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Tue, 08 Sep 2020 05:27:45 -0400 X-MC-Unique: oHFY9PPGPNCCJhtG_cIHOQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 71CE210054FF; Tue, 8 Sep 2020 09:27:43 +0000 (UTC) Received: from T590 (ovpn-12-217.pek2.redhat.com [10.72.12.217]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 117CA5D9E2; Tue, 8 Sep 2020 09:27:34 +0000 (UTC) Date: Tue, 8 Sep 2020 17:27:23 +0800 From: Ming Lei To: Chao Leng Subject: Re: [PATCH V3 2/4] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING Message-ID: <20200908092723.GA1094743@T590> References: <20200908081538.1434936-1-ming.lei@redhat.com> <20200908081538.1434936-3-ming.lei@redhat.com> <6aa24c1e-d127-27fc-9ca7-3299e026aa0a@huawei.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <6aa24c1e-d127-27fc-9ca7-3299e026aa0a@huawei.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200908_052747_950205_9DD83C68 X-CRM114-Status: GOOD ( 35.96 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jens Axboe , Sagi Grimberg , Johannes Thumshirn , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Keith Busch , Christoph Hellwig , Bart Van Assche Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On Tue, Sep 08, 2020 at 05:13:22PM +0800, Chao Leng wrote: > > > 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. No, when request is issued directly, it never be queued to LLD, so we should insert it to scheduler queue, see commit: db03f88fae8a ("blk-mq: insert request not through ->queue_rq into sw/scheduler queue") Thanks, Ming _______________________________________________ Linux-nvme mailing list Linux-nvme@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-nvme