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authorJianfeng Wang <jfwang@google.com>2020-07-30 23:49:16 +0000
committerDavid S. Miller <davem@davemloft.net>2020-08-03 17:54:03 -0700
commit730e700e2c19d87e578ff0e7d8cb1d4a02b036d2 (patch)
treeadd051b1e5c0ad42ff73c0a90d78721b9ab5a9dc /net
parentfbc97de84ef88bc38874c41be151cf220c72a865 (diff)
tcp: apply a floor of 1 for RTT samples from TCP timestamps
For retransmitted packets, TCP needs to resort to using TCP timestamps for computing RTT samples. In the common case where the data and ACK fall in the same 1-millisecond interval, TCP senders with millisecond- granularity TCP timestamps compute a ca_rtt_us of 0. This ca_rtt_us of 0 propagates to rs->rtt_us. This value of 0 can cause performance problems for congestion control modules. For example, in BBR, the zero min_rtt sample can bring the min_rtt and BDP estimate down to 0, reduce snd_cwnd and result in a low throughput. It would be hard to mitigate this with filtering in the congestion control module, because the proper floor to apply would depend on the method of RTT sampling (using timestamp options or internally-saved transmission timestamps). This fix applies a floor of 1 for the RTT sample delta from TCP timestamps, so that seq_rtt_us, ca_rtt_us, and rs->rtt_us will be at least 1 * (USEC_PER_SEC / TCP_TS_HZ). Note that the receiver RTT computation in tcp_rcv_rtt_measure() and min_rtt computation in tcp_update_rtt_min() both already apply a floor of 1 timestamp tick, so this commit makes the code more consistent in avoiding this edge case of a value of 0. Signed-off-by: Jianfeng Wang <jfwang@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Kevin Yang <yyd@google.com> Acked-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_input.c2
1 files changed, 2 insertions, 0 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 739da25b0c23..184ea556f50e 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2950,6 +2950,8 @@ static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
+ if (!delta)
+ delta = 1;
seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
ca_rtt_us = seq_rtt_us;
}