From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 91FC22397A4 for ; Fri, 9 May 2025 03:29:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746761398; cv=none; b=ezebC4LXjEIyFosnGNI39XrLk1X4hFvHH7SVpM7iaUkA9XPI4WLDA/SFwoctjYbh1TqCRGNdJ61qTo+v+cHx+OIDdPw0bhyy2t7RtKQPNXBWki3ZMgsw9d4UpdC+RQhycQkKvC8P6Cr4Fr6oINMwNhbBYriOV74ELKEfDsOdS50= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746761398; c=relaxed/simple; bh=S1yi6rnhNlMVa5dLotn8dcdofZ1p332V+ifvPLisQj8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LT4q5MZiEvI93tp0YNPhi2CpzhfmCdT8UBfWufSjMj7FfCKzxA9vpkWWC6cJtMSFt9inBU6i1OIT0s4RFgrcaZRY6KkJC3E73wflDK3/lYF6ILSkEDIJTkHJuztnLh61uJ9QdYfdV4RL+rLLQMFcIPR6XLwVr+PDkaC/4ZPGMVM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=X54x/TXE; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="X54x/TXE" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1746761395; 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=aX5g4T/ui8zYWG/CR78eBjg3zZKc3gVFjYjjZQSVWQc=; b=X54x/TXE5KZq4gS01fPv50XDRryWneXJ9Mm31dxidVUzo7QAhCcsQgkcrDoWDbErnj43CU V+9HXGsyp+9rWMtKAy69b5ovGU+ZZaV25Gv8ABZutmGOays5b0dR5PF3lQxc4aRp/F/t/c F23jS3+fRrbFDgTDqCoWGMtF5ThVToU= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-194-wz6MvrzNP5OeqswktbK8zQ-1; Thu, 08 May 2025 23:29:49 -0400 X-MC-Unique: wz6MvrzNP5OeqswktbK8zQ-1 X-Mimecast-MFC-AGG-ID: wz6MvrzNP5OeqswktbK8zQ_1746761388 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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 79D1219560AE; Fri, 9 May 2025 03:29:47 +0000 (UTC) Received: from fedora (unknown [10.72.116.120]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7CBA219560AE; Fri, 9 May 2025 03:29:40 +0000 (UTC) Date: Fri, 9 May 2025 11:29:35 +0800 From: Ming Lei To: Uday Shankar Cc: Jens Axboe , Caleb Sander Mateos , Andrew Morton , Shuah Khan , Jonathan Corbet , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH v6 1/8] ublk: have a per-io daemon instead of a per-queue daemon Message-ID: References: <20250507-ublk_task_per_io-v6-0-a2a298783c01@purestorage.com> <20250507-ublk_task_per_io-v6-1-a2a298783c01@purestorage.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250507-ublk_task_per_io-v6-1-a2a298783c01@purestorage.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On Wed, May 07, 2025 at 03:49:35PM -0600, Uday Shankar wrote: > Currently, ublk_drv associates to each hardware queue (hctx) a unique > task (called the queue's ubq_daemon) which is allowed to issue > COMMIT_AND_FETCH commands against the hctx. If any other task attempts > to do so, the command fails immediately with EINVAL. When considered > together with the block layer architecture, the result is that for each > CPU C on the system, there is a unique ublk server thread which is > allowed to handle I/O submitted on CPU C. This can lead to suboptimal > performance under imbalanced load generation. For an extreme example, > suppose all the load is generated on CPUs mapping to a single ublk > server thread. Then that thread may be fully utilized and become the > bottleneck in the system, while other ublk server threads are totally > idle. > > This issue can also be addressed directly in the ublk server without > kernel support by having threads dequeue I/Os and pass them around to > ensure even load. But this solution requires inter-thread communication > at least twice for each I/O (submission and completion), which is > generally a bad pattern for performance. The problem gets even worse > with zero copy, as more inter-thread communication would be required to > have the buffer register/unregister calls to come from the correct > thread. > > Therefore, address this issue in ublk_drv by allowing each I/O to have > its own daemon task. Two I/Os in the same queue are now allowed to be > serviced by different daemon tasks - this was not possible before. > Imbalanced load can then be balanced across all ublk server threads as > follows: > > - specifying the new UBLK_F_RR_TAGS flag when creating the ublk device, > to ensure round-robin tag allocation > - having the ublk server threads issue FETCH_REQs in a round-robin > manner. As a small toy example, consider a system with a single ublk > device having 2 queues, each of depth 4. A ublk server having 4 > threads could issue its FETCH_REQs against this device as follows > (where each entry is the qid,tag pair that the FETCH_REQ targets): > > ublk server thread: T0 T1 T2 T3 > 0,0 0,1 0,2 0,3 > 1,3 1,0 1,1 1,2 > > The combination of these two changes allow a ublk server to balance > load. For example, suppose a program quickly issues 4 I/Os concurrently. > Before this change, all 4 I/Os would have had to be serviced by the same > thread, and that thread may become a bottleneck. With this change, those > 4 I/Os would get tags 0-3 for the same queue, and would thus spread out > evenly over all the ublk server threads. Peak bandwidth in CPU-limited > workloads under imbalanced load thus will increase. > > The UBLK_F_RR_TAGS flag is also added as a feature, so that ublk servers > can essentially test for the presence of this patch and tailor their > behavior accordingly. > > Signed-off-by: Uday Shankar > Reviewed-by: Caleb Sander Mateos > --- > drivers/block/ublk_drv.c | 82 ++++++++++++++++++++++--------------------- > include/uapi/linux/ublk_cmd.h | 8 +++++ > 2 files changed, 50 insertions(+), 40 deletions(-) > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c > index cb612151e9a1de7630c49a2b78d278e005d52856..ff5beab32220ea96a1e15ab26c02f4883a6c3d6e 100644 > --- a/drivers/block/ublk_drv.c > +++ b/drivers/block/ublk_drv.c > @@ -66,7 +66,8 @@ > | UBLK_F_USER_COPY \ > | UBLK_F_ZONED \ > | UBLK_F_USER_RECOVERY_FAIL_IO \ > - | UBLK_F_UPDATE_SIZE) > + | UBLK_F_UPDATE_SIZE \ > + | UBLK_F_RR_TAGS) > > #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \ > | UBLK_F_USER_RECOVERY_REISSUE \ > @@ -148,6 +149,8 @@ struct ublk_io { > /* valid if UBLK_IO_FLAG_OWNED_BY_SRV is set */ > struct request *req; > }; > + > + struct task_struct *task; > }; > > struct ublk_queue { > @@ -155,11 +158,9 @@ struct ublk_queue { > int q_depth; > > unsigned long flags; > - struct task_struct *ubq_daemon; > struct ublksrv_io_desc *io_cmd_buf; > > bool force_abort; > - bool timeout; > bool canceling; > bool fail_io; /* copy of dev->state == UBLK_S_DEV_FAIL_IO */ > unsigned short nr_io_ready; /* how many ios setup */ > @@ -1069,11 +1070,6 @@ static inline struct ublk_uring_cmd_pdu *ublk_get_uring_cmd_pdu( > return io_uring_cmd_to_pdu(ioucmd, struct ublk_uring_cmd_pdu); > } > > -static inline bool ubq_daemon_is_dying(struct ublk_queue *ubq) > -{ > - return !ubq->ubq_daemon || ubq->ubq_daemon->flags & PF_EXITING; > -} > - > /* todo: handle partial completion */ > static inline void __ublk_complete_rq(struct request *req) > { > @@ -1207,13 +1203,13 @@ static void ublk_dispatch_req(struct ublk_queue *ubq, > /* > * Task is exiting if either: > * > - * (1) current != ubq_daemon. > + * (1) current != io->task. > * io_uring_cmd_complete_in_task() tries to run task_work > - * in a workqueue if ubq_daemon(cmd's task) is PF_EXITING. > + * in a workqueue if cmd's task is PF_EXITING. > * > * (2) current->flags & PF_EXITING. > */ > - if (unlikely(current != ubq->ubq_daemon || current->flags & PF_EXITING)) { > + if (unlikely(current != io->task || current->flags & PF_EXITING)) { > __ublk_abort_rq(ubq, req); > return; > } > @@ -1286,13 +1282,10 @@ static void ublk_queue_cmd_list(struct ublk_queue *ubq, struct rq_list *l) > static enum blk_eh_timer_return ublk_timeout(struct request *rq) > { > struct ublk_queue *ubq = rq->mq_hctx->driver_data; > + struct ublk_io *io = &ubq->ios[rq->tag]; > > if (ubq->flags & UBLK_F_UNPRIVILEGED_DEV) { > - if (!ubq->timeout) { > - send_sig(SIGKILL, ubq->ubq_daemon, 0); > - ubq->timeout = true; > - } > - > + send_sig(SIGKILL, io->task, 0); > return BLK_EH_DONE; > } > > @@ -1405,17 +1398,6 @@ static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq) > /* All old ioucmds have to be completed */ > ubq->nr_io_ready = 0; > > - /* > - * old daemon is PF_EXITING, put it now > - * > - * It could be NULL in case of closing one quisced device. > - */ > - if (ubq->ubq_daemon) > - put_task_struct(ubq->ubq_daemon); > - /* We have to reset it to NULL, otherwise ub won't accept new FETCH_REQ */ > - ubq->ubq_daemon = NULL; > - ubq->timeout = false; > - > for (i = 0; i < ubq->q_depth; i++) { > struct ublk_io *io = &ubq->ios[i]; > > @@ -1426,6 +1408,17 @@ static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq) > io->flags &= UBLK_IO_FLAG_CANCELED; > io->cmd = NULL; > io->addr = 0; > + > + /* > + * old task is PF_EXITING, put it now > + * > + * It could be NULL in case of closing one quiesced > + * device. > + */ > + if (io->task) { > + put_task_struct(io->task); > + io->task = NULL; > + } > } > } > > @@ -1447,7 +1440,7 @@ static void ublk_reset_ch_dev(struct ublk_device *ub) > for (i = 0; i < ub->dev_info.nr_hw_queues; i++) > ublk_queue_reinit(ub, ublk_get_queue(ub, i)); > > - /* set to NULL, otherwise new ubq_daemon cannot mmap the io_cmd_buf */ > + /* set to NULL, otherwise new tasks cannot mmap io_cmd_buf */ > ub->mm = NULL; > ub->nr_queues_ready = 0; > ub->nr_privileged_daemon = 0; > @@ -1722,7 +1715,7 @@ static void ublk_uring_cmd_cancel_fn(struct io_uring_cmd *cmd, > return; > > task = io_uring_cmd_get_task(cmd); > - if (WARN_ON_ONCE(task && task != ubq->ubq_daemon)) > + if (WARN_ON_ONCE(task && task != ubq->ios[pdu->tag].task)) > return; > > if (!ubq->canceling) > @@ -1861,8 +1854,6 @@ static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq) > { > ubq->nr_io_ready++; > if (ublk_queue_ready(ubq)) { > - ubq->ubq_daemon = current; > - get_task_struct(ubq->ubq_daemon); > ub->nr_queues_ready++; > > if (capable(CAP_SYS_ADMIN)) > @@ -1995,6 +1986,7 @@ static int ublk_fetch(struct io_uring_cmd *cmd, struct ublk_queue *ubq, > } > > ublk_fill_io_cmd(io, cmd, buf_addr); > + WRITE_ONCE(io->task, get_task_struct(current)); > ublk_mark_io_ready(ub, ubq); > out: > mutex_unlock(&ub->mutex); > @@ -2062,6 +2054,7 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd, > const struct ublksrv_io_cmd *ub_cmd) > { > struct ublk_device *ub = cmd->file->private_data; > + struct task_struct *task; > struct ublk_queue *ubq; > struct ublk_io *io; > u32 cmd_op = cmd->cmd_op; > @@ -2076,13 +2069,14 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd, > goto out; > > ubq = ublk_get_queue(ub, ub_cmd->q_id); > - if (ubq->ubq_daemon && ubq->ubq_daemon != current) > - goto out; > > if (tag >= ubq->q_depth) > goto out; > > io = &ubq->ios[tag]; > + task = READ_ONCE(io->task); > + if (task && task != current) > + goto out; > > /* there is pending io cmd, something must be wrong */ > if (io->flags & UBLK_IO_FLAG_ACTIVE) { > @@ -2332,9 +2326,15 @@ static void ublk_deinit_queue(struct ublk_device *ub, int q_id) > { > int size = ublk_queue_cmd_buf_size(ub, q_id); > struct ublk_queue *ubq = ublk_get_queue(ub, q_id); > + struct ublk_io *io; > + int i; > + > + for (i = 0; i < ubq->q_depth; i++) { > + io = &ubq->ios[i]; > + if (io->task) > + put_task_struct(io->task); > + } > > - if (ubq->ubq_daemon) > - put_task_struct(ubq->ubq_daemon); > if (ubq->io_cmd_buf) > free_pages((unsigned long)ubq->io_cmd_buf, get_order(size)); > } > @@ -2487,6 +2487,8 @@ static int ublk_add_tag_set(struct ublk_device *ub) > ub->tag_set.numa_node = NUMA_NO_NODE; > ub->tag_set.cmd_size = sizeof(struct ublk_rq_data); > ub->tag_set.driver_data = ub; > + if (ub->dev_info.flags & UBLK_F_RR_TAGS) > + ub->tag_set.flags |= BLK_MQ_F_TAG_RR; > return blk_mq_alloc_tag_set(&ub->tag_set); > } > > @@ -3062,14 +3064,14 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub, > int ublksrv_pid = (int)header->data[0]; > int ret = -EINVAL; > > - pr_devel("%s: Waiting for new ubq_daemons(nr: %d) are ready, dev id %d...\n", > - __func__, ub->dev_info.nr_hw_queues, header->dev_id); > - /* wait until new ubq_daemon sending all FETCH_REQ */ > + pr_devel("%s: Waiting for all FETCH_REQs, dev id %d...\n", __func__, > + header->dev_id); > + > if (wait_for_completion_interruptible(&ub->completion)) > return -EINTR; > > - pr_devel("%s: All new ubq_daemons(nr: %d) are ready, dev id %d\n", > - __func__, ub->dev_info.nr_hw_queues, header->dev_id); > + pr_devel("%s: All FETCH_REQs received, dev id %d\n", __func__, > + header->dev_id); > > mutex_lock(&ub->mutex); > if (ublk_nosrv_should_stop_dev(ub)) > diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h > index be5c6c6b16e098838a2bf790e588b79656defdda..3ba8f26d60bc70ade95b4f4d5990157af2bd5b2e 100644 > --- a/include/uapi/linux/ublk_cmd.h > +++ b/include/uapi/linux/ublk_cmd.h > @@ -219,6 +219,14 @@ > */ > #define UBLK_F_UPDATE_SIZE (1ULL << 10) > > +/* > + * Force tags to be allocated round-robin on each queue. If ublk server > + * threads also issue FETCH_REQs against the queues in a round-robin > + * manner, load can be balanced across all threads even if it is issued > + * to the queues in an imbalanced way. > + */ > +#define UBLK_F_RR_TAGS (1ULL << 11) Please make UBLK_F_RR_TAGS as one standalone patch, which is also helpful for evaluating round-robin performance, and doesn't have to be used by per-io task. Thanks, Ming