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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: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Priyaranjan Jha 2019-01-23 12:04:53 -08:00 committed by David S. Miller
parent a1ead2ec3c
commit 232aa8ec3e
1 changed files with 39 additions and 21 deletions
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60
net/ipv4/tcp_bbr.c
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@ -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 */
}