From: Neal Cardwell Date: Tue, 10 Mar 2015 21:17:03 +0000 (-0400) Subject: tcp: fix tcp_cong_avoid_ai() credit accumulation bug with decreases in w X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=9949afa42be0b76f5832db112ce51bb6b35b2abb;p=linux-beck.git 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 814d488c6126. Testing: This patch has been tested in datacenter netperf transfers and live youtube.com and google.com servers. Fixes: 814d488c6126 ("tcp: fix the timid additive increase on stretch ACKs") Signed-off-by: Neal Cardwell Signed-off-by: Yuchung Cheng Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c index d694088214cd..62856e185a93 100644 --- a/net/ipv4/tcp_cong.c +++ b/net/ipv4/tcp_cong.c @@ -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;