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 894B6E9A03B for ; Thu, 19 Feb 2026 15:50:24 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vt6HR-0003nk-9p; Thu, 19 Feb 2026 10:49:29 -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 1vt6HP-0003nC-Dv for qemu-devel@nongnu.org; Thu, 19 Feb 2026 10:49:27 -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 1vt6HN-0002WZ-74 for qemu-devel@nongnu.org; Thu, 19 Feb 2026 10:49:27 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1771516164; 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=MtgxNQdQJWLIUBtmn1jwh5WsRw1wET05NJnYGYnkKVA=; b=Yh7p1AzX/LuBCZo7ezKmPwJun8e5AjfMesRC6Ci74fYVnRTVjSm/XXJI4Su3slVZX2o8JV T/c5daQvlxa3vM7mFyw2ZUcKBNW3snLIeUN9n2uySXwdU/GsXaoe2OqTF02h5razsF4YBV 8iAk6kfYwYbsLbW6pmmvpbe/9A+SjXo= 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-434-zya2MBKYMxiuWhRe2v4ejg-1; Thu, 19 Feb 2026 10:49:19 -0500 X-MC-Unique: zya2MBKYMxiuWhRe2v4ejg-1 X-Mimecast-MFC-AGG-ID: zya2MBKYMxiuWhRe2v4ejg_1771516157 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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E3BEF18003F6; Thu, 19 Feb 2026 15:49:16 +0000 (UTC) Received: from redhat.com (unknown [10.45.224.47]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 050FF1954111; Thu, 19 Feb 2026 15:49:13 +0000 (UTC) Date: Thu, 19 Feb 2026 16:49:11 +0100 From: Kevin Wolf To: Jens Axboe Cc: Stefan Hajnoczi , Fiona Ebner , qemu-block@nongnu.org, qemu-devel@nongnu.org, fam@euphon.net Subject: Re: [PATCH v2] aio-posix: notify main loop when SQEs are queued Message-ID: References: <20260213143225.161043-1-axboe@kernel.dk> <20260213143225.161043-2-axboe@kernel.dk> <20260218161127.GC587447@fedora> <07d701b9-3039-4f9b-99a2-abeae51146a5@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <07d701b9-3039-4f9b-99a2-abeae51146a5@kernel.dk> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 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=-0.045, 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_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_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 18.02.2026 um 17:41 hat Jens Axboe geschrieben: > On 2/18/26 9:19 AM, Jens Axboe wrote: > > On 2/18/26 9:11 AM, Stefan Hajnoczi wrote: > >> On Wed, Feb 18, 2026 at 10:57:02AM +0100, Fiona Ebner wrote: > >>> Am 13.02.26 um 5:05 PM schrieb Kevin Wolf: > >>>> 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? > >>> > >>> Yes, it does fix my issue [0] and the second patch gives another small > >>> improvement :) > >>> > >>> Would it be slightly cleaner to have aio_add_sqe() call aio_notify() > >>> itself? Since aio-posix.c calls downwards into fdmon-io_uring.c, it > >>> would feel nicer to me to not have fdmon-io_uring.c call "back up". I > >>> guess it also depends on whether we expect another future fdmon > >>> implementation with .add_sqe() to also benefit from it. > >> > >> Calling aio_notify() from aio-posix.c:aio_add_sqe() sounds better to me > >> because fdmon-io_uring.c has to be careful about calling aio_*() APIs to > >> avoid loops. > > > > Would anyone care to make that edit? I'm on a plane and gone for a bit, > > so won't get back to this for the next week. But I would love to see a > > fix go in, as this issue has been plaguing me with test timeouts for > > quite a while on the CI front. And seems like I'm not alone, if the > > patches fix Fiona's issues as well. > > Still on a plane but tested this one and it works for me too. Does seem > like a better approach, rather than stuff it in the fdmon part. > > Feel free to run with this one and also to update the commit message if > you want. Thanks! > > > commit a8a94e7a05964d470b8fba50c9d4769489c21752 > Author: Jens Axboe > Date: Fri Feb 13 06:52:14 2026 -0700 > > aio-posix: notify main loop when SQEs are queued > > 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. > > 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 > > diff --git a/util/aio-posix.c b/util/aio-posix.c > index e24b955fd91a..8c7b3795c82d 100644 > --- a/util/aio-posix.c > +++ b/util/aio-posix.c > @@ -813,5 +813,13 @@ void aio_add_sqe(void (*prep_sqe)(struct io_uring_sqe *sqe, void *opaque), > { > AioContext *ctx = qemu_get_current_aio_context(); > ctx->fdmon_ops->add_sqe(ctx, prep_sqe, opaque, 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 I think the comment could even be more generic here. This is not specific to coroutines, but the scenario is just that a vCPU thread holding the BQL performs I/O. > + * actual io_uring_submit() only happens in gsource_prepare(). Without > + * this notify, ppoll() can sleep up to 499ms before submitting. > + */ > + aio_notify(ctx); > } > #endif /* CONFIG_LINUX_IO_URING */ With or without a changed comment to that effect: Reviewed-by: Kevin Wolf