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From:   Leonard Crestez <cdleonard@gmail.com>
To:     Neal Cardwell <ncardwell@google.com>,
        Matt Mathis <mattmathis@google.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Willem de Bruijn <willemb@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        John Heffner <johnwheffner@gmail.com>,
        Leonard Crestez <lcrestez@drivenets.com>,
        Soheil Hassas Yeganeh <soheil@google.com>,
        Roopa Prabhu <roopa@cumulusnetworks.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [RFC 1/3] tcp: Consider mtu probing for tcp_xmit_size_goal
Date:   Tue, 11 May 2021 15:04:16 +0300
Message-Id: <52e63f5b41c9604b909badb7fbc593fe1fe77413.1620733594.git.cdleonard@gmail.com>
In-Reply-To: <cover.1620733594.git.cdleonard@gmail.com>
References: <cover.1620733594.git.cdleonard@gmail.com>
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According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes
in order to accumulate enough data" but linux almost never does that.

Linux checks for (probe_size + (1 + reorder) * mss_cache) bytes to be
available in the send buffer and if that condition is not met it will
send anyway using the current MSS. The feature can be made to work by
sending very large chunks of data from userspace (for example 128k) but
for small writes on fast links tcp mtu probes almost never happen.

This patch tries to take mtu probe into account in tcp_xmit_size_goal, a
function which otherwise attempts to accumulate a packet suitable for
TSO. No delays are introduced beyond existing autocork heuristics.

Suggested-by: Matt Mathis <mattmathis@google.com>
Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
---
 Documentation/networking/ip-sysctl.rst |  5 ++++
 include/net/inet_connection_sock.h     |  4 ++-
 include/net/netns/ipv4.h               |  1 +
 include/net/tcp.h                      |  1 +
 net/ipv4/sysctl_net_ipv4.c             |  7 ++++++
 net/ipv4/tcp.c                         | 11 +++++++-
 net/ipv4/tcp_ipv4.c                    |  1 +
 net/ipv4/tcp_output.c                  | 35 +++++++++++++++++++++++---
 8 files changed, 59 insertions(+), 6 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index c2ecc9894fd0..108a5ee227d3 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -320,10 +320,15 @@ tcp_mtu_probe_floor - INTEGER
 	If MTU probing is enabled this caps the minimum MSS used for search_low
 	for the connection.
 
 	Default : 48
 
+tcp_mtu_probe_autocork - BOOLEAN
+	Take into account mtu probe size when accumulating data via autocorking.
+
+	Default: 1
+
 tcp_min_snd_mss - INTEGER
 	TCP SYN and SYNACK messages usually advertise an ADVMSS option,
 	as described in RFC 1122 and RFC 6691.
 
 	If this ADVMSS option is smaller than tcp_min_snd_mss,
diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 3c8c59471bc1..9a53d698c2e6 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -123,11 +123,13 @@ struct inet_connection_sock {
 		/* Range of MTUs to search */
 		int		  search_high;
 		int		  search_low;
 
 		/* Information on the current probe. */
-		u32		  probe_size:31,
+		u32		  probe_size:30,
+		/* Are we actively accumulating data for an mtu probe? */
+				  wait_data:1,
 		/* Is the MTUP feature enabled for this connection? */
 				  enabled:1;
 
 		u32		  probe_timestamp;
 	} icsk_mtup;
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index f6af8d96d3c6..3a2d8bf2b20a 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -112,10 +112,11 @@ struct netns_ipv4 {
 #ifdef CONFIG_NET_L3_MASTER_DEV
 	u8 sysctl_tcp_l3mdev_accept;
 #endif
 	u8 sysctl_tcp_mtu_probing;
 	int sysctl_tcp_mtu_probe_floor;
+	int sysctl_tcp_mtu_probe_autocork;
 	int sysctl_tcp_base_mss;
 	int sysctl_tcp_min_snd_mss;
 	int sysctl_tcp_probe_threshold;
 	u32 sysctl_tcp_probe_interval;
 
diff --git a/include/net/tcp.h b/include/net/tcp.h
index d05193cb0d99..fb656490c901 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -666,10 +666,11 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
 void tcp_initialize_rcv_mss(struct sock *sk);
 
 int tcp_mtu_to_mss(struct sock *sk, int pmtu);
 int tcp_mss_to_mtu(struct sock *sk, int mss);
 void tcp_mtup_init(struct sock *sk);
+int tcp_mtu_probe_size_needed(struct sock *sk, int *probe_size);
 
 static inline void tcp_bound_rto(const struct sock *sk)
 {
 	if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
 		inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index a62934b9f15a..e19176c17973 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -827,10 +827,17 @@ static struct ctl_table ipv4_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= &tcp_min_snd_mss_min,
 		.extra2		= &tcp_min_snd_mss_max,
 	},
+	{
+		.procname	= "tcp_mtu_probe_autocork",
+		.data		= &init_net.ipv4.sysctl_tcp_mtu_probe_autocork,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
 	{
 		.procname	= "tcp_probe_threshold",
 		.data		= &init_net.ipv4.sysctl_tcp_probe_threshold,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index f1c1f9e3de72..23cfb2db28b4 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -913,10 +913,11 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
 }
 
 static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
 				       int large_allowed)
 {
+	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 new_size_goal, size_goal;
 
 	if (!large_allowed)
 		return mss_now;
@@ -932,11 +933,19 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
 		tp->gso_segs = min_t(u16, new_size_goal / mss_now,
 				     sk->sk_gso_max_segs);
 		size_goal = tp->gso_segs * mss_now;
 	}
 
-	return max(size_goal, mss_now);
+	size_goal = max(size_goal, mss_now);
+
+	if (unlikely(icsk->icsk_mtup.wait_data)) {
+		int mtu_probe_size_needed = tcp_mtu_probe_size_needed(sk, NULL);
+		if (mtu_probe_size_needed > 0)
+			size_goal = max(size_goal, (u32)mtu_probe_size_needed);
+	}
+
+	return size_goal;
 }
 
 int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
 {
 	int mss_now;
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 312184cead57..7e75423c08c9 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2889,10 +2889,11 @@ static int __net_init tcp_sk_init(struct net *net)
 	net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS;
 	net->ipv4.sysctl_tcp_min_snd_mss = TCP_MIN_SND_MSS;
 	net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD;
 	net->ipv4.sysctl_tcp_probe_interval = TCP_PROBE_INTERVAL;
 	net->ipv4.sysctl_tcp_mtu_probe_floor = TCP_MIN_SND_MSS;
+	net->ipv4.sysctl_tcp_mtu_probe_autocork = 1;
 
 	net->ipv4.sysctl_tcp_keepalive_time = TCP_KEEPALIVE_TIME;
 	net->ipv4.sysctl_tcp_keepalive_probes = TCP_KEEPALIVE_PROBES;
 	net->ipv4.sysctl_tcp_keepalive_intvl = TCP_KEEPALIVE_INTVL;
 
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index bde781f46b41..5a320d792ec4 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2311,10 +2311,31 @@ static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
 	}
 
 	return true;
 }
 
+/* Calculate the size of an MTU probe
+ * Probing the MTU requires one packets which is larger that current MSS as well
+ * as enough following mtu-sized packets to ensure that a probe loss can be
+ * detected without a full Retransmit Time out.
+ */
+int tcp_mtu_probe_size_needed(struct sock *sk, int *probe_size)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	int probe_size_val;
+	int size_needed;
+
+	/* This might be a little slow: */
+	probe_size_val = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high + icsk->icsk_mtup.search_low) >> 1);
+	if (probe_size)
+		*probe_size = probe_size_val;
+	size_needed = probe_size_val + (tp->reordering + 1) * tp->mss_cache;
+
+	return size_needed;
+}
+
 /* Create a new MTU probe if we are ready.
  * MTU probe is regularly attempting to increase the path MTU by
  * deliberately sending larger packets.  This discovers routing
  * changes resulting in larger path MTUs.
  *
@@ -2349,13 +2370,12 @@ static int tcp_mtu_probe(struct sock *sk)
 	/* Use binary search for probe_size between tcp_mss_base,
 	 * and current mss_clamp. if (search_high - search_low)
 	 * smaller than a threshold, backoff from probing.
 	 */
 	mss_now = tcp_current_mss(sk);
-	probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
-				    icsk->icsk_mtup.search_low) >> 1);
-	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
+	size_needed = tcp_mtu_probe_size_needed(sk, &probe_size);
+
 	interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
 	/* When misfortune happens, we are reprobing actively,
 	 * and then reprobe timer has expired. We stick with current
 	 * probing process by not resetting search range to its orignal.
 	 */
@@ -2368,11 +2388,11 @@ static int tcp_mtu_probe(struct sock *sk)
 		return -1;
 	}
 
 	/* Have enough data in the send queue to probe? */
 	if (tp->write_seq - tp->snd_nxt < size_needed)
-		return -1;
+		return net->ipv4.sysctl_tcp_mtu_probe_autocork ? 0 : -1;
 
 	if (tp->snd_wnd < size_needed)
 		return -1;
 	if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
 		return 0;
@@ -2596,11 +2616,13 @@ void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
  * but cannot send anything now because of SWS or another problem.
  */
 static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 			   int push_one, gfp_t gfp)
 {
+	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
 	struct sk_buff *skb;
 	unsigned int tso_segs, sent_pkts;
 	int cwnd_quota;
 	int result;
 	bool is_cwnd_limited = false, is_rwnd_limited = false;
@@ -2611,13 +2633,18 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 	tcp_mstamp_refresh(tp);
 	if (!push_one) {
 		/* Do MTU probing. */
 		result = tcp_mtu_probe(sk);
 		if (!result) {
+			if (net->ipv4.sysctl_tcp_mtu_probe_autocork)
+				icsk->icsk_mtup.wait_data = true;
 			return false;
 		} else if (result > 0) {
+			icsk->icsk_mtup.wait_data = false;
 			sent_pkts = 1;
+		} else {
+			icsk->icsk_mtup.wait_data = false;
 		}
 	}
 
 	max_segs = tcp_tso_segs(sk, mss_now);
 	while ((skb = tcp_send_head(sk))) {
-- 
2.25.1