Date:	Fri, 15 May 2015 20:06:51 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
CC:	xen-devel@...ts.xen.org, david.vrabel@...rix.com,
	justing@...ctralogic.com, konrad.wilk@...cle.com,
	paul.durrant@...rix.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] xen/block: add multi-page ring support


On 05/15/2015 07:13 PM, Roger Pau Monné wrote:
> El 12/05/15 a les 13.01, Bob Liu ha escrit:
>> Extend xen/block to support multi-page ring, so that more requests can be issued
>> by using more than one pages as the request ring between blkfront and backend.
>> As a result, the performance can get improved significantly.
>                               ^ s/can get improved/improves/
> 
>>
>> We got some impressive improvements on our highend iscsi storage cluster backend.
>> If using 64 pages as the ring, the IOPS increased about 15 times for the
>> throughput testing and above doubled for the latency testing.
>>
>> The reason was the limit on outstanding requests is 32 if use only one-page
>> ring, but in our case the iscsi lun was spread across about 100 physical drives,
>> 32 was really not enough to keep them busy.
>>
>> Changes in v2:
>>  - Rebased to 4.0-rc6
>>  - Added description on how this protocol works into io/blkif.h
> 
> I don't see any changes to io/blkif.h in this patch, is something missing?
> 

Sorry, I should mention in v3 that these changed were removed because I followed the protocol
already defined in XEN git tree: xen/include/public/io/blkif.h

> Also you use XENBUS_MAX_RING_PAGES which AFAICT it's not defined anywhere.
> 

It was defined in include/xen/xenbus.h.

>>
>> Changes in v3:
>>  - Follow the protocol defined in io/blkif.h on XEN tree
>>
>> Signed-off-by: Bob Liu <bob.liu@...cle.com>
>> ---
>>  drivers/block/xen-blkback/blkback.c |  14 ++++-
>>  drivers/block/xen-blkback/common.h  |   4 +-
>>  drivers/block/xen-blkback/xenbus.c  |  83 ++++++++++++++++++++++-------
>>  drivers/block/xen-blkfront.c        | 102 +++++++++++++++++++++++++++---------
>>  4 files changed, 156 insertions(+), 47 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
>> index 713fc9f..f191083 100644
>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -84,6 +84,12 @@ MODULE_PARM_DESC(max_persistent_grants,
>>                   "Maximum number of grants to map persistently");
>>  
>>  /*
>> + * Maximum number of pages to be used as the ring between front and backend
>> + */
>> +unsigned int xen_blkif_max_ring_pages = XENBUS_MAX_RING_PAGES;
>> +module_param_named(max_ring_pages, xen_blkif_max_ring_pages, int, S_IRUGO);
>> +MODULE_PARM_DESC(max_ring_pages, "Maximum amount of pages to be used as the ring");
>> +/*
>>   * The LRU mechanism to clean the lists of persistent grants needs to
>>   * be executed periodically. The time interval between consecutive executions
>>   * of the purge mechanism is set in ms.
>> @@ -630,7 +636,7 @@ purge_gnt_list:
>>  		}
>>  
>>  		/* Shrink if we have more than xen_blkif_max_buffer_pages */
>> -		shrink_free_pagepool(blkif, xen_blkif_max_buffer_pages);
>> +		shrink_free_pagepool(blkif, xen_blkif_max_buffer_pages * blkif->nr_ring_pages);
> 
> You are greatly increasing the buffer of free (ballooned) pages.
> Possibly making it 32 times bigger than it used to be, is this really
> needed?
> 

Hmm.. it's a bit aggressive.
How about (xen_blkif_max_buffer_pages * blkif->nr_ring_pages) / 2?

>>  
>>  		if (log_stats && time_after(jiffies, blkif->st_print))
>>  			print_stats(blkif);
>> @@ -1435,6 +1441,12 @@ static int __init xen_blkif_init(void)
>>  {
>>  	int rc = 0;
>>  
>> +	if (xen_blkif_max_ring_pages > XENBUS_MAX_RING_PAGES) {
>> +		pr_info("Invalid max_ring_pages (%d), will use default max: %d.\n",
>> +			xen_blkif_max_ring_pages, XENBUS_MAX_RING_PAGES);
>> +		xen_blkif_max_ring_pages = XENBUS_MAX_RING_PAGES;
>> +	}
>> +
>>  	if (!xen_domain())
>>  		return -ENODEV;
>>  
>> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
>> index f620b5d..84a964c 100644
>> --- a/drivers/block/xen-blkback/common.h
>> +++ b/drivers/block/xen-blkback/common.h
>> @@ -44,6 +44,7 @@
>>  #include <xen/interface/io/blkif.h>
>>  #include <xen/interface/io/protocols.h>
>>  
>> +extern unsigned int xen_blkif_max_ring_pages;
>>  /*
>>   * This is the maximum number of segments that would be allowed in indirect
>>   * requests. This value will also be passed to the frontend.
>> @@ -248,7 +249,7 @@ struct backend_info;
>>  #define PERSISTENT_GNT_WAS_ACTIVE	1
>>  
>>  /* Number of requests that we can fit in a ring */
>> -#define XEN_BLKIF_REQS			32
>> +#define XEN_BLKIF_REQS			(32 * XENBUS_MAX_RING_PAGES)
> 
> This should be made a member of xen_blkif and set dynamically, or else
> we are just wasting memory if the frontend doesn't support multipage
> rings. We seem to be doing this for a bunch of features, but allocating
> 32 times the needed amount of requests (if the frontend doesn't support
> multipage rings) seems too much IMHO.
> 

Right, the reason I use static is to simple the code.
Else we should:
1) Move xen_blkif_alloc() from backend_probe() to some place until we read the
negotiated 'num-ring-pages' written by front.
2) Deal with memory alloc/free when domU migrated and 'num-ring-pages' changed.

I'll try to find a suitable place in frontend_changed() but may be in an new patch late.

>>  
>>  struct persistent_gnt {
>>  	struct page *page;
>> @@ -320,6 +321,7 @@ struct xen_blkif {
>>  	struct work_struct	free_work;
>>  	/* Thread shutdown wait queue. */
>>  	wait_queue_head_t	shutdown_wq;
>> +	int nr_ring_pages;
>>  };
>>  
>>  struct seg_buf {
>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
>> index 6ab69ad..909babd 100644
>> --- a/drivers/block/xen-blkback/xenbus.c
>> +++ b/drivers/block/xen-blkback/xenbus.c
>> @@ -198,8 +198,8 @@ fail:
>>  	return ERR_PTR(-ENOMEM);
>>  }
>>  
>> -static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t gref,
>> -			 unsigned int evtchn)
>> +static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t *gref,
>> +			 unsigned int nr_grefs, unsigned int evtchn)
>>  {
>>  	int err;
>>  
>> @@ -207,7 +207,7 @@ static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t gref,
>>  	if (blkif->irq)
>>  		return 0;
>>  
>> -	err = xenbus_map_ring_valloc(blkif->be->dev, &gref, 1,
>> +	err = xenbus_map_ring_valloc(blkif->be->dev, gref, nr_grefs,
>>  				     &blkif->blk_ring);
>>  	if (err < 0)
>>  		return err;
>> @@ -217,21 +217,21 @@ static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t gref,
>>  	{
>>  		struct blkif_sring *sring;
>>  		sring = (struct blkif_sring *)blkif->blk_ring;
>> -		BACK_RING_INIT(&blkif->blk_rings.native, sring, PAGE_SIZE);
>> +		BACK_RING_INIT(&blkif->blk_rings.native, sring, PAGE_SIZE * nr_grefs);
>>  		break;
>>  	}
>>  	case BLKIF_PROTOCOL_X86_32:
>>  	{
>>  		struct blkif_x86_32_sring *sring_x86_32;
>>  		sring_x86_32 = (struct blkif_x86_32_sring *)blkif->blk_ring;
>> -		BACK_RING_INIT(&blkif->blk_rings.x86_32, sring_x86_32, PAGE_SIZE);
>> +		BACK_RING_INIT(&blkif->blk_rings.x86_32, sring_x86_32, PAGE_SIZE * nr_grefs);
>>  		break;
>>  	}
>>  	case BLKIF_PROTOCOL_X86_64:
>>  	{
>>  		struct blkif_x86_64_sring *sring_x86_64;
>>  		sring_x86_64 = (struct blkif_x86_64_sring *)blkif->blk_ring;
>> -		BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE);
>> +		BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE * nr_grefs);
>>  		break;
>>  	}
>>  	default:
>> @@ -597,6 +597,11 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
>>  	if (err)
>>  		goto fail;
>>  
>> +	err = xenbus_printf(XBT_NIL, dev->nodename, "max-ring-pages", "%u",
>> +			    xen_blkif_max_ring_pages);
>> +	if (err)
>> +		pr_warn("%s write out 'max-ring-pages' failed\n", __func__);
>> +
>>  	err = xenbus_switch_state(dev, XenbusStateInitWait);
>>  	if (err)
>>  		goto fail;
>> @@ -860,22 +865,62 @@ again:
>>  static int connect_ring(struct backend_info *be)
>>  {
>>  	struct xenbus_device *dev = be->dev;
>> -	unsigned long ring_ref;
>> -	unsigned int evtchn;
>> +	unsigned int ring_ref[XENBUS_MAX_RING_PAGES];
>> +	unsigned int evtchn, nr_grefs;
>>  	unsigned int pers_grants;
>>  	char protocol[64] = "";
>>  	int err;
>>  
>>  	pr_debug("%s %s\n", __func__, dev->otherend);
>>  
>> -	err = xenbus_gather(XBT_NIL, dev->otherend, "ring-ref", "%lu",
>> -			    &ring_ref, "event-channel", "%u", &evtchn, NULL);
>> -	if (err) {
>> -		xenbus_dev_fatal(dev, err,
>> -				 "reading %s/ring-ref and event-channel",
>> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "event-channel", "%u",
>> +			  &evtchn);
>> +	if (err != 1) {
>> +		err = -EINVAL;
>> +		xenbus_dev_fatal(dev, err, "reading %s/event-channel",
>>  				 dev->otherend);
>>  		return err;
>>  	}
>> +	pr_info("event-channel %u\n", evtchn);
>> +
>> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "num-ring-pages", "%u",
>> +			  &nr_grefs);
>> +	if (err != 1) {
>> +		err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-ref",
>> +				  "%u", &ring_ref[0]);
>> +		if (err != 1) {
>> +			err = -EINVAL;
>> +			xenbus_dev_fatal(dev, err, "reading %s/ring-ref",
>> +					 dev->otherend);
>> +			return err;
>> +		}
>> +		nr_grefs = 1;
>> +		pr_info("%s:using single page: ring-ref %d\n", dev->otherend,
>> +			ring_ref[0]);
>> +	} else {
>> +		unsigned int i;
>> +
>> +		if (nr_grefs > xen_blkif_max_ring_pages) {
>> +			err = -EINVAL;
>> +			xenbus_dev_fatal(dev, err, "%s/request %d ring pages exceed max:%d",
>> +					 dev->otherend, nr_grefs, xen_blkif_max_ring_pages);
>> +			return err;
>> +		}
>> +		for (i = 0; i < nr_grefs; i++) {
>> +			char ring_ref_name[BLKBACK_NAME_LEN];
> 
> I would add a new macro rather than reusing one which might change
> (BLKBACK_RINGREF_LEN?).
> 

Sure.

>> +
>> +			snprintf(ring_ref_name, sizeof(ring_ref_name), "ring-ref%u", i);
>> +			err = xenbus_scanf(XBT_NIL, dev->otherend,
>> +					   ring_ref_name, "%u", &ring_ref[i]);
>> +			if (err != 1) {
>> +				err = -EINVAL;
>> +				xenbus_dev_fatal(dev, err, "reading %s/%s",
>> +						 dev->otherend, ring_ref_name);
>> +				return err;
>> +			}
>> +			pr_info("ring-ref%u: %u\n", i, ring_ref[i]);
>> +		}
>> +	}
>>  
>>  	be->blkif->blk_protocol = BLKIF_PROTOCOL_DEFAULT;
>>  	err = xenbus_gather(XBT_NIL, dev->otherend, "protocol",
>> @@ -900,16 +945,16 @@ static int connect_ring(struct backend_info *be)
>>  
>>  	be->blkif->vbd.feature_gnt_persistent = pers_grants;
>>  	be->blkif->vbd.overflow_max_grants = 0;
>> +	be->blkif->nr_ring_pages = nr_grefs;
>>  
>> -	pr_info("ring-ref %ld, event-channel %d, protocol %d (%s) %s\n",
>> -		ring_ref, evtchn, be->blkif->blk_protocol, protocol,
>> -		pers_grants ? "persistent grants" : "");
>> +	pr_info("ring-pages:%d, event-channel %d, protocol %d (%s) %s\n",
>> +			nr_grefs, evtchn, be->blkif->blk_protocol, protocol,
>> +			pers_grants ? "persistent grants" : "");
>>  
>>  	/* Map the shared frame, irq etc. */
>> -	err = xen_blkif_map(be->blkif, ring_ref, evtchn);
>> +	err = xen_blkif_map(be->blkif, ring_ref, nr_grefs, evtchn);
>>  	if (err) {
>> -		xenbus_dev_fatal(dev, err, "mapping ring-ref %lu port %u",
>> -				 ring_ref, evtchn);
>> +		xenbus_dev_fatal(dev, err, "mapping ring-ref port %u", evtchn);
>>  		return err;
>>  	}
>>  
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index 88e23fd..c4b427f 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -98,7 +98,17 @@ static unsigned int xen_blkif_max_segments = 32;
>>  module_param_named(max, xen_blkif_max_segments, int, S_IRUGO);
>>  MODULE_PARM_DESC(max, "Maximum amount of segments in indirect requests (default is 32)");
>>  
>> -#define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE)
>> +static unsigned int xen_blkif_max_ring_pages = 1;
>> +module_param_named(max_ring_pages, xen_blkif_max_ring_pages, int, S_IRUGO);
>> +MODULE_PARM_DESC(max_ring_pages, "Maximum amount of pages to be used as the ring");
>> +
>> +#define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE * info->nr_ring_pages)
> 
> I would prefer if this macro took a explicit info parameter instead on
> realying that it is always info.
> 

Sorry, I missed your idea here.

>> +#define BLK_MAX_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE * XENBUS_MAX_RING_PAGES)
>> +/*
>> + * ring-ref%i i=(-1UL) would take 11 characters + 'ring-ref' is 8, so 19 characters
>> + * are enough. Define to 20 to keep consist with backend.
>> + */
>> +#define RINGREF_NAME_LEN (20)
>>  
>>  /*
>>   * We have one of these per vbd, whether ide, scsi or 'other'.  They
>> @@ -114,13 +124,14 @@ struct blkfront_info
>>  	int vdevice;
>>  	blkif_vdev_t handle;
>>  	enum blkif_state connected;
>> -	int ring_ref;
>> +	int ring_ref[XENBUS_MAX_RING_PAGES];
>> +	unsigned int nr_ring_pages;
>>  	struct blkif_front_ring ring;
>>  	unsigned int evtchn, irq;
>>  	struct request_queue *rq;
>>  	struct work_struct work;
>>  	struct gnttab_free_callback callback;
>> -	struct blk_shadow shadow[BLK_RING_SIZE];
>> +	struct blk_shadow shadow[BLK_MAX_RING_SIZE];
>>  	struct list_head grants;
>>  	struct list_head indirect_pages;
>>  	unsigned int persistent_gnts_c;
>> @@ -139,8 +150,6 @@ static unsigned int nr_minors;
>>  static unsigned long *minors;
>>  static DEFINE_SPINLOCK(minor_lock);
>>  
>> -#define MAXIMUM_OUTSTANDING_BLOCK_REQS \
>> -	(BLKIF_MAX_SEGMENTS_PER_REQUEST * BLK_RING_SIZE)
>>  #define GRANT_INVALID_REF	0
>>  
>>  #define PARTS_PER_DISK		16
>> @@ -1033,12 +1042,15 @@ free_shadow:
>>  	flush_work(&info->work);
>>  
>>  	/* Free resources associated with old device channel. */
>> -	if (info->ring_ref != GRANT_INVALID_REF) {
>> -		gnttab_end_foreign_access(info->ring_ref, 0,
>> -					  (unsigned long)info->ring.sring);
>> -		info->ring_ref = GRANT_INVALID_REF;
>> -		info->ring.sring = NULL;
>> +	for (i = 0; i < info->nr_ring_pages; i++) {
>> +		if (info->ring_ref[i] != GRANT_INVALID_REF) {
>> +			gnttab_end_foreign_access(info->ring_ref[i], 0, 0);
>> +			info->ring_ref[i] = GRANT_INVALID_REF;
>> +		}
>>  	}
>> +	free_pages((unsigned long)info->ring.sring, get_order(info->nr_ring_pages * PAGE_SIZE));
>> +	info->ring.sring = NULL;
>> +
>>  	if (info->irq)
>>  		unbind_from_irqhandler(info->irq, info);
>>  	info->evtchn = info->irq = 0;
>> @@ -1245,26 +1257,30 @@ static int setup_blkring(struct xenbus_device *dev,
>>  			 struct blkfront_info *info)
>>  {
>>  	struct blkif_sring *sring;
>> -	grant_ref_t gref;
>> -	int err;
>> +	int err, i;
>> +	unsigned long ring_size = info->nr_ring_pages * PAGE_SIZE;
>> +	grant_ref_t gref[XENBUS_MAX_RING_PAGES];
>>  
>> -	info->ring_ref = GRANT_INVALID_REF;
>> +	for (i = 0; i < info->nr_ring_pages; i++)
>> +		info->ring_ref[i] = GRANT_INVALID_REF;
>>  
>> -	sring = (struct blkif_sring *)__get_free_page(GFP_NOIO | __GFP_HIGH);
>> +	sring = (struct blkif_sring *)__get_free_pages(GFP_NOIO | __GFP_HIGH,
>> +						       get_order(ring_size));
>>  	if (!sring) {
>>  		xenbus_dev_fatal(dev, -ENOMEM, "allocating shared ring");
>>  		return -ENOMEM;
>>  	}
>>  	SHARED_RING_INIT(sring);
>> -	FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);
>> +	FRONT_RING_INIT(&info->ring, sring, ring_size);
>>  
>> -	err = xenbus_grant_ring(dev, info->ring.sring, 1, &gref);
>> +	err = xenbus_grant_ring(dev, info->ring.sring, info->nr_ring_pages, gref);
>>  	if (err < 0) {
>>  		free_page((unsigned long)sring);
>>  		info->ring.sring = NULL;
>>  		goto fail;
>>  	}
>> -	info->ring_ref = gref;
>> +	for (i = 0; i < info->nr_ring_pages; i++)
>> +		info->ring_ref[i] = gref[i];
>>  
>>  	err = xenbus_alloc_evtchn(dev, &info->evtchn);
>>  	if (err)
>> @@ -1292,7 +1308,15 @@ static int talk_to_blkback(struct xenbus_device *dev,
>>  {
>>  	const char *message = NULL;
>>  	struct xenbus_transaction xbt;
>> -	int err;
>> +	int err, i;
>> +	unsigned int max_pages = 0;
>> +
>> +	err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
>> +			   "max-ring-pages", "%u", &max_pages);
>> +	if (err != 1)
>> +		info->nr_ring_pages = 1;
>> +	else
>> +		info->nr_ring_pages = min(xen_blkif_max_ring_pages, max_pages);
>>  
>>  	/* Create shared ring, alloc event channel. */
>>  	err = setup_blkring(dev, info);
>> @@ -1306,11 +1330,31 @@ again:
>>  		goto destroy_blkring;
>>  	}
>>  
>> -	err = xenbus_printf(xbt, dev->nodename,
>> -			    "ring-ref", "%u", info->ring_ref);
>> -	if (err) {
>> -		message = "writing ring-ref";
>> -		goto abort_transaction;
>> +	if (info->nr_ring_pages == 1) {
>> +		err = xenbus_printf(xbt, dev->nodename,
>> +				    "ring-ref", "%u", info->ring_ref[0]);
>> +		if (err) {
>> +			message = "writing ring-ref";
>> +			goto abort_transaction;
>> +		}
>> +	} else {
>> +		err = xenbus_printf(xbt, dev->nodename,
>> +				    "num-ring-pages", "%u", info->nr_ring_pages);
>> +		if (err) {
>> +			message = "writing num-ring-pages";
>> +			goto abort_transaction;
>> +		}
>> +
>> +		for (i = 0; i < info->nr_ring_pages; i++) {
>> +			char ring_ref_name[RINGREF_NAME_LEN];
>> +			snprintf(ring_ref_name, sizeof(ring_ref_name), "ring-ref%u", i);
>> +			err = xenbus_printf(xbt, dev->nodename,
>> +					ring_ref_name, "%u", info->ring_ref[i]);
>> +			if (err) {
>> +				message = "writing ring-ref";
>> +				goto abort_transaction;
>> +			}
>> +		}
>>  	}
>>  	err = xenbus_printf(xbt, dev->nodename,
>>  			    "event-channel", "%u", info->evtchn);
>> @@ -1338,6 +1382,7 @@ again:
>>  		goto destroy_blkring;
>>  	}
> 
> Why is this moved to here...
> 

Else the kernel will panic.

>> +	info->shadow[BLK_RING_SIZE-1].req.u.rw.id = 0x0fffffff;
>>  	xenbus_switch_state(dev, XenbusStateInitialised);
>>  
>>  	return 0;
>> @@ -1422,9 +1467,8 @@ static int blkfront_probe(struct xenbus_device *dev,
>>  	info->connected = BLKIF_STATE_DISCONNECTED;
>>  	INIT_WORK(&info->work, blkif_restart_queue);
>>  
>> -	for (i = 0; i < BLK_RING_SIZE; i++)
>> +	for (i = 0; i < BLK_MAX_RING_SIZE; i++)
>>  		info->shadow[i].req.u.rw.id = i+1;
>> -	info->shadow[BLK_RING_SIZE-1].req.u.rw.id = 0x0fffffff;
> 
> from here. Isn't the previous location suitable anymore?
> 

See:
#define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE * info->nr_ring_pages)

Because we haven't get info->nr_ring_pages yet here(in probe()), info->nr_ring_pages
is read out in talk_to_blkback() which I already moved to blkback_changed().

Thanks for your review.

-Bob
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