tcp: Don't reset ACK_TO_TAP_DUE on any ACK, reschedule timer as needed2023_03_21.1ee2f7c

This is mostly symmetric with commit cc6d8286d104 ("tcp: Reset
ACK_FROM_TAP_DUE flag only as needed, update timer"): we shouldn't
reset the ACK_TO_TAP_DUE flag on any inbound ACK segment, but only
once we acknowledge everything we received from the guest or the
container.

If we don't, a client might unnecessarily hold off further data,
especially during slow start, and in general we won't converge to the
usable bandwidth.

This is very visible especially with traffic tests on links with
non-negligible latency, such as in the reported issue. There, a
public iperf3 server sometimes aborts the test due do what appears to
be a low iperf3's --rcv-timeout (probably less than a second). Even
if this doesn't happen, the throughput will converge to a fraction of
the usable bandwidth.

Clear ACK_TO_TAP_DUE if we acknowledged everything, set it if we
didn't, and reschedule the timer in case the flag is still set as the
timer expires.

While at it, decrease the ACK timer interval to 10ms.

A 50ms interval is short enough for any bandwidth-delay product I had
in mind (local connections, or non-local connections with limited
bandwidth), but here I am, testing 1gbps transfers to a peer with
100ms RTT.

Indeed, we could eventually make the timer interval dependent on the
current window and estimated bandwidth-delay product, but at least
for the moment being, 10ms should be long enough to avoid any
measurable syscall overhead, yet usable for any real-world
application.

Reported-by: Lukas Mrtvy <lukas.mrtvy@gmail.com>
Link: https://bugs.passt.top/show_bug.cgi?id=44
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/tcp.c b/tcp.c
index 590b08a..f156287 100644
--- a/tcp.c
+++ b/tcp.c
@@ -364,7 +364,7 @@ struct tcp6_l2_head {	/* For MSS6 macro: keep in sync with tcp6_l2_buf_t */
 # define KERNEL_REPORTS_SND_WND(c)	(0 && (c))
 #endif
 
-#define ACK_INTERVAL			50		/* ms */
+#define ACK_INTERVAL			10		/* ms */
 #define SYN_TIMEOUT			10		/* s */
 #define ACK_TIMEOUT			2
 #define FIN_TIMEOUT			60
@@ -1730,8 +1730,12 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 	iov->iov_len = tcp_l2_buf_fill_headers(c, conn, p, optlen,
 					       NULL, conn->seq_to_tap);
 
-	if (th->ack)
-		conn_flag(c, conn, ~ACK_TO_TAP_DUE);
+	if (th->ack) {
+		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
+			conn_flag(c, conn, ~ACK_TO_TAP_DUE);
+		else
+			conn_flag(c, conn, ACK_TO_TAP_DUE);
+	}
 
 	if (th->fin)
 		conn_flag(c, conn, ACK_FROM_TAP_DUE);
@@ -2820,7 +2824,7 @@ static void tcp_timer_handler(struct ctx *c, union epoll_ref ref)
 
 	if (conn->flags & ACK_TO_TAP_DUE) {
 		tcp_send_flag(c, conn, ACK_IF_NEEDED);
-		conn_flag(c, conn, ~ACK_TO_TAP_DUE);
+		tcp_timer_ctl(c, conn);
 	} else if (conn->flags & ACK_FROM_TAP_DUE) {
 		if (!(conn->events & ESTABLISHED)) {
 			debug("TCP: index %li, handshake timeout", CONN_IDX(conn));