[PATCH RFC 1/8] SUNRPC: Use atomic_t for XID allocation

[Chuck Lever]

xprt_alloc_xid() acquires reserve_lock to increment a simple
counter. Under a high-IOPS NFSv3 workload on 100GbE RDMA,
profiling shows 1.06% of system-wide CPU cycles contending on
this lock in xprt_request_init, as ~150 RPC worker threads
serialize on the counter.

reserve_lock protects the slot table and backlog queue, but
XID allocation is an independent operation that does not require
synchronization with either.

Signed-off-by: Chuck Lever <cel@xxxxxxxxxx>
---
 include/linux/sunrpc/xprt.h | 2 +-
 net/sunrpc/xprt.c           | 9 ++-------
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
index a82045804d34..0d6c3f6bf97e 100644
--- a/include/linux/sunrpc/xprt.h
+++ b/include/linux/sunrpc/xprt.h
@@ -273,7 +273,7 @@ struct rpc_xprt {
 	spinlock_t		transport_lock;	/* lock transport info */
 	spinlock_t		reserve_lock;	/* lock slot table */
 	spinlock_t		queue_lock;	/* send/receive queue lock */
-	u32			xid;		/* Next XID value to use */
+	atomic_t		xid;		/* Most recently issued XID */
 	struct rpc_task *	snd_task;	/* Task blocked in send */
 
 	struct list_head	xmit_queue;	/* Send queue */
diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
index 48a3618cbb29..186c14f0f928 100644
--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -1882,18 +1882,13 @@ xprt_init_connect_cookie(struct rpc_rqst *req, struct rpc_xprt *xprt)
 static __be32
 xprt_alloc_xid(struct rpc_xprt *xprt)
 {
-	__be32 xid;
-
-	spin_lock(&xprt->reserve_lock);
-	xid = (__force __be32)xprt->xid++;
-	spin_unlock(&xprt->reserve_lock);
-	return xid;
+	return (__force __be32)atomic_inc_return(&xprt->xid);
 }
 
 static void
 xprt_init_xid(struct rpc_xprt *xprt)
 {
-	xprt->xid = get_random_u32();
+	atomic_set(&xprt->xid, get_random_u32());
 }
 
 static void

-- 
2.55.0