Re: [PATCH v3 2/2] xen/block: add multi-page ring support

From: Bob Liu
Date: Fri May 15 2015 - 08:07:10 EST


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@xxxxxxxxxx>
>> ---
>> 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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