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Commit e114a710 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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tcp: fix cwnd limited checking to improve congestion control 



Yuchung discovered tcp_is_cwnd_limited() was returning false in
slow start phase even if the application filled the socket write queue.

All congestion modules take into account tcp_is_cwnd_limited()
before increasing cwnd, so this behavior limits slow start from
probing the bandwidth at full speed.

The problem is that even if write queue is full (aka we are _not_
application limited), cwnd can be under utilized if TSO should auto
defer or TCP Small queues decided to hold packets.

So the in_flight can be kept to smaller value, and we can get to the
point tcp_is_cwnd_limited() returns false.

With TCP Small Queues and FQ/pacing, this issue is more visible.

We fix this by having tcp_cwnd_validate(), which is supposed to track
such things, take into account unsent_segs, the number of segs that we
are not sending at the moment due to TSO or TSQ, but intend to send
real soon. Then when we are cwnd-limited, remember this fact while we
are processing the window of ACKs that comes back.

For example, suppose we have a brand new connection with cwnd=10; we
are in slow start, and we send a flight of 9 packets. By the time we
have received ACKs for all 9 packets we want our cwnd to be 18.
We implement this by setting tp->lsnd_pending to 9, and
considering ourselves to be cwnd-limited while cwnd is less than
twice tp->lsnd_pending (2*9 -> 18).

This makes tcp_is_cwnd_limited() more understandable, by removing
the GSO/TSO kludge, that tried to work around the issue.

Note the in_flight parameter can be removed in a followup cleanup
patch.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 4e8bbb81

include/linux/tcp.h

+1 −0
Original line number Diff line number Diff line
@@ -230,6 +230,7 @@ struct tcp_sock { 
	u32	snd_cwnd_clamp; /* Do not allow snd_cwnd to grow above this */

	u32	snd_cwnd_used;

	u32	snd_cwnd_stamp;

	u32	lsnd_pending;	/* packets inflight or unsent since last xmit */

	u32	prior_cwnd;	/* Congestion window at start of Recovery. */

	u32	prr_delivered;	/* Number of newly delivered packets to

				 * receiver in Recovery. */

include/net/tcp.h

+21 −1
Original line number Diff line number Diff line
@@ -974,7 +974,27 @@ static inline u32 tcp_wnd_end(const struct tcp_sock *tp) 
{

	return tp->snd_una + tp->snd_wnd;

}

bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight);



/* We follow the spirit of RFC2861 to validate cwnd but implement a more

 * flexible approach. The RFC suggests cwnd should not be raised unless

 * it was fully used previously. But we allow cwnd to grow as long as the

 * application has used half the cwnd.

 * Example :

 *    cwnd is 10 (IW10), but application sends 9 frames.

 *    We allow cwnd to reach 18 when all frames are ACKed.

 * This check is safe because it's as aggressive as slow start which already

 * risks 100% overshoot. The advantage is that we discourage application to

 * either send more filler packets or data to artificially blow up the cwnd

 * usage, and allow application-limited process to probe bw more aggressively.

 *

 * TODO: remove in_flight once we can fix all callers, and their callers...

 */

static inline bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)

{

	const struct tcp_sock *tp = tcp_sk(sk);



	return tp->snd_cwnd < 2 * tp->lsnd_pending;

}



static inline void tcp_check_probe_timer(struct sock *sk)

{

net/ipv4/tcp_cong.c

+0 −20
Original line number Diff line number Diff line
@@ -276,26 +276,6 @@ int tcp_set_congestion_control(struct sock *sk, const char *name) 
	return err;

}



/* RFC2861 Check whether we are limited by application or congestion window

 * This is the inverse of cwnd check in tcp_tso_should_defer

 */

bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)

{

	const struct tcp_sock *tp = tcp_sk(sk);

	u32 left;



	if (in_flight >= tp->snd_cwnd)

		return true;



	left = tp->snd_cwnd - in_flight;

	if (sk_can_gso(sk) &&

	    left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&

	    left < tp->xmit_size_goal_segs)

		return true;

	return left <= tcp_max_tso_deferred_mss(tp);

}

EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);



/* Slow start is used when congestion window is no greater than the slow start

 * threshold. We base on RFC2581 and also handle stretch ACKs properly.

 * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but

net/ipv4/tcp_output.c

+14 −7
Original line number Diff line number Diff line
@@ -1402,12 +1402,13 @@ static void tcp_cwnd_application_limited(struct sock *sk) 
	tp->snd_cwnd_stamp = tcp_time_stamp;

}



/* Congestion window validation. (RFC2861) */

static void tcp_cwnd_validate(struct sock *sk)

static void tcp_cwnd_validate(struct sock *sk, u32 unsent_segs)

{

	struct tcp_sock *tp = tcp_sk(sk);



	if (tp->packets_out >= tp->snd_cwnd) {

	tp->lsnd_pending = tp->packets_out + unsent_segs;



	if (tcp_is_cwnd_limited(sk, 0)) {

		/* Network is feed fully. */

		tp->snd_cwnd_used = 0;

		tp->snd_cwnd_stamp = tcp_time_stamp;
@@ -1880,7 +1881,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, 
{

	struct tcp_sock *tp = tcp_sk(sk);

	struct sk_buff *skb;

	unsigned int tso_segs, sent_pkts;

	unsigned int tso_segs, sent_pkts, unsent_segs = 0;

	int cwnd_quota;

	int result;
@@ -1924,7 +1925,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, 
				break;

		} else {

			if (!push_one && tcp_tso_should_defer(sk, skb))

				break;

				goto compute_unsent_segs;

		}



		/* TCP Small Queues :
@@ -1949,9 +1950,15 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, 
			 * there is no smp_mb__after_set_bit() yet

			 */

			smp_mb__after_clear_bit();

			if (atomic_read(&sk->sk_wmem_alloc) > limit)

			if (atomic_read(&sk->sk_wmem_alloc) > limit) {

				u32 unsent_bytes;



compute_unsent_segs:

				unsent_bytes = tp->write_seq - tp->snd_nxt;

				unsent_segs = DIV_ROUND_UP(unsent_bytes, mss_now);

				break;

			}

		}



		limit = mss_now;

		if (tso_segs > 1 && !tcp_urg_mode(tp))
@@ -1990,7 +1997,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, 
		/* Send one loss probe per tail loss episode. */

		if (push_one != 2)

			tcp_schedule_loss_probe(sk);

		tcp_cwnd_validate(sk);

		tcp_cwnd_validate(sk, unsent_segs);

		return false;

	}

	return (push_one == 2) || (!tp->packets_out && tcp_send_head(sk));