net/smc: add autocorking support

This patch adds autocorking support for SMC which could improve
throughput for small message by x3+.

The main idea is borrowed from TCP autocorking with some RDMA
specific modification:
1. The first message should never cork to make sure we won't
bring extra latency
2. If we have posted any Tx WRs to the NIC that have not
completed, cork the new messages until:
a) Receive CQE for the last Tx WR
b) We have corked enough message on the connection
3. Try to push the corked data out when we receive CQE of
the last Tx WR to prevent the corked messages hang in
the send queue.

Both SMC autocorking and TCP autocorking check the TX completion
to decide whether we should cork or not. The difference is
when we got a SMC Tx WR completion, the data have been confirmed
by the RNIC while TCP TX completion just tells us the data
have been sent out by the local NIC.

Add an atomic variable tx_pushing in smc_connection to make
sure only one can send to let it cork more and save CDC slot.

SMC autocorking should not bring extra latency since the first
message will always been sent out immediately.

The qperf tcp_bw test shows more than x4 increase under small
message size with Mellanox connectX4-Lx, same result with other
throughput benchmarks like sockperf/netperf.
The qperf tcp_lat test shows SMC autocorking has not increase any
ping-pong latency.

Test command:
client: smc_run taskset -c 1 qperf smc-server -oo msg_size:1:64K:*2 \
-t 30 -vu tcp_{bw|lat}
server: smc_run taskset -c 1 qperf

=== Bandwidth ====
MsgSize(Bytes) SMC-NoCork TCP SMC-AutoCorking
1 0.578 MB/s 2.392 MB/s(313.57%) 2.647 MB/s(357.72%)
2 1.159 MB/s 4.780 MB/s(312.53%) 5.153 MB/s(344.71%)
4 2.283 MB/s 10.266 MB/s(349.77%) 10.363 MB/s(354.02%)
8 4.668 MB/s 19.040 MB/s(307.86%) 21.215 MB/s(354.45%)
16 9.147 MB/s 38.904 MB/s(325.31%) 41.740 MB/s(356.32%)
32 18.369 MB/s 79.587 MB/s(333.25%) 82.392 MB/s(348.52%)
64 36.562 MB/s 148.668 MB/s(306.61%) 161.564 MB/s(341.89%)
128 72.961 MB/s 274.913 MB/s(276.80%) 325.363 MB/s(345.94%)
256 144.705 MB/s 512.059 MB/s(253.86%) 633.743 MB/s(337.96%)
512 288.873 MB/s 884.977 MB/s(206.35%) 1250.681 MB/s(332.95%)
1024 574.180 MB/s 1337.736 MB/s(132.98%) 2246.121 MB/s(291.19%)
2048 1095.192 MB/s 1865.952 MB/s( 70.38%) 2057.767 MB/s( 87.89%)
4096 2066.157 MB/s 2380.337 MB/s( 15.21%) 2173.983 MB/s( 5.22%)
8192 3717.198 MB/s 2733.073 MB/s(-26.47%) 3491.223 MB/s( -6.08%)
16384 4742.221 MB/s 2958.693 MB/s(-37.61%) 4637.692 MB/s( -2.20%)
32768 5349.550 MB/s 3061.285 MB/s(-42.77%) 5385.796 MB/s( 0.68%)
65536 5162.919 MB/s 3731.408 MB/s(-27.73%) 5223.890 MB/s( 1.18%)
==== Latency ====
MsgSize(Bytes) SMC-NoCork TCP SMC-AutoCorking
1 10.540 us 11.938 us( 13.26%) 10.573 us( 0.31%)
2 10.996 us 11.992 us( 9.06%) 10.269 us( -6.61%)
4 10.229 us 11.687 us( 14.25%) 10.240 us( 0.11%)
8 10.203 us 11.653 us( 14.21%) 10.402 us( 1.95%)
16 10.530 us 11.313 us( 7.44%) 10.599 us( 0.66%)
32 10.241 us 11.586 us( 13.13%) 10.223 us( -0.18%)
64 10.693 us 11.652 us( 8.97%) 10.251 us( -4.13%)
128 10.597 us 11.579 us( 9.27%) 10.494 us( -0.97%)
256 10.409 us 11.957 us( 14.87%) 10.710 us( 2.89%)
512 11.088 us 12.505 us( 12.78%) 10.547 us( -4.88%)
1024 11.240 us 12.255 us( 9.03%) 10.787 us( -4.03%)
2048 11.485 us 16.970 us( 47.76%) 11.256 us( -1.99%)
4096 12.077 us 13.948 us( 15.49%) 12.230 us( 1.27%)
8192 13.683 us 16.693 us( 22.00%) 13.786 us( 0.75%)
16384 16.470 us 23.615 us( 43.38%) 16.459 us( -0.07%)
32768 22.540 us 40.966 us( 81.75%) 23.284 us( 3.30%)
65536 34.192 us 73.003 us(113.51%) 34.233 us( 0.12%)

With SMC autocorking support, we can archive better throughput
than TCP in most message sizes without any latency trade-off.

Signed-off-by: Dust Li <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/smc/smc.h

···
 #define SMC_MAX_ISM_DEVS	8	/* max # of proposed non-native ISM
 					 * devices
 					 */
+#define SMC_AUTOCORKING_DEFAULT_SIZE	0x10000	/* 64K by default */
 
 extern struct proto smc_proto;
 extern struct proto smc_proto6;
···
 						 * - dec on polled tx cqe
 						 */
 	wait_queue_head_t	cdc_pend_tx_wq; /* wakeup on no cdc_pend_tx_wr*/
+	atomic_t		tx_pushing;     /* nr_threads trying tx push */
 	struct delayed_work	tx_work;	/* retry of smc_cdc_msg_send */
 	u32			tx_off;		/* base offset in peer rmb */
 

net/smc/smc_cdc.c

···
 		conn->tx_cdc_seq_fin = cdcpend->ctrl_seq;
 	}
 
-	if (atomic_dec_and_test(&conn->cdc_pend_tx_wr) &&
-	    unlikely(wq_has_sleeper(&conn->cdc_pend_tx_wq)))
-		wake_up(&conn->cdc_pend_tx_wq);
+	if (atomic_dec_and_test(&conn->cdc_pend_tx_wr)) {
+		/* If this is the last pending WR complete, we must push to
+		 * prevent hang when autocork enabled.
+		 */
+		smc_tx_sndbuf_nonempty(conn);
+		if (unlikely(wq_has_sleeper(&conn->cdc_pend_tx_wq)))
+			wake_up(&conn->cdc_pend_tx_wq);
+	}
 	WARN_ON(atomic_read(&conn->cdc_pend_tx_wr) < 0);
 
 	smc_tx_sndbuf_nonfull(smc);

net/smc/smc_tx.c

···
 	return (tp->nonagle & TCP_NAGLE_CORK) ? true : false;
 }
 
+/* If we have pending CDC messages, do not send:
+ * Because CQE of this CDC message will happen shortly, it gives
+ * a chance to coalesce future sendmsg() payload in to one RDMA Write,
+ * without need for a timer, and with no latency trade off.
+ * Algorithm here:
+ *  1. First message should never cork
+ *  2. If we have pending Tx CDC messages, wait for the first CDC
+ *     message's completion
+ *  3. Don't cork to much data in a single RDMA Write to prevent burst
+ *     traffic, total corked message should not exceed sendbuf/2
+ */
+static bool smc_should_autocork(struct smc_sock *smc)
+{
+	struct smc_connection *conn = &smc->conn;
+	int corking_size;
+
+	corking_size = min(SMC_AUTOCORKING_DEFAULT_SIZE,
+			   conn->sndbuf_desc->len >> 1);
+
+	if (atomic_read(&conn->cdc_pend_tx_wr) == 0 ||
+	    smc_tx_prepared_sends(conn) > corking_size)
+		return false;
+	return true;
+}
+
+static bool smc_tx_should_cork(struct smc_sock *smc, struct msghdr *msg)
+{
+	struct smc_connection *conn = &smc->conn;
+
+	if (smc_should_autocork(smc))
+		return true;
+
+	/* for a corked socket defer the RDMA writes if
+	 * sndbuf_space is still available. The applications
+	 * should known how/when to uncork it.
+	 */
+	if ((msg->msg_flags & MSG_MORE ||
+	     smc_tx_is_corked(smc) ||
+	     msg->msg_flags & MSG_SENDPAGE_NOTLAST) &&
+	    atomic_read(&conn->sndbuf_space))
+		return true;
+
+	return false;
+}
+
 /* sndbuf producer: main API called by socket layer.
  * called under sock lock.
  */
···
 		 */
 		if ((msg->msg_flags & MSG_OOB) && !send_remaining)
 			conn->urg_tx_pend = true;
-		/* for a corked socket defer the RDMA writes if
-		 * sndbuf_space is still available. The applications
-		 * should known how/when to uncork it.
+		/* If we need to cork, do nothing and wait for the next
+		 * sendmsg() call or push on tx completion
 		 */
-		if (!((msg->msg_flags & MSG_MORE || smc_tx_is_corked(smc) ||
-		       msg->msg_flags & MSG_SENDPAGE_NOTLAST) &&
-		      atomic_read(&conn->sndbuf_space)))
+		if (!smc_tx_should_cork(smc, msg))
 			smc_tx_sndbuf_nonempty(conn);
 
 		trace_smc_tx_sendmsg(smc, copylen);
···
 	return rc;
 }
 
-int smc_tx_sndbuf_nonempty(struct smc_connection *conn)
+static int __smc_tx_sndbuf_nonempty(struct smc_connection *conn)
 {
-	int rc;
+	struct smc_sock *smc = container_of(conn, struct smc_sock, conn);
+	int rc = 0;
+
+	/* No data in the send queue */
+	if (unlikely(smc_tx_prepared_sends(conn) <= 0))
+		goto out;
+
+	/* Peer don't have RMBE space */
+	if (unlikely(atomic_read(&conn->peer_rmbe_space) <= 0)) {
+		SMC_STAT_RMB_TX_PEER_FULL(smc, !conn->lnk);
+		goto out;
+	}
 
 	if (conn->killed ||
-	    conn->local_rx_ctrl.conn_state_flags.peer_conn_abort)
-		return -EPIPE;	/* connection being aborted */
+	    conn->local_rx_ctrl.conn_state_flags.peer_conn_abort) {
+		rc = -EPIPE;    /* connection being aborted */
+		goto out;
+	}
 	if (conn->lgr->is_smcd)
 		rc = smcd_tx_sndbuf_nonempty(conn);
 	else
···
 
 	if (!rc) {
 		/* trigger socket release if connection is closing */
-		struct smc_sock *smc = container_of(conn, struct smc_sock,
-						    conn);
 		smc_close_wake_tx_prepared(smc);
 	}
+
+out:
+	return rc;
+}
+
+int smc_tx_sndbuf_nonempty(struct smc_connection *conn)
+{
+	int rc;
+
+	/* This make sure only one can send simultaneously to prevent wasting
+	 * of CPU and CDC slot.
+	 * Record whether someone has tried to push while we are pushing.
+	 */
+	if (atomic_inc_return(&conn->tx_pushing) > 1)
+		return 0;
+
+again:
+	atomic_set(&conn->tx_pushing, 1);
+	smp_wmb(); /* Make sure tx_pushing is 1 before real send */
+	rc = __smc_tx_sndbuf_nonempty(conn);
+
+	/* We need to check whether someone else have added some data into
+	 * the send queue and tried to push but failed after the atomic_set()
+	 * when we are pushing.
+	 * If so, we need to push again to prevent those data hang in the send
+	 * queue.
+	 */
+	if (unlikely(!atomic_dec_and_test(&conn->tx_pushing)))
+		goto again;
+
 	return rc;
 }