Commit 9949afa4 authored Mar 10, 2015 by Neal Cardwell Committed by David S. Miller Mar 11, 2015

tcp: fix tcp_cong_avoid_ai() credit accumulation bug with decreases in w



The recent change to tcp_cong_avoid_ai() to handle stretch ACKs
introduced a bug where snd_cwnd_cnt could accumulate a very large
value while w was large, and then if w was reduced snd_cwnd could be
incremented by a large delta, leading to a large burst and high packet
loss. This was tickled when CUBIC's bictcp_update() sets "ca->cnt =
100 * cwnd".

This bug crept in while preparing the upstream version of
814d488c.

Testing: This patch has been tested in datacenter netperf transfers
and live youtube.com and google.com servers.

Fixes: 814d488c ("tcp: fix the timid additive increase on stretch ACKs")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 366c1bd1

net/ipv4/tcp_cong.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -378,6 +378,12 @@ EXPORT_SYMBOL_GPL(tcp_slow_start);
		*/
		void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
		{
		/* If credits accumulated at a higher w, apply them gently now. */
		if (tp->snd_cwnd_cnt >= w) {
		tp->snd_cwnd_cnt = 0;
		tp->snd_cwnd++;
		}

		tp->snd_cwnd_cnt += acked;
		if (tp->snd_cwnd_cnt >= w) {
		u32 delta = tp->snd_cwnd_cnt / w;

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