xen/blkback: make pool of persistent grants and free pages per-queue

Make pool of persistent grants and free pages per-queue/ring instead of
per-device to get better scalability.

Test was done based on null_blk driver:
dom0: v4.2-rc8 16vcpus 10GB "modprobe null_blk"
domu: v4.2-rc8 16vcpus 10GB

[test]
rw=read
direct=1
ioengine=libaio
bs=4k
time_based
runtime=30
filename=/dev/xvdb
numjobs=16
iodepth=64
iodepth_batch=64
iodepth_batch_complete=64
group_reporting

Results:
iops1: After patch "xen/blkfront: make persistent grants per-queue".
iops2: After this patch.

Queues:			  1 	   4 	  	  8 	 	 16
Iops orig(k):		810 	1064 		780 		700
Iops1(k):		810     1230(~20%)	1024(~20%)	850(~20%)
Iops2(k):		810     1410(~35%)	1354(~75%)      1440(~100%)

With 4 queues after this commit we can get ~75% increase in IOPS, and
performance won't drop if increasing queue numbers.

Please find the respective chart in this link:
https://www.dropbox.com/s/agrcy2pbzbsvmwv/iops.png?dl=0

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 16 insertions, 16 deletions

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 847444dc1df4..3c244ecf22a4 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -291,6 +291,22 @@ struct xen_blkif_ring {
         spinlock_t              pending_free_lock;
         wait_queue_head_t       pending_free_wq;
 
+        /* Tree to store persistent grants. */
+        spinlock_t              pers_gnts_lock;
+        struct rb_root          persistent_gnts;
+        unsigned int            persistent_gnt_c;
+        atomic_t                persistent_gnt_in_use;
+        unsigned long           next_lru;
+
+        /* Used by the kworker that offload work from the persistent purge. */
+        struct list_head        persistent_purge_list;
+        struct work_struct      persistent_purge_work;
+
+        /* Buffer of free pages to map grant refs. */
+        spinlock_t              free_pages_lock;
+        int                     free_pages_num;
+        struct list_head        free_pages;
+
         struct work_struct      free_work;
         /* Thread shutdown wait queue. */
         wait_queue_head_t       shutdown_wq;
@@ -312,22 +328,6 @@ struct xen_blkif {
         struct completion       drain_complete;
         atomic_t                drain;
 
-        /* tree to store persistent grants */
-        spinlock_t              pers_gnts_lock;
-        struct rb_root          persistent_gnts;
-        unsigned int            persistent_gnt_c;
-        atomic_t                persistent_gnt_in_use;
-        unsigned long           next_lru;
-
-        /* used by the kworker that offload work from the persistent purge */
-        struct list_head        persistent_purge_list;
-        struct work_struct      persistent_purge_work;
-
-        /* buffer of free pages to map grant refs */
-        spinlock_t              free_pages_lock;
-        int                     free_pages_num;
-        struct list_head        free_pages;
-
         /* statistics */
         unsigned long           st_print;
         unsigned long long                      st_rd_req;