blkcg: make request_queue bypassing on allocation (b82d4b19) · Commits · 戴 / test

With the previous change to guarantee bypass visiblity for RCU read lock regions, entering bypass mode involves non-trivial overhead and future changes are scheduled to make use of bypass mode during init path. Combined it may end up adding noticeable delay during boot. This patch makes request_queue start its life in bypass mode, which is ended on queue init completion at the end of blk_init_allocated_queue(), and updates blk_queue_bypass_start() such that draining and RCU synchronization are performed only when the queue actually enters bypass mode. This avoids unnecessarily switching in and out of bypass mode during init avoiding the overhead and any nasty surprises which may step from leaving bypass mode on half-initialized queues. The boot time overhead was pointed out by Vivek. Signed-off-by:

Tejun Heo <tj@kernel.org> Cc: Vivek Goyal <vgoyal@redhat.com> Signed-off-by:

Jens Axboe <axboe@kernel.dk>

block/blk-core.c

+25 −12

Original line number	Diff line number	Diff line
		@@ -421,15 +421,19 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
		*/
		void blk_queue_bypass_start(struct request_queue *q)
		{
		bool drain;

		spin_lock_irq(q->queue_lock);
		q->bypass_depth++;
		drain = !q->bypass_depth++;
		queue_flag_set(QUEUE_FLAG_BYPASS, q);
		spin_unlock_irq(q->queue_lock);

		if (drain) {
		blk_drain_queue(q, false);
		/* ensure blk_queue_bypass() is %true inside RCU read lock */
		synchronize_rcu();
		}
		}
		EXPORT_SYMBOL_GPL(blk_queue_bypass_start);

		/**
		@@ -577,6 +581,15 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
		*/
		q->queue_lock = &q->__queue_lock;

		/*
		* A queue starts its life with bypass turned on to avoid
		* unnecessary bypass on/off overhead and nasty surprises during
		* init. The initial bypass will be finished at the end of
		* blk_init_allocated_queue().
		*/
		q->bypass_depth = 1;
		__set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);

		if (blkcg_init_queue(q))
		goto fail_id;

		@@ -672,15 +685,15 @@ blk_init_allocated_queue(struct request_queue q, request_fn_proc rfn,

		q->sg_reserved_size = INT_MAX;

		/*
		* all done
		*/
		if (!elevator_init(q, NULL)) {
		/* init elevator */
		if (elevator_init(q, NULL))
		return NULL;

		blk_queue_congestion_threshold(q);
		return q;
		}

		return NULL;
		/* all done, end the initial bypass */
		blk_queue_bypass_end(q);
		return q;
		}
		EXPORT_SYMBOL(blk_init_allocated_queue);

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