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Subject: Re: [PATCH V3 1/2] blk-mq: init hctx sched after update ctx and hctx
 mapping
To:     Jianchao Wang <jianchao.w.wang@oracle.com>
Cc:     tom.leiming@gmail.com, bart.vanassche@wdc.com,
        keith.busch@linux.intel.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <1534749606-7311-1-git-send-email-jianchao.w.wang@oracle.com>
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <048cdf08-6c9e-f54a-1e5c-f1d40fb2ecdb@kernel.dk>
Date:   Mon, 20 Aug 2018 08:45:26 -0600
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On 8/20/18 1:20 AM, Jianchao Wang wrote:
> @@ -2913,6 +2906,29 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
>  	list_for_each_entry(q, &set->tag_list, tag_set_list)
>  		blk_mq_freeze_queue(q);
>  
> +	/*
> +	 * switch io scheduler to NULL to clean up the data in it.
> +	 * will get it back after update mapping between cpu and hw queues.
> +	 */

"Switch IO scheduler to 'none', cleaning up the data associated with
the previous scheduler. We'll switch back once we're done updating
the new sw to hw queue mappings.

> +	list_for_each_entry(q, &set->tag_list, tag_set_list) {
> +		if (!q->elevator) {
> +			q->elv_type = NULL;
> +			continue;
> +		}
> +		q->elv_type = q->elevator->type;
> +		mutex_lock(&q->sysfs_lock);
> +		/*
> +		 * After elevator_switch_mq, the previous elevator_queue will be
> +		 * released by elevator_release. The reference of the io scheduler
> +		 * module get by elevator_get will also be put. So we need to get
> +		 * a reference of the io scheduler module here to prevent it to be
> +		 * removed.
> +		 */
> +		__module_get(q->elv_type->elevator_owner);
> +		elevator_switch_mq(q, NULL);
> +		mutex_unlock(&q->sysfs_lock);
> +	}

I don't hate this, but I also find it pretty annoying to put local state
into the queue itself. It would be much nicer _not_ to have ->elv_type
in the queue itself. That would need an allocation, however...

Apart from those two things, looks fine to me.

-- 
Jens Axboe