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Date: Tue, 3 Nov 2015 09:12:16 +0800
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Subject: Re: [patch, v2] blk-mq: avoid excessive boot delays with large lun counts
From: Ming Lei <tom.leiming@gmail.com>
To: Jeff Moyer <jmoyer@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, Jason Luo <zhangqing.luo@oracle.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Guru Anbalagane <guru.anbalagane@oracle.com>,
        Feng Jin <joe.jin@oracle.com>, Tejun Heo <tj@kernel.org>
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On Mon, Nov 2, 2015 at 10:04 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Ming Lei <tom.leiming@gmail.com> writes:
>
>> You can add
>>          Reviewed-by: Ming Lei <ming.lei@canonical.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.

OK, it is correct, then v1 is fine.

Reviewed-by: Ming Lei <ming.lei@canonical.com>

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