Re: [PATCH v2 net-next] tcp: fix ABC in tcp_slow_start()
From: Yuchung Cheng <hidden>
Date: 2012-07-20 15:08:00
On Fri, Jul 20, 2012 at 8:02 AM, Eric Dumazet [off-list ref] wrote:
From: Eric Dumazet <edumazet@google.com> When/if sysctl_tcp_abc > 1, we expect to increase cwnd by 2 if the received ACK acknowledges more than 2*MSS bytes, in tcp_slow_start() Problem is this RFC 3465 statement is not correctly coded, as the while () loop increases snd_cwnd one by one. Add a new variable to avoid this off-by one error. Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <redacted>
quoted hunk ↗ jump to hunk
Cc: Tom Herbert <redacted> Cc: Yuchung Cheng <redacted> Cc: Neal Cardwell <ncardwell@google.com> Cc: Nandita Dukkipati <redacted> Cc: John Heffner <redacted> Cc: Stephen Hemminger <redacted> --- v2: added John suggestion net/ipv4/tcp_cong.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c index 04dbd7a..4d4db16 100644 --- a/net/ipv4/tcp_cong.c +++ b/net/ipv4/tcp_cong.c@@ -307,6 +307,7 @@ EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited); void tcp_slow_start(struct tcp_sock *tp) { int cnt; /* increase in packets */ + unsigned int delta = 0; /* RFC3465: ABC Slow start * Increase only after a full MSS of bytes is acked@@ -333,9 +334,9 @@ void tcp_slow_start(struct tcp_sock *tp) tp->snd_cwnd_cnt += cnt; while (tp->snd_cwnd_cnt >= tp->snd_cwnd) { tp->snd_cwnd_cnt -= tp->snd_cwnd; - if (tp->snd_cwnd < tp->snd_cwnd_clamp) - tp->snd_cwnd++; + delta++;
Nice! this also removes wasteful iteration when clamp << cwnd_cnt.
} + tp->snd_cwnd = min(tp->snd_cwnd + delta, tp->snd_cwnd_clamp); } EXPORT_SYMBOL_GPL(tcp_slow_start);