Re: [PATCH V3 2/4] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

Chao Leng
2020-09-08T09:13:57Z

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 <ming.lei@redhat.com>
> Cc: Sagi Grimberg <sagi@grimberg.me>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
> Cc: Chao Leng <lengchao@huawei.com>
> ---
> 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 <linux/blkdev.h>
> #include <linux/sbitmap.h>
> -#include <linux/srcu.h>
>
> 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;
>