linux-kernel - Re: [PATCH RFC v2 12/24] hpsa: use reserved commands

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Date:   Mon, 30 Mar 2020 14:42:45 +0100
From:   John Garry <john.garry@...wei.com>
To:     Hannes Reinecke <hare@...e.de>, Ming Lei <ming.lei@...hat.com>
CC:     <axboe@...nel.dk>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <bvanassche@....org>,
        <hch@...radead.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <esc.storagedev@...rosemi.com>, <chenxiang66@...ilicon.com>,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [PATCH RFC v2 12/24] hpsa: use reserved commands

On 17/03/2020 09:48, Hannes Reinecke wrote:
> On 3/17/20 10:38 AM, John Garry wrote:
>> On 11/03/2020 08:10, Ming Lei wrote:
>>>> ands(struct ctlr_info *h)
>>>> @@ -5803,6 +5803,7 @@ static int hpsa_scsi_host_alloc(struct 
>>>> ctlr_info *h)
>>>>       sh->max_lun = HPSA_MAX_LUN;
>>>>       sh->max_id = HPSA_MAX_LUN;
>>>>       sh->can_queue = h->nr_cmds - HPSA_NRESERVED_CMDS;
>>>> +    sh->nr_reserved_cmds = HPSA_NRESERVED_CMDS;
>>> Now .nr_reserved_cmds has been passed to blk-mq, you need to increase
>>> sh->can_queue to h->nr_cmds, because .can_queue is the whole queue depth
>>> (include the part of reserved tags), otherwise, IO tags will be
>>> decreased.
>>>
>>
>> Sounds correct.
>>
> I will have having a look at the patchset; I thought I did a patch to 
> modify .can_queue so that it would cover only the usable tags, not the 
> reserved ones.
> 

To me, it makes sense to leave .can_queue unmodified, carry it down to 
blk-mq and allow blk_mq_init_bitmap_tags() find the queue depth:

static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
   int node, int alloc_policy)
{
	unsigned int depth = tags->nr_tags - tags->nr_reserved_tags; *
	bool round_robin = alloc_policy == BLK_TAG_ALLOC_RR;

	if (bt_alloc(&tags->bitmap_tags, depth, round_robin, node))
		goto free_tags;

Cheers,
John