From c589e69b508d29ed8e644dfecda453f71c02ec27 Mon Sep 17 00:00:00 2001 From: Neal Cardwell Date: Thu, 7 Dec 2017 12:43:30 -0500 Subject: [PATCH 1/3] tcp_bbr: record "full bw reached" decision in new full_bw_reached bit This commit records the "full bw reached" decision in a new full_bw_reached bit. This is a pure refactor that does not change the current behavior, but enables subsequent fixes and improvements. In particular, this enables simple and clean fixes because the full_bw and full_bw_cnt can be unconditionally zeroed without worrying about forgetting that we estimated we filled the pipe in Startup. And it enables future improvements because multiple code paths can be used for estimating that we filled the pipe in Startup; any new code paths only need to set this bit when they think the pipe is full. Note that this fix intentionally reduces the width of the full_bw_cnt counter, since we have never used the most significant bit. Signed-off-by: Neal Cardwell Reviewed-by: Yuchung Cheng Acked-by: Soheil Hassas Yeganeh Signed-off-by: David S. Miller --- net/ipv4/tcp_bbr.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c index 69ee877574d0..3089c956b9f9 100644 --- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -110,7 +110,8 @@ struct bbr { u32 lt_last_lost; /* LT intvl start: tp->lost */ u32 pacing_gain:10, /* current gain for setting pacing rate */ cwnd_gain:10, /* current gain for setting cwnd */ - full_bw_cnt:3, /* number of rounds without large bw gains */ + full_bw_reached:1, /* reached full bw in Startup? */ + full_bw_cnt:2, /* number of rounds without large bw gains */ cycle_idx:3, /* current index in pacing_gain cycle array */ has_seen_rtt:1, /* have we seen an RTT sample yet? */ unused_b:5; @@ -180,7 +181,7 @@ static bool bbr_full_bw_reached(const struct sock *sk) { const struct bbr *bbr = inet_csk_ca(sk); - return bbr->full_bw_cnt >= bbr_full_bw_cnt; + return bbr->full_bw_reached; } /* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */ @@ -717,6 +718,7 @@ static void bbr_check_full_bw_reached(struct sock *sk, return; } ++bbr->full_bw_cnt; + bbr->full_bw_reached = bbr->full_bw_cnt >= bbr_full_bw_cnt; } /* If pipe is probably full, drain the queue and then enter steady-state. */ @@ -850,6 +852,7 @@ static void bbr_init(struct sock *sk) bbr->restore_cwnd = 0; bbr->round_start = 0; bbr->idle_restart = 0; + bbr->full_bw_reached = 0; bbr->full_bw = 0; bbr->full_bw_cnt = 0; bbr->cycle_mstamp = 0; From 2f6c498e4f15d27852c04ed46d804a39137ba364 Mon Sep 17 00:00:00 2001 From: Neal Cardwell Date: Thu, 7 Dec 2017 12:43:31 -0500 Subject: [PATCH 2/3] tcp_bbr: reset full pipe detection on loss recovery undo Fix BBR so that upon notification of a loss recovery undo BBR resets the full pipe detection (STARTUP exit) state machine. Under high reordering, reordering events can be interpreted as loss. If the reordering and spurious loss estimates are high enough, this could previously cause BBR to spuriously estimate that the pipe is full. Since spurious loss recovery means that our overall sending will have slowed down spuriously, this commit gives a flow more time to probe robustly for bandwidth and decide the pipe is really full. Signed-off-by: Neal Cardwell Reviewed-by: Yuchung Cheng Acked-by: Soheil Hassas Yeganeh Signed-off-by: David S. Miller --- net/ipv4/tcp_bbr.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c index 3089c956b9f9..ab3ff14ea7f7 100644 --- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -874,6 +874,10 @@ static u32 bbr_sndbuf_expand(struct sock *sk) */ static u32 bbr_undo_cwnd(struct sock *sk) { + struct bbr *bbr = inet_csk_ca(sk); + + bbr->full_bw = 0; /* spurious slow-down; reset full pipe detection */ + bbr->full_bw_cnt = 0; return tcp_sk(sk)->snd_cwnd; } From 600647d467c6d04b3954b41a6ee1795b5ae00550 Mon Sep 17 00:00:00 2001 From: Neal Cardwell Date: Thu, 7 Dec 2017 12:43:32 -0500 Subject: [PATCH 3/3] tcp_bbr: reset long-term bandwidth sampling on loss recovery undo Fix BBR so that upon notification of a loss recovery undo BBR resets long-term bandwidth sampling. Under high reordering, reordering events can be interpreted as loss. If the reordering and spurious loss estimates are high enough, this can cause BBR to spuriously estimate that we are seeing loss rates high enough to trigger long-term bandwidth estimation. To avoid that problem, this commit resets long-term bandwidth sampling on loss recovery undo events. Signed-off-by: Neal Cardwell Reviewed-by: Yuchung Cheng Acked-by: Soheil Hassas Yeganeh Signed-off-by: David S. Miller --- net/ipv4/tcp_bbr.c | 1 + 1 file changed, 1 insertion(+) diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c index ab3ff14ea7f7..8322f26e770e 100644 --- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -878,6 +878,7 @@ static u32 bbr_undo_cwnd(struct sock *sk) bbr->full_bw = 0; /* spurious slow-down; reset full pipe detection */ bbr->full_bw_cnt = 0; + bbr_reset_lt_bw_sampling(sk); return tcp_sk(sk)->snd_cwnd; }