse I/O latency since they use > MMIO (not ioeventfd like virtio) and have no other mechanism to wake the > main loop after queuing block I/O. > > This is usually a bit hard to detect, as it also relies on the ppoll > loop not waking up for other activity, and micro benchmarks tend not to > see it because they don't have any real processing time. With a > synthetic test case that has a few usleep() to simulate processing of > read data, it's very noticeable. The below example reads 128MB with > O_DIRECT in 128KB chunks in batches of 16, and has a 1ms delay before > each batch submit, and a 1ms delay after processing each completion. > Running it on /dev/sda yields: > > time sudo ./iotest /dev/sda > > ________________________________________________________ > Executed in 25.76 secs fish external > usr time 6.19 millis 783.00 micros 5.41 millis > sys time 12.43 millis 642.00 micros 11.79 millis > > while on a virtio-blk or NVMe device we get: > > time sudo ./iotest /dev/vdb > > ________________________________________________________ > Executed in 1.25 secs fish external > usr time 1.40 millis 0.30 millis 1.10 millis > sys time 17.61 millis 1.43 millis 16.18 millis > > time sudo ./iotest /dev/nvme0n1 > > ________________________________________________________ > Executed in 1.26 secs fish external > usr time 6.11 millis 0.52 millis 5.59 millis > sys time 13.94 millis 1.50 millis 12.43 millis > > where the latter are consistent. If we run the same test but keep the > socket for the ssh connection active by having activity there, then > the sda test looks as follows: > > time sudo ./iotest /dev/sda > > ________________________________________________________ > Executed in 1.23 secs fish external > usr time 2.70 millis 39.00 micros 2.66 millis > sys time 4.97 millis 977.00 micros 3.99 millis > > as now the ppoll loop is woken all the time anyway. > > After this fix, on an idle system: > > time sudo ./iotest /dev/sda > > ________________________________________________________ > Executed in 1.30 secs fish external > usr time 2.14 millis 0.14 millis 2.00 millis > sys time 16.93 millis 1.16 millis 15.76 millis > > Signed-off-by: Jens Axboe > --- > util/fdmon-io_uring.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c > index d0b56127c670..96392876b490 100644 > --- a/util/fdmon-io_uring.c > +++ b/util/fdmon-io_uring.c > @@ -181,6 +181,14 @@ static void fdmon_io_uring_add_sqe(AioContext *ctx, > > trace_fdmon_io_uring_add_sqe(ctx, opaque, sqe->opcode, sqe->fd, sqe->off, > cqe_handler); > + > + /* > + * Wake the main loop if it is sleeping in ppoll(). When a vCPU thread > + * runs a coroutine inline (holding BQL), it queues SQEs here but the > + * actual io_uring_submit() only happens in gsource_prepare(). Without > + * this notify, ppoll() can sleep up to 499ms before submitting. > + */ > + aio_notify(ctx); > } Makes sense to me. At first I wondered if we should use defer_call() for the aio_notify() to batch the submission, but of course holding the BQL will already take care of that. And in iothreads where there is no BQL, the aio_notify() shouldn't make a difference anyway because we're already in the right thread. I suppose the other variation could be have another io_uring_enter() call here (but then probably really through defer_call()) to avoid waiting for another CPU to submit the request in its main loop. But I don't really have an intuition if that would make things better or worse in the common case. Fiona, does this fix your case, too? Kevin