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=-12.9 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 652CFC433E2 for ; Tue, 8 Sep 2020 09:13:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 346EB2166E for ; Tue, 8 Sep 2020 09:13:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728705AbgIHJN4 (ORCPT ); Tue, 8 Sep 2020 05:13:56 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:3154 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727995AbgIHJNy (ORCPT ); Tue, 8 Sep 2020 05:13:54 -0400 Received: from DGGEMM404-HUB.china.huawei.com (unknown [172.30.72.56]) by Forcepoint Email with ESMTP id D725E62D58891F56C74D; Tue, 8 Sep 2020 17:13:51 +0800 (CST) Received: from dggema772-chm.china.huawei.com (10.1.198.214) by DGGEMM404-HUB.china.huawei.com (10.3.20.212) with Microsoft SMTP Server (TLS) id 14.3.487.0; Tue, 8 Sep 2020 17:13:35 +0800 Received: from [10.169.42.93] (10.169.42.93) by dggema772-chm.china.huawei.com (10.1.198.214) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1913.5; Tue, 8 Sep 2020 17:13:34 +0800 Subject: Re: [PATCH V3 2/4] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING To: Ming Lei , Jens Axboe , , , "Christoph Hellwig" , Keith Busch CC: Sagi Grimberg , Bart Van Assche , Johannes Thumshirn References: <20200908081538.1434936-1-ming.lei@redhat.com> <20200908081538.1434936-3-ming.lei@redhat.com> From: Chao Leng Message-ID: <6aa24c1e-d127-27fc-9ca7-3299e026aa0a@huawei.com> Date: Tue, 8 Sep 2020 17:13:22 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200908081538.1434936-3-ming.lei@redhat.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.169.42.93] X-ClientProxiedBy: dggeme718-chm.china.huawei.com (10.1.199.114) To dggema772-chm.china.huawei.com (10.1.198.214) X-CFilter-Loop: Reflected Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2020/9/8 16:15, Ming Lei 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: Sagi Grimberg > Cc: Bart Van Assche > Cc: Johannes Thumshirn > Cc: Chao Leng > --- > block/blk-mq-sysfs.c | 2 - > block/blk-mq.c | 130 +++++++++++++++++++++-------------------- > block/blk-sysfs.c | 6 +- > include/linux/blk-mq.h | 8 --- > include/linux/blkdev.h | 4 ++ > 5 files changed, 77 insertions(+), 73 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 13cc10b89629..60630a720449 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -220,26 +220,22 @@ 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; > + bool blocking = !!(q->tag_set->flags & BLK_MQ_F_BLOCKING); > > mutex_lock(&q->mq_quiesce_lock); > > - if (blk_queue_quiesced(q)) > - goto exit; > - > - 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 (!blk_queue_quiesced(q)) { > + blk_mq_quiesce_queue_nowait(q); > + if (blocking) > + percpu_ref_kill(&q->dispatch_counter); > } > - if (rcu) > + > + if (blocking) > + wait_event(q->mq_quiesce_wq, > + percpu_ref_is_zero(&q->dispatch_counter)); > + else > synchronize_rcu(); > - exit: > + > mutex_unlock(&q->mq_quiesce_lock); > } > EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue); > @@ -255,7 +251,12 @@ void blk_mq_unquiesce_queue(struct request_queue *q) > { > mutex_lock(&q->mq_quiesce_lock); > > - blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q); > + if (blk_queue_quiesced(q)) { > + blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q); > + > + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) > + percpu_ref_resurrect(&q->dispatch_counter); > + } > > /* dispatch requests which are inserted during quiescing */ > blk_mq_run_hw_queues(q, true); > @@ -710,24 +711,21 @@ 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(); > + if (hctx->flags & BLK_MQ_F_BLOCKING) > + percpu_ref_put(&hctx->queue->dispatch_counter); > else > - srcu_read_unlock(hctx->srcu, srcu_idx); > + rcu_read_unlock(); > } > > -static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx) > - __acquires(hctx->srcu) > +/* Returning false means that queue is being quiesced */ > +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(); > - } else > - *srcu_idx = srcu_read_lock(hctx->srcu); > + if (hctx->flags & BLK_MQ_F_BLOCKING) > + return percpu_ref_tryget_live(&hctx->queue->dispatch_counter); > + rcu_read_lock(); > + return true; > } > > /** > @@ -1506,8 +1504,6 @@ 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. > @@ -1541,9 +1537,10 @@ 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); > + } > } > > static inline int blk_mq_first_mapped_cpu(struct blk_mq_hw_ctx *hctx) > @@ -1655,7 +1652,6 @@ 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; > > /* > @@ -1666,10 +1662,12 @@ 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); > @@ -2009,7 +2007,7 @@ 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, > @@ -2057,11 +2055,14 @@ 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); > + /* Insert request to queue in case of being quiesced */ > + if (!hctx_lock(hctx)) { > + blk_mq_sched_insert_request(rq, false, false, false); Suggest: use blk_mq_request_bypass_insert, the rq should do first. > + return; > + } > > ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false, true); > if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) > @@ -2069,19 +2070,22 @@ 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); > + /* Insert request to queue in case of being quiesced */ > + if (!hctx_lock(hctx)) { > + blk_mq_sched_insert_request(rq, false, false, false); Same here. > + return BLK_STS_OK; > + } > ret = __blk_mq_try_issue_directly(hctx, rq, &unused_cookie, true, last); > - hctx_unlock(hctx, srcu_idx); > + hctx_unlock(hctx); > > return ret; > } > @@ -2612,20 +2616,6 @@ 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) > @@ -2663,7 +2653,7 @@ 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; > > @@ -2706,8 +2696,6 @@ 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; > @@ -3187,6 +3175,13 @@ 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) > @@ -3203,6 +3198,14 @@ 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); > > @@ -3211,7 +3214,7 @@ 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); > @@ -3245,6 +3248,9 @@ 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 b23eeca4d677..df642055f02c 100644 > --- a/include/linux/blk-mq.h > +++ b/include/linux/blk-mq.h > @@ -4,7 +4,6 @@ > > #include > #include > -#include > > struct blk_mq_tags; > struct blk_flush_queue; > @@ -173,13 +172,6 @@ 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 cc6fb4d0d078..f27819574e86 100644 > --- a/include/linux/blkdev.h > +++ b/include/linux/blkdev.h > @@ -574,6 +574,10 @@ struct request_queue { > > struct mutex mq_quiesce_lock; > > + /* only used for BLK_MQ_F_BLOCKING */ > + struct percpu_ref dispatch_counter; > + wait_queue_head_t mq_quiesce_wq; > + > struct blk_mq_tag_set *tag_set; > struct list_head tag_set_list; > struct bio_set bio_split; >