Commit 749fefe6 authored Sep 20, 2012 by Tejun Heo Committed by Jens Axboe Sep 21, 2012

block: lift the initial queue bypass mode on blk_register_queue() instead of...


block: lift the initial queue bypass mode on blk_register_queue() instead of blk_init_allocated_queue()

b82d4b19 ("blkcg: make request_queue bypassing on allocation") made
request_queues bypassed on allocation to avoid switching on and off
bypass mode on a queue being initialized.  Some drivers allocate and
then destroy a lot of queues without fully initializing them and
incurring bypass latency overhead on each of them could add upto
significant overhead.

Unfortunately, blk_init_allocated_queue() is never used by queues of
bio-based drivers, which means that all bio-based driver queues are in
bypass mode even after initialization and registration complete
successfully.

Due to the limited way request_queues are used by bio drivers, this
problem is hidden pretty well but it shows up when blk-throttle is
used in combination with a bio-based driver.  Trying to configure
(echoing to cgroupfs file) blk-throttle for a bio-based driver hangs
indefinitely in blkg_conf_prep() waiting for bypass mode to end.

This patch moves the initial blk_queue_bypass_end() call from
blk_init_allocated_queue() to blk_register_queue() which is called for
any userland-visible queues regardless of its type.

I believe this is correct because I don't think there is any block
driver which needs or wants working elevator and blk-cgroup on a queue
which isn't visible to userland.  If there are such users, we need a
different solution.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Joseph Glanville <joseph.glanville@orionvm.com.au>
Cc: stable@vger.kernel.org
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 66ba32dc

block/blk-core.c

+2 −5

Original line number	Diff line number	Diff line
		@@ -608,8 +608,8 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
		/*
		* 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().
		* init. The initial bypass will be finished when the queue is
		* registered by blk_register_queue().
		*/
		q->bypass_depth = 1;
		__set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
		@@ -712,9 +712,6 @@ blk_init_allocated_queue(struct request_queue q, request_fn_proc rfn,
		/* init elevator */
		if (elevator_init(q, NULL))
		return NULL;

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

block/blk-sysfs.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -561,6 +561,12 @@ int blk_register_queue(struct gendisk *disk)
		if (WARN_ON(!q))
		return -ENXIO;

		/*
		* Initialization must be complete by now. Finish the initial
		* bypass from queue allocation.
		*/
		blk_queue_bypass_end(q);

		ret = blk_trace_init_sysfs(dev);
		if (ret)
		return ret;

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