| From foo@baz Sun Nov 22 12:00:04 PM CET 2020 |
| From: Ryan Sharpelletti <sharpelletti@google.com> |
| Date: Mon, 16 Nov 2020 17:44:13 +0000 |
| Subject: tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate |
| |
| From: Ryan Sharpelletti <sharpelletti@google.com> |
| |
| [ Upstream commit 1b9e2a8c99a5c021041bfb2d512dc3ed92a94ffd ] |
| |
| During loss recovery, retransmitted packets are forced to use TCP |
| timestamps to calculate the RTT samples, which have a millisecond |
| granularity. BBR is designed using a microsecond granularity. As a |
| result, multiple RTT samples could be truncated to the same RTT value |
| during loss recovery. This is problematic, as BBR will not enter |
| PROBE_RTT if the RTT sample is <= the current min_rtt sample, meaning |
| that if there are persistent losses, PROBE_RTT will constantly be |
| pushed off and potentially never re-entered. This patch makes sure |
| that BBR enters PROBE_RTT by checking if RTT sample is < the current |
| min_rtt sample, rather than <=. |
| |
| The Netflix transport/TCP team discovered this bug in the Linux TCP |
| BBR code during lab tests. |
| |
| Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control") |
| Signed-off-by: Ryan Sharpelletti <sharpelletti@google.com> |
| Signed-off-by: Neal Cardwell <ncardwell@google.com> |
| Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com> |
| Signed-off-by: Yuchung Cheng <ycheng@google.com> |
| Link: https://lore.kernel.org/r/20201116174412.1433277-1-sharpelletti.kdev@gmail.com |
| Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| net/ipv4/tcp_bbr.c | 2 +- |
| 1 file changed, 1 insertion(+), 1 deletion(-) |
| |
| --- a/net/ipv4/tcp_bbr.c |
| +++ b/net/ipv4/tcp_bbr.c |
| @@ -917,7 +917,7 @@ static void bbr_update_min_rtt(struct so |
| filter_expired = after(tcp_jiffies32, |
| bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ); |
| if (rs->rtt_us >= 0 && |
| - (rs->rtt_us <= bbr->min_rtt_us || |
| + (rs->rtt_us < bbr->min_rtt_us || |
| (filter_expired && !rs->is_ack_delayed))) { |
| bbr->min_rtt_us = rs->rtt_us; |
| bbr->min_rtt_stamp = tcp_jiffies32; |
