Re: [PATCH v3] xen/blkfront: convert to blk-mq APIs

[Bob Liu]

Hi Rafal,

Please have a try adding "--iodepth_batch=32 --iodepth_batch_complete=32" to the fio command line.
I didn't see this issue any more, neither for domU.

Thanks,
-Bob

On 08/21/2015 04:46 PM, Rafal Mielniczuk wrote:
> On 19/08/15 12:12, Bob Liu wrote:
>> Hi Jens & Christoph,
>>
>> Rafal reported an issue about this patch, that's after this patch no more 
>> merges happen and the performance dropped if "modprobe null_blk irqmode=2 completion_nsec=1000000", 
>> but works fine if "modprobe null_blk".
>>
>> I'm not sure whether it's as expect or not.
>> Do you have any suggestions? Thank you!
>>
>> Here is the test result:
>>
>> fio --name=test --ioengine=libaio --rw=read --numjobs=8 --iodepth=32 \
>>     --time_based=1 --runtime=30 --bs=4KB --filename=/dev/xvdb \
>>     --direct=1 --group_reporting=1 --iodepth_batch=16
>>
>> ========================================================================
>> modprobe null_blk
>> ========================================================================
>> ------------------------------------------------------------------------
>> *no patch* (avgrq-sz = 8.00 avgqu-sz=5.00)
>> ------------------------------------------------------------------------
>> READ: io=10655MB, aggrb=363694KB/s, minb=363694KB/s, maxb=363694KB/s, mint=30001msec, maxt=30001msec
>>
>> Disk stats (read/write):
>>   xvdb: ios=2715852/0, merge=1089/0, ticks=126572/0, in_queue=127456, util=100.00%
>>
>> ------------------------------------------------------------------------
>> *with patch* (avgrq-sz = 8.00 avgqu-sz=8.00)
>> ------------------------------------------------------------------------
>> READ: io=20655MB, aggrb=705010KB/s, minb=705010KB/s, maxb=705010KB/s, mint=30001msec, maxt=30001msec
>>
>> Disk stats (read/write):
>>   xvdb: ios=5274633/0, merge=22/0, ticks=243208/0, in_queue=242908, util=99.98%
>>
>> ========================================================================
>> modprobe null_blk irqmode=2 completion_nsec=1000000
>> ========================================================================
>> ------------------------------------------------------------------------
>> *no patch* (avgrq-sz = 34.00 avgqu-sz=38.00)
>> ------------------------------------------------------------------------
>> READ: io=10372MB, aggrb=354008KB/s, minb=354008KB/s, maxb=354008KB/s, mint=30003msec, maxt=30003msec
>>
>> Disk stats (read/write):
>>   xvdb: ios=621760/0, *merge=1988170/0*, ticks=1136700/0, in_queue=1146020, util=99.76%
>>
>> ------------------------------------------------------------------------
>> *with patch* (avgrq-sz = 8.00 avgqu-sz=28.00)
>> ------------------------------------------------------------------------
>> READ: io=2876.8MB, aggrb=98187KB/s, minb=98187KB/s, maxb=98187KB/s, mint=30002msec, maxt=30002msec
>>
>> Disk stats (read/write):
>>   xvdb: ios=734048/0, merge=0/0, ticks=843584/0, in_queue=843080, util=99.72%
>>
>> Regards,
>> -Bob
> 
> Hello,
> 
> We got a problem with the lack of merges also when we tested on null_blk device in dom0 directly.
> When we enabled multi queue block-layer we got no merges, even when we set the number of submission queues to 1.
> 
> If I don't miss anything, that could suggest the problem lays somewhere in the blk-mq layer itself?
> 
> Please take a look at the results below:
> 
> fio --name=test --ioengine=libaio --rw=read --numjobs=8 --iodepth=32 \
>     --time_based=1 --runtime=30 --bs=4KB --filename=/dev/nullb0 \
>     --direct=1 --group_reporting=1
> 
> ========================================================================
> modprobe null_blk irqmode=2 completion_nsec=1000000 queue_mode=1 submit_queues=1
> ========================================================================
>    READ: io=13692MB, aggrb=467320KB/s, minb=467320KB/s, maxb=467320KB/s, mint=30002msec, maxt=30002msec
> 
> Disk stats (read/write):
>   nullb0: ios=991026/0, merge=2499524/0, ticks=1846952/0, in_queue=900012, util=100.00%
> 
> ========================================================================
> modprobe null_blk irqmode=2 completion_nsec=1000000 queue_mode=2 submit_queues=1
> ========================================================================
>    READ: io=6839.1MB, aggrb=233452KB/s, minb=233452KB/s, maxb=233452KB/s, mint=30002msec, maxt=30002msec
> 
> Disk stats (read/write):
>   nullb0: ios=1743967/0, merge=0/0, ticks=1712900/0, in_queue=1839072, util=100.00%
> 
> Thanks,
> Rafal
> 
>>
>> On 07/13/2015 05:55 PM, Bob Liu wrote:
>>> Note: This patch is based on original work of Arianna's internship for
>>> GNOME's Outreach Program for Women.
>>>
>>> Only one hardware queue is used now, so there is no performance change.
>>>
>>> The legacy non-mq code is deleted completely which is the same as other
>>> drivers like virtio, mtip, and nvme.
>>>
>>> Also dropped one unnecessary holding of info->io_lock when calling
>>> blk_mq_stop_hw_queues().
>>>
>>> Changes in v2:
>>>  - Reorganized blk_mq_queue_rq()
>>>  - Restored most io_locks in place
>>>
>>> Change in v3:
>>>  - Rename blk_mq_queue_rq to blkif_queue_rq
>>>
>>> Signed-off-by: Arianna Avanzini <avanzini.arianna@xxxxxxxxx>
>>> Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx>
>>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>>> Acked-by: Jens Axboe <axboe@xxxxxx>
>>> ---
>>>  drivers/block/xen-blkfront.c |  146 +++++++++++++++++-------------------------
>>>  1 file changed, 60 insertions(+), 86 deletions(-)
>>>
>>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>>> index 6d89ed3..5b45ee5 100644
>>> --- a/drivers/block/xen-blkfront.c
>>> +++ b/drivers/block/xen-blkfront.c
>>> @@ -37,6 +37,7 @@
>>>  
>>>  #include <linux/interrupt.h>
>>>  #include <linux/blkdev.h>
>>> +#include <linux/blk-mq.h>
>>>  #include <linux/hdreg.h>
>>>  #include <linux/cdrom.h>
>>>  #include <linux/module.h>
>>> @@ -148,6 +149,7 @@ struct blkfront_info
>>>  	unsigned int feature_persistent:1;
>>>  	unsigned int max_indirect_segments;
>>>  	int is_ready;
>>> +	struct blk_mq_tag_set tag_set;
>>>  };
>>>  
>>>  static unsigned int nr_minors;
>>> @@ -616,54 +618,41 @@ static inline bool blkif_request_flush_invalid(struct request *req,
>>>  		 !(info->feature_flush & REQ_FUA)));
>>>  }
>>>  
>>> -/*
>>> - * do_blkif_request
>>> - *  read a block; request is in a request queue
>>> - */
>>> -static void do_blkif_request(struct request_queue *rq)
>>> +static int blkif_queue_rq(struct blk_mq_hw_ctx *hctx,
>>> +			   const struct blk_mq_queue_data *qd)
>>>  {
>>> -	struct blkfront_info *info = NULL;
>>> -	struct request *req;
>>> -	int queued;
>>> -
>>> -	pr_debug("Entered do_blkif_request\n");
>>> -
>>> -	queued = 0;
>>> +	struct blkfront_info *info = qd->rq->rq_disk->private_data;
>>>  
>>> -	while ((req = blk_peek_request(rq)) != NULL) {
>>> -		info = req->rq_disk->private_data;
>>> -
>>> -		if (RING_FULL(&info->ring))
>>> -			goto wait;
>>> +	blk_mq_start_request(qd->rq);
>>> +	spin_lock_irq(&info->io_lock);
>>> +	if (RING_FULL(&info->ring))
>>> +		goto out_busy;
>>>  
>>> -		blk_start_request(req);
>>> +	if (blkif_request_flush_invalid(qd->rq, info))
>>> +		goto out_err;
>>>  
>>> -		if (blkif_request_flush_invalid(req, info)) {
>>> -			__blk_end_request_all(req, -EOPNOTSUPP);
>>> -			continue;
>>> -		}
>>> +	if (blkif_queue_request(qd->rq))
>>> +		goto out_busy;
>>>  
>>> -		pr_debug("do_blk_req %p: cmd %p, sec %lx, "
>>> -			 "(%u/%u) [%s]\n",
>>> -			 req, req->cmd, (unsigned long)blk_rq_pos(req),
>>> -			 blk_rq_cur_sectors(req), blk_rq_sectors(req),
>>> -			 rq_data_dir(req) ? "write" : "read");
>>> -
>>> -		if (blkif_queue_request(req)) {
>>> -			blk_requeue_request(rq, req);
>>> -wait:
>>> -			/* Avoid pointless unplugs. */
>>> -			blk_stop_queue(rq);
>>> -			break;
>>> -		}
>>> +	flush_requests(info);
>>> +	spin_unlock_irq(&info->io_lock);
>>> +	return BLK_MQ_RQ_QUEUE_OK;
>>>  
>>> -		queued++;
>>> -	}
>>> +out_err:
>>> +	spin_unlock_irq(&info->io_lock);
>>> +	return BLK_MQ_RQ_QUEUE_ERROR;
>>>  
>>> -	if (queued != 0)
>>> -		flush_requests(info);
>>> +out_busy:
>>> +	spin_unlock_irq(&info->io_lock);
>>> +	blk_mq_stop_hw_queue(hctx);
>>> +	return BLK_MQ_RQ_QUEUE_BUSY;
>>>  }
>>>  
>>> +static struct blk_mq_ops blkfront_mq_ops = {
>>> +	.queue_rq = blkif_queue_rq,
>>> +	.map_queue = blk_mq_map_queue,
>>> +};
>>> +
>>>  static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size,
>>>  				unsigned int physical_sector_size,
>>>  				unsigned int segments)
>>> @@ -671,9 +660,22 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size,
>>>  	struct request_queue *rq;
>>>  	struct blkfront_info *info = gd->private_data;
>>>  
>>> -	rq = blk_init_queue(do_blkif_request, &info->io_lock);
>>> -	if (rq == NULL)
>>> +	memset(&info->tag_set, 0, sizeof(info->tag_set));
>>> +	info->tag_set.ops = &blkfront_mq_ops;
>>> +	info->tag_set.nr_hw_queues = 1;
>>> +	info->tag_set.queue_depth =  BLK_RING_SIZE(info);
>>> +	info->tag_set.numa_node = NUMA_NO_NODE;
>>> +	info->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
>>> +	info->tag_set.cmd_size = 0;
>>> +	info->tag_set.driver_data = info;
>>> +
>>> +	if (blk_mq_alloc_tag_set(&info->tag_set))
>>>  		return -1;
>>> +	rq = blk_mq_init_queue(&info->tag_set);
>>> +	if (IS_ERR(rq)) {
>>> +		blk_mq_free_tag_set(&info->tag_set);
>>> +		return -1;
>>> +	}
>>>  
>>>  	queue_flag_set_unlocked(QUEUE_FLAG_VIRT, rq);
>>>  
>>> @@ -901,19 +903,15 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
>>>  static void xlvbd_release_gendisk(struct blkfront_info *info)
>>>  {
>>>  	unsigned int minor, nr_minors;
>>> -	unsigned long flags;
>>>  
>>>  	if (info->rq == NULL)
>>>  		return;
>>>  
>>> -	spin_lock_irqsave(&info->io_lock, flags);
>>> -
>>>  	/* No more blkif_request(). */
>>> -	blk_stop_queue(info->rq);
>>> +	blk_mq_stop_hw_queues(info->rq);
>>>  
>>>  	/* No more gnttab callback work. */
>>>  	gnttab_cancel_free_callback(&info->callback);
>>> -	spin_unlock_irqrestore(&info->io_lock, flags);
>>>  
>>>  	/* Flush gnttab callback work. Must be done with no locks held. */
>>>  	flush_work(&info->work);
>>> @@ -925,20 +923,18 @@ static void xlvbd_release_gendisk(struct blkfront_info *info)
>>>  	xlbd_release_minors(minor, nr_minors);
>>>  
>>>  	blk_cleanup_queue(info->rq);
>>> +	blk_mq_free_tag_set(&info->tag_set);
>>>  	info->rq = NULL;
>>>  
>>>  	put_disk(info->gd);
>>>  	info->gd = NULL;
>>>  }
>>>  
>>> +/* Must be called with io_lock holded */
>>>  static void kick_pending_request_queues(struct blkfront_info *info)
>>>  {
>>> -	if (!RING_FULL(&info->ring)) {
>>> -		/* Re-enable calldowns. */
>>> -		blk_start_queue(info->rq);
>>> -		/* Kick things off immediately. */
>>> -		do_blkif_request(info->rq);
>>> -	}
>>> +	if (!RING_FULL(&info->ring))
>>> +		blk_mq_start_stopped_hw_queues(info->rq, true);
>>>  }
>>>  
>>>  static void blkif_restart_queue(struct work_struct *work)
>>> @@ -963,7 +959,7 @@ static void blkif_free(struct blkfront_info *info, int suspend)
>>>  		BLKIF_STATE_SUSPENDED : BLKIF_STATE_DISCONNECTED;
>>>  	/* No more blkif_request(). */
>>>  	if (info->rq)
>>> -		blk_stop_queue(info->rq);
>>> +		blk_mq_stop_hw_queues(info->rq);
>>>  
>>>  	/* Remove all persistent grants */
>>>  	if (!list_empty(&info->grants)) {
>>> @@ -1144,7 +1140,6 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>>>  	RING_IDX i, rp;
>>>  	unsigned long flags;
>>>  	struct blkfront_info *info = (struct blkfront_info *)dev_id;
>>> -	int error;
>>>  
>>>  	spin_lock_irqsave(&info->io_lock, flags);
>>>  
>>> @@ -1185,37 +1180,37 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>>>  			continue;
>>>  		}
>>>  
>>> -		error = (bret->status == BLKIF_RSP_OKAY) ? 0 : -EIO;
>>> +		req->errors = (bret->status == BLKIF_RSP_OKAY) ? 0 : -EIO;
>>>  		switch (bret->operation) {
>>>  		case BLKIF_OP_DISCARD:
>>>  			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
>>>  				struct request_queue *rq = info->rq;
>>>  				printk(KERN_WARNING "blkfront: %s: %s op failed\n",
>>>  					   info->gd->disk_name, op_name(bret->operation));
>>> -				error = -EOPNOTSUPP;
>>> +				req->errors = -EOPNOTSUPP;
>>>  				info->feature_discard = 0;
>>>  				info->feature_secdiscard = 0;
>>>  				queue_flag_clear(QUEUE_FLAG_DISCARD, rq);
>>>  				queue_flag_clear(QUEUE_FLAG_SECDISCARD, rq);
>>>  			}
>>> -			__blk_end_request_all(req, error);
>>> +			blk_mq_complete_request(req);
>>>  			break;
>>>  		case BLKIF_OP_FLUSH_DISKCACHE:
>>>  		case BLKIF_OP_WRITE_BARRIER:
>>>  			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
>>>  				printk(KERN_WARNING "blkfront: %s: %s op failed\n",
>>>  				       info->gd->disk_name, op_name(bret->operation));
>>> -				error = -EOPNOTSUPP;
>>> +				req->errors = -EOPNOTSUPP;
>>>  			}
>>>  			if (unlikely(bret->status == BLKIF_RSP_ERROR &&
>>>  				     info->shadow[id].req.u.rw.nr_segments == 0)) {
>>>  				printk(KERN_WARNING "blkfront: %s: empty %s op failed\n",
>>>  				       info->gd->disk_name, op_name(bret->operation));
>>> -				error = -EOPNOTSUPP;
>>> +				req->errors = -EOPNOTSUPP;
>>>  			}
>>> -			if (unlikely(error)) {
>>> -				if (error == -EOPNOTSUPP)
>>> -					error = 0;
>>> +			if (unlikely(req->errors)) {
>>> +				if (req->errors == -EOPNOTSUPP)
>>> +					req->errors = 0;
>>>  				info->feature_flush = 0;
>>>  				xlvbd_flush(info);
>>>  			}
>>> @@ -1226,7 +1221,7 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>>>  				dev_dbg(&info->xbdev->dev, "Bad return from blkdev data "
>>>  					"request: %x\n", bret->status);
>>>  
>>> -			__blk_end_request_all(req, error);
>>> +			blk_mq_complete_request(req);
>>>  			break;
>>>  		default:
>>>  			BUG();
>>> @@ -1555,28 +1550,6 @@ static int blkif_recover(struct blkfront_info *info)
>>>  
>>>  	kfree(copy);
>>>  
>>> -	/*
>>> -	 * Empty the queue, this is important because we might have
>>> -	 * requests in the queue with more segments than what we
>>> -	 * can handle now.
>>> -	 */
>>> -	spin_lock_irq(&info->io_lock);
>>> -	while ((req = blk_fetch_request(info->rq)) != NULL) {
>>> -		if (req->cmd_flags &
>>> -		    (REQ_FLUSH | REQ_FUA | REQ_DISCARD | REQ_SECURE)) {
>>> -			list_add(&req->queuelist, &requests);
>>> -			continue;
>>> -		}
>>> -		merge_bio.head = req->bio;
>>> -		merge_bio.tail = req->biotail;
>>> -		bio_list_merge(&bio_list, &merge_bio);
>>> -		req->bio = NULL;
>>> -		if (req->cmd_flags & (REQ_FLUSH | REQ_FUA))
>>> -			pr_alert("diskcache flush request found!\n");
>>> -		__blk_end_request_all(req, 0);
>>> -	}
>>> -	spin_unlock_irq(&info->io_lock);
>>> -
>>>  	xenbus_switch_state(info->xbdev, XenbusStateConnected);
>>>  
>>>  	spin_lock_irq(&info->io_lock);
>>> @@ -1591,9 +1564,10 @@ static int blkif_recover(struct blkfront_info *info)
>>>  		/* Requeue pending requests (flush or discard) */
>>>  		list_del_init(&req->queuelist);
>>>  		BUG_ON(req->nr_phys_segments > segs);
>>> -		blk_requeue_request(info->rq, req);
>>> +		blk_mq_requeue_request(req);
>>>  	}
>>>  	spin_unlock_irq(&info->io_lock);
>>> +	blk_mq_kick_requeue_list(info->rq);
>>>  
>>>  	while ((bio = bio_list_pop(&bio_list)) != NULL) {
>>>  		/* Traverse the list of pending bios and re-queue them */
>>>
--
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