Commit 3aff3b4b authored Jan 31, 2018 by Neal Cardwell Committed by David S. Miller Feb 01, 2018

tcp_bbr: fix pacing_gain to always be unity when using lt_bw



This commit fixes the pacing_gain to remain at BBR_UNIT (1.0) when
using lt_bw and returning from the PROBE_RTT state to PROBE_BW.

Previously, when using lt_bw, upon exiting PROBE_RTT and entering
PROBE_BW the bbr_reset_probe_bw_mode() code could sometimes randomly
end up with a cycle_idx of 0 and hence have bbr_advance_cycle_phase()
set a pacing gain above 1.0. In such cases this would result in a
pacing rate that is 1.25x higher than intended, potentially resulting
in a high loss rate for a little while until we stop using the lt_bw a
bit later.

This commit is a stable candidate for kernels back as far as 4.9.

Fixes: 0f8782ea ("tcp_bbr: add BBR congestion control")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Reported-by: Beyers Cronje <bcronje@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 2e85283d

net/ipv4/tcp_bbr.c

+3 −3

Original line number	Original line	Diff line number	Diff line
	@@ -481,7 +481,8 @@ static void bbr_advance_cycle_phase(struct sock *sk)

	bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);		bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);
	bbr->cycle_mstamp = tp->delivered_mstamp;		bbr->cycle_mstamp = tp->delivered_mstamp;
	bbr->pacing_gain = bbr_pacing_gain[bbr->cycle_idx];		bbr->pacing_gain = bbr->lt_use_bw ? BBR_UNIT :
			bbr_pacing_gain[bbr->cycle_idx];
	}		}

	/* Gain cycling: cycle pacing gain to converge to fair share of available bw. */		/* Gain cycling: cycle pacing gain to converge to fair share of available bw. */
	@@ -490,8 +491,7 @@ static void bbr_update_cycle_phase(struct sock *sk,
	{		{
	struct bbr *bbr = inet_csk_ca(sk);		struct bbr *bbr = inet_csk_ca(sk);

	if ((bbr->mode == BBR_PROBE_BW) && !bbr->lt_use_bw &&		if (bbr->mode == BBR_PROBE_BW && bbr_is_next_cycle_phase(sk, rs))
	bbr_is_next_cycle_phase(sk, rs))
	bbr_advance_cycle_phase(sk);		bbr_advance_cycle_phase(sk);
	}		}

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