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Date: Thu, 10 Apr 2025 17:34:18 +0200
From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
	Nilay Shroff <nilay@linux.ibm.com>,
	Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
	Thomas =?iso-8859-1?Q?Hellstr=F6m?= <thomas.hellstrom@linux.intel.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 05/15] block: simplify elevator reset for updating
 nr_hw_queues
Message-ID: <20250410153417.GA12430@lst.de>
References: <20250410133029.2487054-1-ming.lei@redhat.com> <20250410133029.2487054-6-ming.lei@redhat.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Thu, Apr 10, 2025 at 09:30:17PM +0800, Ming Lei wrote:
> @@ -5071,9 +4984,15 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
>  		blk_mq_debugfs_register_hctxs(q);
>  	}
>  
> -switch_back:
> -	list_for_each_entry(q, &set->tag_list, tag_set_list)
> -		blk_mq_elv_switch_back(&head, q);
> +	list_for_each_entry(q, &set->tag_list, tag_set_list) {
> +		const char *name = "none";
> +
> +		mutex_lock(&q->elevator_lock);
> +		if (q->elevator && !blk_queue_dying(q))
> +			name = q->elevator->type->elevator_name;
> +		__elevator_change(q, name, true);
> +		mutex_unlock(&q->elevator_lock);
> +	}

Coming back to this after looking through the next patches.

Why do we even need the __elevator_change call here?  We've not
actually disabled the elevator, and we prevent other callers
from changing it.

As you pass in the force argument this now always calls
elevator_switch and thus blk_mq_init_sched.  But why?