From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <jianchao.w.wang@oracle.com>
Subject: Re: [RFC PATCH 09/14] SCSI: create admin queue for each host
To: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, Alan Stern <stern@rowland.harvard.edu>,
        Christoph Hellwig <hch@lst.de>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        Hannes Reinecke <hare@suse.de>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        Adrian Hunter <adrian.hunter@intel.com>,
        "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        linux-scsi@vger.kernel.org
References: <20180807174433.8374-1-ming.lei@redhat.com>
 <20180807174433.8374-10-ming.lei@redhat.com>
From: "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <dd553e06-738e-37cb-ec4a-c8b695cb1b79@oracle.com>
Date: Wed, 8 Aug 2018 13:57:01 +0800
MIME-Version: 1.0
In-Reply-To: <20180807174433.8374-10-ming.lei@redhat.com>
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List-ID: <linux-block@vger.kernel.org>

Hi Ming

On 08/08/2018 01:44 AM, Ming Lei wrote:
>  
> +static struct request_queue *scsi_mq_alloc_admin_queue(struct Scsi_Host *shost)
> +{
> +	struct request_queue *q = __blk_mq_init_queue(&shost->tag_set,
> +			QUEUE_FLAG_MQ_NO_SCHED_DEFAULT);
> +	if (IS_ERR(q))
> +		return NULL;
> +
> +	q->mq_ops = &scsi_mq_admin_ops;
> +
> +	__scsi_init_queue(shost, q);
> +
> +	return q;
> +}

In your patch set, the logical adminq per host is standalone request_queue which share
the tagset with other request_queue.

Due to the hctx_may_queue, 
If only one LUN, the adminq will take away half of the driver tags when
the adminq is active.
And when multiple LUNs, all of the LUNs have to share the limited budget of tags
of the adminq.
This is unacceptable.

And also, not all the admin request is send out through scsi_execute, maybe SG_IO.
So the admin queue here looks more like the pm queue ?

Thanks
Jianchao