[PATCH RFC 10/12] xen-blkback: make the queue of free requests per backend

From: Roger Pau Monne
Date: Thu Feb 28 2013 - 05:30:54 EST

Remove the last dependency from blkbk by moving the list of free
requests to blkif. This change reduces the contention on the list of
available requests.

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxx
---
drivers/block/xen-blkback/blkback.c | 123 +++++++----------------------------
drivers/block/xen-blkback/common.h | 27 ++++++++
drivers/block/xen-blkback/xenbus.c | 17 +++++
3 files changed, 67 insertions(+), 100 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index c43de8a..04ad2aa 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -50,18 +50,14 @@
#include "common.h"

/*
- * These are rather arbitrary. They are fairly large because adjacent requests
- * pulled from a communication ring are quite likely to end up being part of
- * the same scatter/gather request at the disc.
- *
- * ** TRY INCREASING 'xen_blkif_reqs' IF WRITE SPEEDS SEEM TOO LOW **
- *
- * This will increase the chances of being able to write whole tracks.
- * 64 should be enough to keep us competitive with Linux.
+ * This is the number of requests that will be pre-allocated for each backend.
+ * For better performance this is set to RING_SIZE (32), so requests
+ * in the ring will never have to wait for a free pending_req.
*/
-static int xen_blkif_reqs = 64;
+
+int xen_blkif_reqs = 32;
module_param_named(reqs, xen_blkif_reqs, int, 0);
-MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");
+MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate per backend");

/*
* Maximum number of grants to map persistently in blkback. For maximum
@@ -120,50 +116,8 @@ MODULE_PARM_DESC(max_buffer_pages,
static unsigned int log_stats;
module_param(log_stats, int, 0644);

-/*
- * Each outstanding request that we've passed to the lower device layers has a
- * 'pending_req' allocated to it. Each buffer_head that completes decrements
- * the pendcnt towards zero. When it hits zero, the specified domain has a
- * response queued for it, with the saved 'id' passed back.
- */
-struct pending_req {
- struct xen_blkif *blkif;
- u64 id;
- int nr_pages;
- atomic_t pendcnt;
- unsigned short operation;
- int status;
- struct list_head free_list;
- struct persistent_gnt *persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
- struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
- grant_handle_t grant_handles[BLKIF_MAX_SEGMENTS_PER_REQUEST];
-};
-
#define BLKBACK_INVALID_HANDLE (~0)

-struct xen_blkbk {
- struct pending_req *pending_reqs;
- /* List of all 'pending_req' available */
- struct list_head pending_free;
- /* And its spinlock. */
- spinlock_t pending_free_lock;
- wait_queue_head_t pending_free_wq;
-};
-
-static struct xen_blkbk *blkbk;
-
-/*
- * Little helpful macro to figure out the index and virtual address of the
- * pending_pages[..]. For each 'pending_req' we have have up to
- * BLKIF_MAX_SEGMENTS_PER_REQUEST (11) pages. The seg would be from 0 through
- * 10 and would index in the pending_pages[..].
- */
-static inline int vaddr_pagenr(struct pending_req *req, int seg)
-{
- return (req - blkbk->pending_reqs) *
- BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
-}
-
static inline int get_free_page(struct xen_blkif *blkif, struct page **page)
{
unsigned long flags;
@@ -400,18 +354,18 @@ finished:
/*
* Retrieve from the 'pending_reqs' a free pending_req structure to be used.
*/
-static struct pending_req *alloc_req(void)
+static struct pending_req *alloc_req(struct xen_blkif *blkif)
{
struct pending_req *req = NULL;
unsigned long flags;

- spin_lock_irqsave(&blkbk->pending_free_lock, flags);
- if (!list_empty(&blkbk->pending_free)) {
- req = list_entry(blkbk->pending_free.next, struct pending_req,
+ spin_lock_irqsave(&blkif->pending_free_lock, flags);
+ if (!list_empty(&blkif->pending_free)) {
+ req = list_entry(blkif->pending_free.next, struct pending_req,
free_list);
list_del(&req->free_list);
}
- spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
+ spin_unlock_irqrestore(&blkif->pending_free_lock, flags);
return req;
}

@@ -419,17 +373,17 @@ static struct pending_req *alloc_req(void)
* Return the 'pending_req' structure back to the freepool. We also
* wake up the thread if it was waiting for a free page.
*/
-static void free_req(struct pending_req *req)
+static void free_req(struct xen_blkif *blkif, struct pending_req *req)
{
unsigned long flags;
int was_empty;

- spin_lock_irqsave(&blkbk->pending_free_lock, flags);
- was_empty = list_empty(&blkbk->pending_free);
- list_add(&req->free_list, &blkbk->pending_free);
- spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
+ spin_lock_irqsave(&blkif->pending_free_lock, flags);
+ was_empty = list_empty(&blkif->pending_free);
+ list_add(&req->free_list, &blkif->pending_free);
+ spin_unlock_irqrestore(&blkif->pending_free_lock, flags);
if (was_empty)
- wake_up(&blkbk->pending_free_wq);
+ wake_up(&blkif->pending_free_wq);
}

/*
@@ -564,8 +518,8 @@ int xen_blkif_schedule(void *arg)
if (timeout == 0)
goto purge_gnt_list;
timeout = wait_event_interruptible_timeout(
- blkbk->pending_free_wq,
- !list_empty(&blkbk->pending_free) ||
+ blkif->pending_free_wq,
+ !list_empty(&blkif->pending_free) ||
kthread_should_stop(),
timeout);
if (timeout == 0)
@@ -886,7 +840,7 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
if (atomic_read(&pending_req->blkif->drain))
complete(&pending_req->blkif->drain_complete);
}
- free_req(pending_req);
+ free_req(pending_req->blkif, pending_req);
}
}

@@ -929,7 +883,7 @@ __do_block_io_op(struct xen_blkif *blkif)
break;
}

- pending_req = alloc_req();
+ pending_req = alloc_req(blkif);
if (NULL == pending_req) {
blkif->st_oo_req++;
more_to_do = 1;
@@ -954,7 +908,7 @@ __do_block_io_op(struct xen_blkif *blkif)
/* Apply all sanity checks to /private copy/ of request. */
barrier();
if (unlikely(req.operation == BLKIF_OP_DISCARD)) {
- free_req(pending_req);
+ free_req(blkif, pending_req);
if (dispatch_discard_io(blkif, &req))
break;
} else if (dispatch_rw_block_io(blkif, &req, pending_req))
@@ -1157,7 +1111,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
fail_response:
/* Haven't submitted any bio's yet. */
make_response(blkif, req->u.rw.id, req->operation, BLKIF_RSP_ERROR);
- free_req(pending_req);
+ free_req(blkif, pending_req);
msleep(1); /* back off a bit */
return -EIO;

@@ -1213,51 +1167,20 @@ static void make_response(struct xen_blkif *blkif, u64 id,

static int __init xen_blkif_init(void)
{
- int i;
int rc = 0;

if (!xen_domain())
return -ENODEV;

- blkbk = kzalloc(sizeof(struct xen_blkbk), GFP_KERNEL);
- if (!blkbk) {
- pr_alert(DRV_PFX "%s: out of memory!\n", __func__);
- return -ENOMEM;
- }
-
-
- blkbk->pending_reqs = kzalloc(sizeof(blkbk->pending_reqs[0]) *
- xen_blkif_reqs, GFP_KERNEL);
-
- if (!blkbk->pending_reqs) {
- rc = -ENOMEM;
- goto out_of_memory;
- }
-
rc = xen_blkif_interface_init();
if (rc)
goto failed_init;

- INIT_LIST_HEAD(&blkbk->pending_free);
- spin_lock_init(&blkbk->pending_free_lock);
- init_waitqueue_head(&blkbk->pending_free_wq);
-
- for (i = 0; i < xen_blkif_reqs; i++)
- list_add_tail(&blkbk->pending_reqs[i].free_list,
- &blkbk->pending_free);
-
rc = xen_blkif_xenbus_init();
if (rc)
goto failed_init;

- return 0;
-
- out_of_memory:
- pr_alert(DRV_PFX "%s: out of memory\n", __func__);
failed_init:
- kfree(blkbk->pending_reqs);
- kfree(blkbk);
- blkbk = NULL;
return rc;
}

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 604bd30..0b0ad3f 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -214,6 +214,14 @@ struct xen_blkif {
int free_pages_num;
struct list_head free_pages;

+ /* Allocation of pending_reqs */
+ struct pending_req *pending_reqs;
+ /* List of all 'pending_req' available */
+ struct list_head pending_free;
+ /* And its spinlock. */
+ spinlock_t pending_free_lock;
+ wait_queue_head_t pending_free_wq;
+
/* statistics */
unsigned long st_print;
int st_rd_req;
@@ -227,6 +235,25 @@ struct xen_blkif {
wait_queue_head_t waiting_to_free;
};

+/*
+ * Each outstanding request that we've passed to the lower device layers has a
+ * 'pending_req' allocated to it. Each buffer_head that completes decrements
+ * the pendcnt towards zero. When it hits zero, the specified domain has a
+ * response queued for it, with the saved 'id' passed back.
+ */
+struct pending_req {
+ struct xen_blkif *blkif;
+ u64 id;
+ int nr_pages;
+ atomic_t pendcnt;
+ unsigned short operation;
+ int status;
+ struct list_head free_list;
+ struct persistent_gnt *persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+ struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+ grant_handle_t grant_handles[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+};
+

#define vbd_sz(_v) ((_v)->bdev->bd_part ? \
(_v)->bdev->bd_part->nr_sects : \
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index d7926ec..8f929cb 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -30,6 +30,8 @@ struct backend_info {
char *mode;
};

+extern int xen_blkif_reqs;
+
static struct kmem_cache *xen_blkif_cachep;
static void connect(struct backend_info *);
static int connect_ring(struct backend_info *);
@@ -104,6 +106,7 @@ static void xen_update_blkif_status(struct xen_blkif *blkif)
static struct xen_blkif *xen_blkif_alloc(domid_t domid)
{
struct xen_blkif *blkif;
+ int i;

blkif = kmem_cache_zalloc(xen_blkif_cachep, GFP_KERNEL);
if (!blkif)
@@ -122,6 +125,19 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
spin_lock_init(&blkif->free_pages_lock);
INIT_LIST_HEAD(&blkif->free_pages);
blkif->free_pages_num = 0;
+ blkif->pending_reqs = kzalloc(sizeof(blkif->pending_reqs[0]) *
+ xen_blkif_reqs, GFP_KERNEL);
+ if (!blkif->pending_reqs) {
+ kmem_cache_free(xen_blkif_cachep, blkif);
+ return ERR_PTR(-ENOMEM);
+ }
+ INIT_LIST_HEAD(&blkif->pending_free);
+ spin_lock_init(&blkif->pending_free_lock);
+ init_waitqueue_head(&blkif->pending_free_wq);
+
+ for (i = 0; i < xen_blkif_reqs; i++)
+ list_add_tail(&blkif->pending_reqs[i].free_list,
+ &blkif->pending_free);

return blkif;
}
@@ -204,6 +220,7 @@ static void xen_blkif_free(struct xen_blkif *blkif)
{
if (!atomic_dec_and_test(&blkif->refcnt))
BUG();
+ kfree(blkif->pending_reqs);
kmem_cache_free(xen_blkif_cachep, blkif);
}

--
1.7.7.5 (Apple Git-26)

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