SUNRPC: Improve latency for interactive tasks

One of the intentions with the priority queues was to ensure that no
single process can hog the transport. The field task->tk_owner therefore
identifies the RPC call's origin, and is intended to allow the RPC layer
to organise queues for fairness.
This commit therefore modifies the transmit queue to group requests
by task->tk_owner, and ensures that we round robin among those groups.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

include/linux/sunrpc/xprt.h

@@ -89,6 +89,7 @@ struct rpc_rqst {
 	};
 
 	struct list_head	rq_xmit;	/* Send queue */
+	struct list_head	rq_xmit2;	/* Send queue */
 
 	void			*rq_buffer;	/* Call XDR encode buffer */
 	size_t			rq_callsize;

net/sunrpc/xprt.c

@@ -1053,12 +1053,21 @@ xprt_request_need_enqueue_transmit(struct rpc_task *task, struct rpc_rqst *req)
 void
 xprt_request_enqueue_transmit(struct rpc_task *task)
 {
-	struct rpc_rqst *req = task->tk_rqstp;
+	struct rpc_rqst *pos, *req = task->tk_rqstp;
 	struct rpc_xprt *xprt = req->rq_xprt;
 
 	if (xprt_request_need_enqueue_transmit(task, req)) {
 		spin_lock(&xprt->queue_lock);
+		list_for_each_entry(pos, &xprt->xmit_queue, rq_xmit) {
+			if (pos->rq_task->tk_owner != task->tk_owner)
+				continue;
+			list_add_tail(&req->rq_xmit2, &pos->rq_xmit2);
+			INIT_LIST_HEAD(&req->rq_xmit);
+			goto out;
+		}
 		list_add_tail(&req->rq_xmit, &xprt->xmit_queue);
+		INIT_LIST_HEAD(&req->rq_xmit2);
+out:
 		set_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate);
 		spin_unlock(&xprt->queue_lock);
 	}
@@ -1074,8 +1083,20 @@ xprt_request_enqueue_transmit(struct rpc_task *task)
 static void
 xprt_request_dequeue_transmit_locked(struct rpc_task *task)
 {
-	if (test_and_clear_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
-		list_del(&task->tk_rqstp->rq_xmit);
+	struct rpc_rqst *req = task->tk_rqstp;
+
+	if (!test_and_clear_bit(RPC_TASK_NEED_XMIT, &task->tk_runstate))
+		return;
+	if (!list_empty(&req->rq_xmit)) {
+		list_del(&req->rq_xmit);
+		if (!list_empty(&req->rq_xmit2)) {
+			struct rpc_rqst *next = list_first_entry(&req->rq_xmit2,
+					struct rpc_rqst, rq_xmit2);
+			list_del(&req->rq_xmit2);
+			list_add_tail(&next->rq_xmit, &next->rq_xprt->xmit_queue);
+		}
+	} else
+		list_del(&req->rq_xmit2);
 }
 
 /**