C|网络|TCP-BBR拥塞控制剖析

static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	bool filter_expired;

	/* Track min RTT seen in the min_rtt_win_sec filter window: */
	filter_expired = after(tcp_jiffies32,
			       bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
	if (rs->rtt_us >= 0 &&
	    (rs->rtt_us <= bbr->min_rtt_us ||
	     (filter_expired && !rs->is_ack_delayed))) {
		bbr->min_rtt_us = rs->rtt_us;
		bbr->min_rtt_stamp = tcp_jiffies32;
	}
}

	if (bbr_probe_rtt_mode_ms > 0 && filter_expired &&
	    !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) {
		bbr->mode = BBR_PROBE_RTT;  /* dip, drain queue */
		bbr_save_cwnd(sk);  /* note cwnd so we can restore it */
		bbr->probe_rtt_done_stamp = 0;
	}

	if (bbr->mode == BBR_PROBE_RTT) {
		/* Ignore low rate samples during this mode. */
		tp->app_limited =
			(tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
		/* Maintain min packets in flight for max(200 ms, 1 round). */
		if (!bbr->probe_rtt_done_stamp &&
		    tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) {
			bbr->probe_rtt_done_stamp = tcp_jiffies32 +
				msecs_to_jiffies(bbr_probe_rtt_mode_ms);
			bbr->probe_rtt_round_done = 0;
			bbr->next_rtt_delivered = tp->delivered;
		} else if (bbr->probe_rtt_done_stamp) {
			if (bbr->round_start)
				bbr->probe_rtt_round_done = 1;
			if (bbr->probe_rtt_round_done)
				bbr_check_probe_rtt_done(sk);
		}
	}
	/* Restart after idle ends only once we process a new S/ACK for data */
	if (rs->delivered > 0)
		bbr->idle_restart = 0;

static void bbr_check_probe_rtt_done(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);

	if (!(bbr->probe_rtt_done_stamp &&
	      after(tcp_jiffies32, bbr->probe_rtt_done_stamp)))
		return;

	bbr->min_rtt_stamp = tcp_jiffies32;  /* wait a while until PROBE_RTT */
	tp->snd_cwnd = max(tp->snd_cwnd, bbr->prior_cwnd);
	bbr_reset_mode(sk);
}

/* Estimate the bandwidth based on how fast packets are delivered */
static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u64 bw;
	bbr->round_start = 0;
	if (rs->delivered < 0 || rs->interval_us <= 0)
		return; /* Not a valid observation */
	if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) {
		bbr->next_rtt_delivered = tp->delivered;
		bbr->rtt_cnt++;
		bbr->round_start = 1;
		bbr->packet_conservation = 0;
	}
	bbr_lt_bw_sampling(sk, rs);
	bw = div64_long((u64)rs->delivered * BW_UNIT, rs->interval_us);
	if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) {
		/* Incorporate new sample into our max bw filter. */
		minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw);
	}
}

/* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */
static u32 bbr_max_bw(const struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);
	return minmax_get(&bbr->bw);
}
/* Return the estimated bandwidth of the path, in pkts/uS << BW_SCALE. */
static u32 bbr_bw(const struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);
	return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk);
}
/* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of
 * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and
 * explicitly models their policed rate, to reduce unnecessary losses. We
 * estimate that we're policed if we see 2 consecutive sampling intervals with
 * consistent throughput and high packet loss. If we think we're being policed,
 * set lt_bw to the "long-term" average delivery rate from those 2 intervals.
 */

static u32 bbr_bdp(struct sock *sk, u32 bw, int gain)
{
	struct bbr *bbr = inet_csk_ca(sk);
	u32 bdp;
	u64 w;
	if (unlikely(bbr->min_rtt_us == ~0U))	 /* no valid RTT samples yet? */
		return TCP_INIT_CWND;  /* be safe: cap at default initial cwnd*/
	w = (u64)bw * bbr->min_rtt_us;
	/* Apply a gain to the given value, remove the BW_SCALE shift, and
	 * round the value up to avoid a negative feedback loop.
	 */
	bdp = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
	return bdp;
}

pacing_rate = pacing_gain * bottleneck_bandwidth
cwnd = max(cwnd_gain * bottleneck_bandwidth * min_rtt, 4)

static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
{
	bbr_update_bw(sk, rs);
	bbr_update_ack_aggregation(sk, rs);
	bbr_update_cycle_phase(sk, rs);
	bbr_check_full_bw_reached(sk, rs);
	bbr_check_drain(sk, rs);
	bbr_update_min_rtt(sk, rs);
	bbr_update_gains(sk);
}