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 X-Spam-Level: X-Spam-Status: No, score=-9.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 064D5C433E1 for ; Sat, 22 Aug 2020 13:40:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C1AD72078D for ; Sat, 22 Aug 2020 13:40:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="iSiPcFdK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728020AbgHVNkW (ORCPT ); Sat, 22 Aug 2020 09:40:22 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:37311 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727994AbgHVNkS (ORCPT ); Sat, 22 Aug 2020 09:40:18 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1598103615; 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=IUgcbEqo3OaCczU5QhsHJbekyLWNUhLek3xSpMLNotU=; b=iSiPcFdKCzjdHbsxl571K1/iC7pKYlb3OByOfRTg9mxwPwMuhqVq+HSRK3UXNj3BIRRhBn kKA6Cp07G9V/9OsQ1EUMncBM18Z1zqmv6eXHlBQ8M1DStaegBMZBDDHTW/kxdX0ZQ8H1dj m9KhprO9ycwyiwADRyRNpnesykV4w1Y= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-456-12itMRiHM5CNjZMiT1PdVQ-1; Sat, 22 Aug 2020 09:40:13 -0400 X-MC-Unique: 12itMRiHM5CNjZMiT1PdVQ-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 170E01885D8F; Sat, 22 Aug 2020 13:40:11 +0000 (UTC) Received: from T590 (ovpn-12-54.pek2.redhat.com [10.72.12.54]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 767D719728; Sat, 22 Aug 2020 13:39:58 +0000 (UTC) Date: Sat, 22 Aug 2020 21:39:54 +0800 From: Ming Lei To: Sagi Grimberg Cc: Jens Axboe , linux-block@vger.kernel.org, Lai Jiangshan , "Paul E . McKenney" , Josh Triplett , Bart Van Assche , Johannes Thumshirn , Chao Leng , Christoph Hellwig Subject: Re: [PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING Message-ID: <20200822133954.GC3189453@T590> References: <20200820030248.2809559-1-ming.lei@redhat.com> <856f6108-2227-67e8-e913-fdef296a2d26@grimberg.me> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <856f6108-2227-67e8-e913-fdef296a2d26@grimberg.me> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Aug 21, 2020 at 01:14:41PM -0700, Sagi Grimberg wrote: > > > In case of BLK_MQ_F_BLOCKING, blk-mq uses SRCU to mark read critical > > section during dispatching request, then request queue quiesce is based on > > SRCU. What we want to get is low cost added in fast path. > > > > With percpu-ref, it is cleaner and simpler & enough for implementing queue > > quiesce. The main requirement is to make sure all read sections to observe > > QUEUE_FLAG_QUIESCED once blk_mq_quiesce_queue() returns. > > > > Also it becomes much easier to add interface of async queue quiesce. > > > > Meantime memory footprint can be reduced with per-request-queue percpu-ref. > > > > From implementation viewpoint, in fast path, not see percpu_ref is > > slower than SRCU, and srcu tree(default option in most distributions) > > could be slower since memory barrier is required in both lock & unlock, > > and rcu_read_lock()/rcu_read_unlock() should be much cheap than > > smp_mb(). > > > > 1) percpu_ref just hold the rcu_read_lock, then run a check & > > increase/decrease on the percpu variable: > > > > rcu_read_lock() > > if (__ref_is_percpu(ref, &percpu_count)) > > this_cpu_inc(*percpu_count); > > rcu_read_unlock() > > > > 2) srcu tree: > > idx = READ_ONCE(ssp->srcu_idx) & 0x1; > > this_cpu_inc(ssp->sda->srcu_lock_count[idx]); > > smp_mb(); /* B */ /* Avoid leaking the critical section. */ > > > > Also from my test on null_blk(blocking), not observe percpu-ref performs > > worse than srcu, see the following test: > > > > 1) test steps: > > > > rmmod null_blk > /dev/null 2>&1 > > modprobe null_blk nr_devices=1 submit_queues=1 blocking=1 > > fio --bs=4k --size=512G --rw=randread --norandommap --direct=1 --ioengine=libaio \ > > --iodepth=64 --runtime=60 --group_reporting=1 --name=nullb0 \ > > --filename=/dev/nullb0 --numjobs=32 > > > > test machine: HP DL380, 16 cpu cores, 2 threads per core, dual > > sockets/numa, Intel(R) Xeon(R) Silver 4110 CPU @ 2.10GHz > > > > 2) test result: > > - srcu quiesce: 6063K IOPS > > - percpu-ref quiesce: 6113K IOPS > > > > Signed-off-by: Ming Lei > > Cc: Lai Jiangshan > > Cc: Paul E. McKenney > > Cc: Josh Triplett > > Cc: Sagi Grimberg > > Cc: Bart Van Assche > > Cc: Johannes Thumshirn > > Cc: Chao Leng > > Cc: Christoph Hellwig > > > > --- > > V1: > > - remove SRCU related comment > > - remove RFC > > - not dispatch when the dispatch percpu ref becomes not live > > - add test result on commit log > > > > block/blk-mq-sysfs.c | 2 - > > block/blk-mq.c | 104 ++++++++++++++++++++--------------------- > > block/blk-sysfs.c | 6 ++- > > include/linux/blk-mq.h | 7 --- > > include/linux/blkdev.h | 4 ++ > > 5 files changed, 61 insertions(+), 62 deletions(-) > > > > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c > > index 062229395a50..799db7937105 100644 > > --- a/block/blk-mq-sysfs.c > > +++ b/block/blk-mq-sysfs.c > > @@ -38,8 +38,6 @@ static void blk_mq_hw_sysfs_release(struct kobject *kobj) > > cancel_delayed_work_sync(&hctx->run_work); > > - if (hctx->flags & BLK_MQ_F_BLOCKING) > > - cleanup_srcu_struct(hctx->srcu); > > blk_free_flush_queue(hctx->fq); > > sbitmap_free(&hctx->ctx_map); > > free_cpumask_var(hctx->cpumask); > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > index 6294fa5c7ed9..e198bd565109 100644 > > --- a/block/blk-mq.c > > +++ b/block/blk-mq.c > > @@ -220,19 +220,13 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait); > > */ > > void blk_mq_quiesce_queue(struct request_queue *q) > > { > > - struct blk_mq_hw_ctx *hctx; > > - unsigned int i; > > - bool rcu = false; > > - > > blk_mq_quiesce_queue_nowait(q); > > - queue_for_each_hw_ctx(q, hctx, i) { > > - if (hctx->flags & BLK_MQ_F_BLOCKING) > > - synchronize_srcu(hctx->srcu); > > - else > > - rcu = true; > > - } > > - if (rcu) > > + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) { > > + percpu_ref_kill(&q->dispatch_counter); > > + wait_event(q->mq_quiesce_wq, > > + percpu_ref_is_zero(&q->dispatch_counter)); > > Looking at the q_usage_counter percpu, it's protected with the > mq_freeze_lock and mq_freeze_depth, the fact that it's not protected > here makes this non-nesting, which scares me... We had issues before > in this area... In theory, percpu_ref_kill() could be called nested, since percpu_ref_switch_lock covers it. However, the warning in percpu_ref_kill_and_confirm() may be triggered. So looks we just need one mutex to cover quiesce and unquiesce. If queue is being quiesced, return immediately from blk_mq_quiesce_queue(). > > > + } else > > synchronize_rcu(); > > } > > EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue); > > @@ -248,6 +242,9 @@ void blk_mq_unquiesce_queue(struct request_queue *q) > > { > > blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q); > > + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) > > + percpu_ref_resurrect(&q->dispatch_counter); > > Same comment here... > > > + > > /* dispatch requests which are inserted during quiescing */ > > blk_mq_run_hw_queues(q, true); > > } > > @@ -699,24 +696,21 @@ void blk_mq_complete_request(struct request *rq) > > } > > EXPORT_SYMBOL(blk_mq_complete_request); > > -static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx) > > - __releases(hctx->srcu) > > +static void hctx_unlock(struct blk_mq_hw_ctx *hctx) > > { > > if (!(hctx->flags & BLK_MQ_F_BLOCKING)) > > rcu_read_unlock(); > > else > > - srcu_read_unlock(hctx->srcu, srcu_idx); > > + percpu_ref_put(&hctx->queue->dispatch_counter); > > } > > -static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx) > > - __acquires(hctx->srcu) > > +static inline bool hctx_lock(struct blk_mq_hw_ctx *hctx) > > { > > if (!(hctx->flags & BLK_MQ_F_BLOCKING)) { > > - /* shut up gcc false positive */ > > - *srcu_idx = 0; > > rcu_read_lock(); > > + return true; > > } else > > - *srcu_idx = srcu_read_lock(hctx->srcu); > > + return percpu_ref_tryget_live(&hctx->queue->dispatch_counter); > > } > > /** > > @@ -1495,8 +1489,6 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list, > > */ > > static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx) > > { > > - int srcu_idx; > > - > > /* > > * We should be running this queue from one of the CPUs that > > * are mapped to it. > > @@ -1530,9 +1522,10 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx) > > might_sleep_if(hctx->flags & BLK_MQ_F_BLOCKING); > > - hctx_lock(hctx, &srcu_idx); > > - blk_mq_sched_dispatch_requests(hctx); > > - hctx_unlock(hctx, srcu_idx); > > + if (hctx_lock(hctx)) { > > + blk_mq_sched_dispatch_requests(hctx); > > + hctx_unlock(hctx); > > + } > > Maybe invert? > if (!hctx_lock(hctx)) > return; > blk_mq_sched_dispatch_requests(hctx); > hctx_unlock(hctx); > > I think we need a comment to why this is OK, both in the change log > and in the code. Fine, it is because hctx_lock() failure can be thought as queue being quiesced. > > > } > > static inline int blk_mq_first_mapped_cpu(struct blk_mq_hw_ctx *hctx) > > @@ -1644,7 +1637,6 @@ EXPORT_SYMBOL(blk_mq_delay_run_hw_queue); > > */ > > void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) > > { > > - int srcu_idx; > > bool need_run; > > /* > > @@ -1655,10 +1647,12 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) > > * And queue will be rerun in blk_mq_unquiesce_queue() if it is > > * quiesced. > > */ > > - hctx_lock(hctx, &srcu_idx); > > + if (!hctx_lock(hctx)) > > + return; > > + > > need_run = !blk_queue_quiesced(hctx->queue) && > > blk_mq_hctx_has_pending(hctx); > > - hctx_unlock(hctx, srcu_idx); > > + hctx_unlock(hctx); > > if (need_run) > > __blk_mq_delay_run_hw_queue(hctx, async, 0); > > @@ -1997,7 +1991,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx, > > bool run_queue = true; > > /* > > - * RCU or SRCU read lock is needed before checking quiesced flag. > > + * hctx_lock() is needed before checking quiesced flag. > > * > > * When queue is stopped or quiesced, ignore 'bypass_insert' from > > * blk_mq_request_issue_directly(), and return BLK_STS_OK to caller, > > @@ -2045,11 +2039,13 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx, > > struct request *rq, blk_qc_t *cookie) > > { > > blk_status_t ret; > > - int srcu_idx; > > might_sleep_if(hctx->flags & BLK_MQ_F_BLOCKING); > > - hctx_lock(hctx, &srcu_idx); > > + if (!hctx_lock(hctx)) { > > + blk_mq_sched_insert_request(rq, false, false, false); > > + return; > > + } > > I think we want the same flow for both modes. Maybe a preparation patch > that lifts the checks in __blk_mq_try_issue_directly to do this, and > then have the same flow with the hctx_lock failure (with a comment > explaining this). Looks a good idea. > > > ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false, true); > > if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) > > @@ -2057,19 +2053,21 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx, > > else if (ret != BLK_STS_OK) > > blk_mq_end_request(rq, ret); > > - hctx_unlock(hctx, srcu_idx); > > + hctx_unlock(hctx); > > } > > blk_status_t blk_mq_request_issue_directly(struct request *rq, bool last) > > { > > blk_status_t ret; > > - int srcu_idx; > > blk_qc_t unused_cookie; > > struct blk_mq_hw_ctx *hctx = rq->mq_hctx; > > - hctx_lock(hctx, &srcu_idx); > > + if (!hctx_lock(hctx)) { > > + blk_mq_sched_insert_request(rq, false, false, false); > > + return BLK_STS_OK; > > + } > > Same comment here. > > > ret = __blk_mq_try_issue_directly(hctx, rq, &unused_cookie, true, last); > > - hctx_unlock(hctx, srcu_idx); > > + hctx_unlock(hctx); > > return ret; > > } > > @@ -2600,20 +2598,6 @@ static void blk_mq_exit_hw_queues(struct request_queue *q, > > } > > } > > -static int blk_mq_hw_ctx_size(struct blk_mq_tag_set *tag_set) > > -{ > > - int hw_ctx_size = sizeof(struct blk_mq_hw_ctx); > > - > > - BUILD_BUG_ON(ALIGN(offsetof(struct blk_mq_hw_ctx, srcu), > > - __alignof__(struct blk_mq_hw_ctx)) != > > - sizeof(struct blk_mq_hw_ctx)); > > - > > - if (tag_set->flags & BLK_MQ_F_BLOCKING) > > - hw_ctx_size += sizeof(struct srcu_struct); > > - > > - return hw_ctx_size; > > -} > > - > > static int blk_mq_init_hctx(struct request_queue *q, > > struct blk_mq_tag_set *set, > > struct blk_mq_hw_ctx *hctx, unsigned hctx_idx) > > @@ -2651,7 +2635,7 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set, > > struct blk_mq_hw_ctx *hctx; > > gfp_t gfp = GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY; > > - hctx = kzalloc_node(blk_mq_hw_ctx_size(set), gfp, node); > > + hctx = kzalloc_node(sizeof(struct blk_mq_hw_ctx), gfp, node); > > if (!hctx) > > goto fail_alloc_hctx; > > @@ -2693,8 +2677,6 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set, > > if (!hctx->fq) > > goto free_bitmap; > > - if (hctx->flags & BLK_MQ_F_BLOCKING) > > - init_srcu_struct(hctx->srcu); > > blk_mq_hctx_kobj_init(hctx); > > return hctx; > > @@ -3171,6 +3153,13 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, > > mutex_unlock(&q->sysfs_lock); > > } > > +static void blk_mq_dispatch_counter_release(struct percpu_ref *ref) > > +{ > > + struct request_queue *q = container_of(ref, struct request_queue, > > + dispatch_counter); > > + wake_up_all(&q->mq_quiesce_wq); > > +} > > + > > struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set, > > struct request_queue *q, > > bool elevator_init) > > @@ -3187,6 +3176,14 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set, > > if (blk_mq_alloc_ctxs(q)) > > goto err_poll; > > + if (set->flags & BLK_MQ_F_BLOCKING) { > > + init_waitqueue_head(&q->mq_quiesce_wq); > > + if (percpu_ref_init(&q->dispatch_counter, > > + blk_mq_dispatch_counter_release, > > + PERCPU_REF_ALLOW_REINIT, GFP_KERNEL)) > > + goto err_hctxs; > > + } > > + > > /* init q->mq_kobj and sw queues' kobjects */ > > blk_mq_sysfs_init(q); > > @@ -3195,7 +3192,7 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set, > > blk_mq_realloc_hw_ctxs(set, q); > > if (!q->nr_hw_queues) > > - goto err_hctxs; > > + goto err_dispatch_counter; > > INIT_WORK(&q->timeout_work, blk_mq_timeout_work); > > blk_queue_rq_timeout(q, set->timeout ? set->timeout : 30 * HZ); > > @@ -3229,6 +3226,9 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set, > > return q; > > +err_dispatch_counter: > > + if (set->flags & BLK_MQ_F_BLOCKING) > > + percpu_ref_exit(&q->dispatch_counter); > > err_hctxs: > > kfree(q->queue_hw_ctx); > > q->nr_hw_queues = 0; > > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c > > index 7dda709f3ccb..56b6c045e30c 100644 > > --- a/block/blk-sysfs.c > > +++ b/block/blk-sysfs.c > > @@ -941,9 +941,13 @@ static void blk_release_queue(struct kobject *kobj) > > blk_queue_free_zone_bitmaps(q); > > - if (queue_is_mq(q)) > > + if (queue_is_mq(q)) { > > blk_mq_release(q); > > + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) > > + percpu_ref_exit(&q->dispatch_counter); > > + } > > + > > blk_trace_shutdown(q); > > mutex_lock(&q->debugfs_mutex); > > debugfs_remove_recursive(q->debugfs_dir); > > diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h > > index 9d2d5ad367a4..ea3461298de5 100644 > > --- a/include/linux/blk-mq.h > > +++ b/include/linux/blk-mq.h > > @@ -169,13 +169,6 @@ struct blk_mq_hw_ctx { > > * q->unused_hctx_list. > > */ > > struct list_head hctx_list; > > - > > - /** > > - * @srcu: Sleepable RCU. Use as lock when type of the hardware queue is > > - * blocking (BLK_MQ_F_BLOCKING). Must be the last member - see also > > - * blk_mq_hw_ctx_size(). > > - */ > > - struct srcu_struct srcu[]; > > }; > > /** > > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h > > index bb5636cc17b9..5fa8bc1bb7a8 100644 > > --- a/include/linux/blkdev.h > > +++ b/include/linux/blkdev.h > > @@ -572,6 +572,10 @@ struct request_queue { > > struct list_head tag_set_list; > > struct bio_set bio_split; > > + /* only used for BLK_MQ_F_BLOCKING */ > > + struct percpu_ref dispatch_counter; > > Can this be moved to be next to the q_usage_counter? they > will be taken together for blocking drivers... I don't think it is a good idea, at least hctx->srcu is put at the end of hctx, do you want to move it at beginning of hctx? .q_usage_counter should have been put in the 1st cacheline of request queue. If it is moved to the 1st cacheline of request queue, we shouldn't put 'dispatch_counter' there, because it may hurt other non-blocking drivers. > > Also maybe a better name is needed here since it's just > for blocking hctxs. > > > + wait_queue_head_t mq_quiesce_wq; > > + > > struct dentry *debugfs_dir; > > #ifdef CONFIG_BLK_DEBUG_FS > > > > What I think is needed here is at a minimum test quiesce/unquiesce loops > during I/O. code auditing is not enough, there may be driver assumptions > broken with this change (although I hope there shouldn't be). We have elevator switch / updating nr_request stress test, and both relies on quiesce/unquiesce, and I did run such test with this patch. thanks, Ming