linux-kernel - Re: [patch, v2] blk-mq: avoid excessive boot delays with large lun counts

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Message-ID: <x49ziywqyyy.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 02 Nov 2015 09:04:37 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Jason Luo <zhangqing.luo@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Guru Anbalagane <guru.anbalagane@...cle.com>,
	Feng Jin <joe.jin@...cle.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [patch, v2] blk-mq: avoid excessive boot delays with large lun counts

Ming Lei <tom.leiming@...il.com> writes:

> You can add
>          Reviewed-by: Ming Lei <ming.lei@...onical.com>
> if the following trivial issues(especially the 2nd one) are addressed.

[snip]

>> @@ -1891,7 +1890,12 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>>
>>         mutex_lock(&set->tag_list_lock);
>>         list_del_init(&q->tag_set_list);
>> -       blk_mq_update_tag_set_depth(set);
>> +       if (set->tag_list.next == set->tag_list.prev) {
>
> list_is_singular() should be better.

Didn't even know that existed.  Thanks.

>> +               /* just transitioned to unshared */
>> +               set->flags &= ~BLK_MQ_F_TAG_SHARED;
>> +               /* update existing queue */
>> +               blk_mq_update_tag_set_depth(set, false);
>> +       }
>>         mutex_unlock(&set->tag_list_lock);
>>  }
>>
>> @@ -1901,8 +1905,17 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
>>         q->tag_set = set;
>>
>>         mutex_lock(&set->tag_list_lock);
>> +
>> +       /* Check to see if we're transitioning to shared (from 1 to 2 queues). */
>> +       if (!list_empty(&set->tag_list) && !(set->flags & BLK_MQ_F_TAG_SHARED)) {
>> +               set->flags |= BLK_MQ_F_TAG_SHARED;
>> +               /* update existing queue */
>> +               blk_mq_update_tag_set_depth(set, true);
>> +       }
>> +       if (set->flags & BLK_MQ_F_TAG_SHARED)
>
> The above should be 'else if', otherwise the current queue will be set
> twice.

I moved the list add below this to avoid that very issue.  See:

>> +               queue_set_hctx_shared(q, true);
>>         list_add_tail(&q->tag_set_list, &set->tag_list);
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This seemed the cleanest way to structure the code to avoid the double
walking of the hctx list for the current q.

-Jeff

>> -       blk_mq_update_tag_set_depth(set);
>> +
>>         mutex_unlock(&set->tag_list_lock);
>>  }
>>
--
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