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auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="v9Ux+11Zm5mwPlX6" Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=stefanha@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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Zhanghailiang , qemu-block@nongnu.org, Zhenyu Ye , qemu-devel@nongnu.org, xiexiangyou@huawei.com, armbru@redhat.com, Glauber Costa , Paolo Bonzini , mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --v9Ux+11Zm5mwPlX6 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Oct 22, 2020 at 05:29:16PM +0100, Fam Zheng wrote: > On Tue, 2020-10-20 at 09:34 +0800, Zhenyu Ye wrote: > > On 2020/10/19 21:25, Paolo Bonzini wrote: > > > On 19/10/20 14:40, Zhenyu Ye wrote: > > > > The kernel backtrace for io_submit in GUEST is: > > > >=20 > > > > =09guest# ./offcputime -K -p `pgrep -nx fio` > > > > =09 b'finish_task_switch' > > > > =09 b'__schedule' > > > > =09 b'schedule' > > > > =09 b'io_schedule' > > > > =09 b'blk_mq_get_tag' > > > > =09 b'blk_mq_get_request' > > > > =09 b'blk_mq_make_request' > > > > =09 b'generic_make_request' > > > > =09 b'submit_bio' > > > > =09 b'blkdev_direct_IO' > > > > =09 b'generic_file_read_iter' > > > > =09 b'aio_read' > > > > =09 b'io_submit_one' > > > > =09 b'__x64_sys_io_submit' > > > > =09 b'do_syscall_64' > > > > =09 b'entry_SYSCALL_64_after_hwframe' > > > > =09 - fio (1464) > > > > =09=0940031912 > > > >=20 > > > > And Linux io_uring can avoid the latency problem. >=20 > Thanks for the info. What this tells us is basically the inflight > requests are high. It's sad that the linux-aio is in practice > implemented as a blocking API. >=20 > Host side backtrace will be of more help. Can you get that too? I guess Linux AIO didn't set the BLK_MQ_REQ_NOWAIT flag so the task went to sleep when it ran out of blk-mq tags. The easiest solution is to move to io_uring. Linux AIO is broken - it's not AIO :). If we know that no other process is writing to the host block device then maybe we can determine the blk-mq tags limit (the queue depth) and avoid sending more requests. That way QEMU doesn't block, but I don't think this approach works when other processes are submitting I/O to the same host block device :(. Fam's original suggestion of invoking io_submit(2) from a worker thread is an option, but I'm afraid it will slow down the uncontended case. I'm CCing Glauber in case he battled this in the past in ScyllaDB. 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