| Message ID | 20201116174412.1433277-1-sharpelletti.kdev@gmail.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [net] tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate | expand |
LGTM. Thanks! On Mon, Nov 16, 2020 at 12:44 PM Ryan Sharpelletti <sharpelletti.kdev@gmail.com> wrote: > > From: Ryan Sharpelletti <sharpelletti@google.com> > > 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> > --- > net/ipv4/tcp_bbr.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c > index 6c4d79baff26..6ea3dc2e4219 100644 > --- a/net/ipv4/tcp_bbr.c > +++ b/net/ipv4/tcp_bbr.c > @@ -945,7 +945,7 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs) > 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; > -- > 2.29.2.299.gdc1121823c-goog >
Hello: This patch was applied to netdev/net.git (refs/heads/master): On Mon, 16 Nov 2020 17:44:13 +0000 you wrote: > From: Ryan Sharpelletti <sharpelletti@google.com> > > 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 <=. > > [...] Here is the summary with links: - [net] tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate https://git.kernel.org/netdev/net/c/1b9e2a8c99a5 You are awesome, thank you! -- Deet-doot-dot, I am a bot. https://korg.docs.kernel.org/patchwork/pwbot.html
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c index 6c4d79baff26..6ea3dc2e4219 100644 --- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -945,7 +945,7 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs) 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;