| From | Sasha Levin <> | Subject | [PATCH 5.7 038/265] bpf: tcp: bpf_cubic: fix spurious HYSTART_DELAY exit upon drop in min RTT | Date | Mon, 29 Jun 2020 11:14:31 -0400 |
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From: Neal Cardwell <ncardwell@google.com>
[ Upstream commit 7d21d54d624777358ab6c7be7ff778808fef70ba ]
Apply the fix from: "tcp_cubic: fix spurious HYSTART_DELAY exit upon drop in min RTT" to the BPF implementation of TCP CUBIC congestion control.
Repeating the commit description here for completeness:
Mirja Kuehlewind reported a bug in Linux TCP CUBIC Hystart, where Hystart HYSTART_DELAY mechanism can exit Slow Start spuriously on an ACK when the minimum rtt of a connection goes down. From inspection it is clear from the existing code that this could happen in an example like the following:
o The first 8 RTT samples in a round trip are 150ms, resulting in a curr_rtt of 150ms and a delay_min of 150ms.
o The 9th RTT sample is 100ms. The curr_rtt does not change after the first 8 samples, so curr_rtt remains 150ms. But delay_min can be lowered at any time, so delay_min falls to 100ms. The code executes the HYSTART_DELAY comparison between curr_rtt of 150ms and delay_min of 100ms, and the curr_rtt is declared far enough above delay_min to force a (spurious) exit of Slow start.
The fix here is simple: allow every RTT sample in a round trip to lower the curr_rtt.
Fixes: 6de4a9c430b5 ("bpf: tcp: Add bpf_cubic example") Reported-by: Mirja Kuehlewind <mirja.kuehlewind@ericsson.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: David S. Miller <davem@davemloft.net> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> --- tools/testing/selftests/bpf/progs/bpf_cubic.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/tools/testing/selftests/bpf/progs/bpf_cubic.c b/tools/testing/selftests/bpf/progs/bpf_cubic.c index 7897c8f4d363b..ef574087f1e1f 100644 --- a/tools/testing/selftests/bpf/progs/bpf_cubic.c +++ b/tools/testing/selftests/bpf/progs/bpf_cubic.c @@ -480,10 +480,9 @@ static __always_inline void hystart_update(struct sock *sk, __u32 delay) if (hystart_detect & HYSTART_DELAY) { /* obtain the minimum delay of more than sampling packets */ + if (ca->curr_rtt > delay) + ca->curr_rtt = delay; if (ca->sample_cnt < HYSTART_MIN_SAMPLES) { - if (ca->curr_rtt > delay) - ca->curr_rtt = delay; - ca->sample_cnt++; } else { if (ca->curr_rtt > ca->delay_min + -- 2.25.1
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