linux-kernel - Re: [RESEND PATCH] xen/blkfront: convert to blk-mq APIs

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Date:	Sat, 11 Jul 2015 16:18:42 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	linux-kernel@...r.kernel.org, axboe@...com, hch@...radead.org,
	xen-devel@...ts.xenproject.org, avanzini.arianna@...il.com,
	david.vrabel@...rix.com, marcus.granado@...rix.com,
	roger.pau@...rix.com
Subject: Re: [RESEND PATCH] xen/blkfront: convert to blk-mq APIs


On 07/11/2015 03:57 AM, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 06, 2015 at 05:56:48PM +0800, Bob Liu wrote:
>> From: Arianna Avanzini <avanzini.arianna@...il.com>
>>
>> This patch converts xen-blkfront driver to use the block multiqueue APIs.
>> Only one hardware queue is used now, so there is no performance change.
>>
>> The legacy non-mq code was deleted completely which is the same as other drivers
>> like virtio, mtip, and nvme.
>>
>> Also dropped unnecessary holding of info->io_lock when calling into blk-mq APIs.
> 
> Yeey!
> 
> Two points:
> 
>  - The io_lock is now used to guard against concurrent access to the ring.
>    We should rename it to 'ring_lock'.
> 

Sure.

>  - The kick_pending_request_queues should have an extra argument - 'bool locked'.
>    This is so that you don't drop and immediately grab the lock from the blkif_interrupt.
> 

Then where to drop the lock?

In kick_pending_request_queues(), the lock have to be dropped before calling blk_mq_start_stopped_hw_queues().

 static void kick_pending_request_queues(struct blkfront_info *info)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&info->io_lock, flags);
 	if (!RING_FULL(&info->ring)) {
-		/* Re-enable calldowns. */
-		blk_start_queue(info->rq);
-		/* Kick things off immediately. */
-		do_blkif_request(info->rq);
+		spin_unlock_irqrestore(&info->io_lock, flags);
+		blk_mq_start_stopped_hw_queues(info->rq, true);
+		return;
 	}

>    See:
> 
>> @@ -1243,9 +1243,8 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>>  	} else
>>  		info->ring.sring->rsp_event = i + 1;
>>  
>> -	kick_pending_request_queues(info);
>> -
>>  	spin_unlock_irqrestore(&info->io_lock, flags);
>> +	kick_pending_request_queues(info);
>>  
>>  	return IRQ_HANDLED;
>>  }
> 
> Otherwise Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> 

-- 
Regards,
-Bob
--
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