Commit 232aa8ec authored by Priyaranjan Jha's avatar Priyaranjan Jha Committed by David S. Miller
Browse files

tcp_bbr: refactor bbr_target_cwnd() for general inflight provisioning 



Because bbr_target_cwnd() is really a general-purpose BBR helper for
computing some volume of inflight data as a function of the estimated
BDP, refactor it into following helper functions:
- bbr_bdp()
- bbr_quantization_budget()
- bbr_inflight()

Signed-off-by: default avatarPriyaranjan Jha <priyarjha@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 a1ead2ec

net/ipv4/tcp_bbr.c

+39 −21
Original line number Diff line number Diff line
@@ -315,30 +315,19 @@ static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event) 
	}

}



/* Find target cwnd. Right-size the cwnd based on min RTT and the

 * estimated bottleneck bandwidth:

/* Calculate bdp based on min RTT and the estimated bottleneck bandwidth:

 *

 * cwnd = bw * min_rtt * gain = BDP * gain

 * bdp = bw * min_rtt * gain

 *

 * The key factor, gain, controls the amount of queue. While a small gain

 * builds a smaller queue, it becomes more vulnerable to noise in RTT

 * measurements (e.g., delayed ACKs or other ACK compression effects). This

 * noise may cause BBR to under-estimate the rate.

 *

 * To achieve full performance in high-speed paths, we budget enough cwnd to

 * fit full-sized skbs in-flight on both end hosts to fully utilize the path:

 *   - one skb in sending host Qdisc,

 *   - one skb in sending host TSO/GSO engine

 *   - one skb being received by receiver host LRO/GRO/delayed-ACK engine

 * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because

 * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets,

 * which allows 2 outstanding 2-packet sequences, to try to keep pipe

 * full even with ACK-every-other-packet delayed ACKs.

 */

static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain)

static u32 bbr_bdp(struct sock *sk, u32 bw, int gain)

{

	struct bbr *bbr = inet_csk_ca(sk);

	u32 cwnd;

	u32 bdp;

	u64 w;



	/* If we've never had a valid RTT sample, cap cwnd at the initial
@@ -353,7 +342,24 @@ static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain) 
	w = (u64)bw * bbr->min_rtt_us;



	/* Apply a gain to the given value, then remove the BW_SCALE shift. */

	cwnd = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;

	bdp = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;



	return bdp;

}



/* To achieve full performance in high-speed paths, we budget enough cwnd to

 * fit full-sized skbs in-flight on both end hosts to fully utilize the path:

 *   - one skb in sending host Qdisc,

 *   - one skb in sending host TSO/GSO engine

 *   - one skb being received by receiver host LRO/GRO/delayed-ACK engine

 * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because

 * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets,

 * which allows 2 outstanding 2-packet sequences, to try to keep pipe

 * full even with ACK-every-other-packet delayed ACKs.

 */

static u32 bbr_quantization_budget(struct sock *sk, u32 cwnd, int gain)

{

	struct bbr *bbr = inet_csk_ca(sk);



	/* Allow enough full-sized skbs in flight to utilize end systems. */

	cwnd += 3 * bbr_tso_segs_goal(sk);
@@ -368,6 +374,17 @@ static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain) 
	return cwnd;

}



/* Find inflight based on min RTT and the estimated bottleneck bandwidth. */

static u32 bbr_inflight(struct sock *sk, u32 bw, int gain)

{

	u32 inflight;



	inflight = bbr_bdp(sk, bw, gain);

	inflight = bbr_quantization_budget(sk, inflight, gain);



	return inflight;

}



/* With pacing at lower layers, there's often less data "in the network" than

 * "in flight". With TSQ and departure time pacing at lower layers (e.g. fq),

 * we often have several skbs queued in the pacing layer with a pre-scheduled
@@ -462,7 +479,8 @@ static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs, 
		goto done;



	/* If we're below target cwnd, slow start cwnd toward target cwnd. */

	target_cwnd = bbr_target_cwnd(sk, bw, gain);

	target_cwnd = bbr_bdp(sk, bw, gain);

	target_cwnd = bbr_quantization_budget(sk, target_cwnd, gain);

	if (bbr_full_bw_reached(sk))  /* only cut cwnd if we filled the pipe */

		cwnd = min(cwnd + acked, target_cwnd);

	else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND)
@@ -503,14 +521,14 @@ static bool bbr_is_next_cycle_phase(struct sock *sk, 
	if (bbr->pacing_gain > BBR_UNIT)

		return is_full_length &&

			(rs->losses ||  /* perhaps pacing_gain*BDP won't fit */

			 inflight >= bbr_target_cwnd(sk, bw, bbr->pacing_gain));

			 inflight >= bbr_inflight(sk, bw, bbr->pacing_gain));



	/* A pacing_gain < 1.0 tries to drain extra queue we added if bw

	 * probing didn't find more bw. If inflight falls to match BDP then we

	 * estimate queue is drained; persisting would underutilize the pipe.

	 */

	return is_full_length ||

		inflight <= bbr_target_cwnd(sk, bw, BBR_UNIT);

		inflight <= bbr_inflight(sk, bw, BBR_UNIT);

}



static void bbr_advance_cycle_phase(struct sock *sk)
@@ -762,11 +780,11 @@ static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs) 
	if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) {

		bbr->mode = BBR_DRAIN;	/* drain queue we created */

		tcp_sk(sk)->snd_ssthresh =

				bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT);

				bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT);

	}	/* fall through to check if in-flight is already small: */

	if (bbr->mode == BBR_DRAIN &&

	    bbr_packets_in_net_at_edt(sk, tcp_packets_in_flight(tcp_sk(sk))) <=

	    bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT))

	    bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT))

		bbr_reset_probe_bw_mode(sk);  /* we estimate queue is drained */

}

CRA Git | Maintained and supported by SUSTech CRA and CCSE