linux-kernel - Re: [patch, v3] blk-mq: avoid excessive boot delays with large lun counts

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Message-ID: <5638D6BC.4050607@kernel.dk>
Date:	Tue, 3 Nov 2015 08:46:04 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Ming Lei <ming.lei@...onical.com>,
	Jason Luo <zhangqing.luo@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch, v3] blk-mq: avoid excessive boot delays with large lun
 counts

On 11/03/2015 08:40 AM, Jeff Moyer wrote:
> Hi,
>
> Zhangqing Luo reported long boot times on a system with thousands of
> LUNs when scsi-mq was enabled.  He narrowed the problem down to
> blk_mq_add_queue_tag_set, where every queue is frozen in order to set
> the BLK_MQ_F_TAG_SHARED flag.  Each added device will freeze all queues
> added before it in sequence, which involves waiting for an RCU grace
> period for each one.  We don't need to do this.  After the second queue
> is added, only new queues need to be initialized with the shared tag.
> We can do that by percolating the flag up to the blk_mq_tag_set, and
> updating the newly added queue's hctxs if the flag is set.
>
> This problem was introduced by commit 0d2602ca30e41 (blk-mq: improve
> support for shared tags maps).

Applied, thanks Jeff and Ming.

-- 
Jens Axboe

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