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42f3a8aaae
After switching ca->delay_min to usec resolution, we exit slow start prematurely for very low RTT flows, setting snd_ssthresh to 20. The reason is that delay_min is fed with RTT of small packet trains. Then as cwnd is increased, TCP sends bigger TSO packets. LRO/GRO aggregation and/or interrupt mitigation strategies on receiver tend to inflate RTT samples. Fix this by adding to delay_min the expected delay of two TSO packets, given current pacing rate. Tested: Sender uses pfifo_fast qdisc Before : $ nstat -n;for f in {1..10}; do ./super_netperf 1 -H lpaa24 -l -4000000; done;nstat|egrep "Hystart" 11348 11707 11562 11428 11773 11534 9878 11693 10597 10968 TcpExtTCPHystartTrainDetect 10 0.0 TcpExtTCPHystartTrainCwnd 200 0.0 After : $ nstat -n;for f in {1..10}; do ./super_netperf 1 -H lpaa24 -l -4000000; done;nstat|egrep "Hystart" 14877 14517 15797 18466 17376 14833 17558 17933 16039 18059 TcpExtTCPHystartTrainDetect 10 0.0 TcpExtTCPHystartTrainCwnd 1670 0.0 Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>