On Tue, 17 Nov 2020 10:05:24 +0000 <erik.tarald...@telenor.com> wrote:
> Thank you for the response Neal
Yes. And it is impressive how many highly qualified people are on the
bufferbloat list.
> old_hw # uname -r
> 5.3.0-64-generic
> (Ubuntu 19.10 on xenon workstation, integrated network card, 1Gbit
> GPON access. Used as proof of concept from the lab at work)
>
>
> new_hw # uname -r
> 4.18.0-193.19.1.el8_2.x86_64
> (Centos 8.2 on xenon rack server, discrete 10Gbit network card,
> 40Gbit server farm link (low utilization on link), intended as fully
> supported and run service. Not possible to have newer kernel and
> still get service agreement in my organization)
Let me help out here. The CentOS/RHEL8 kernels have a huge amount of
backports. I've attached a patch/diff of net/ipv4/tcp_bbr.c changes
missing in RHEL8.
It looks like these patches are missing in CentOS/RHEL8:
[1] https://git.kernel.org/torvalds/c/78dc70ebaa38aa3
[2] https://git.kernel.org/torvalds/c/a87c83d5ee25cf7
Could missing patch [1] result in the issue Erik is seeing?
(It explicitly mentions improvements for WiFi...)
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
--- /home/hawk/git/redhat/kernel-rhel8/net/ipv4/tcp_bbr.c 2020-01-30 17:38:20.832726582 +0100
+++ /home/hawk/git/kernel/net-next/net/ipv4/tcp_bbr.c 2020-11-17 15:38:22.665729797 +0100
@@ -115,6 +115,14 @@ struct bbr {
unused_b:5;
u32 prior_cwnd; /* prior cwnd upon entering loss recovery */
u32 full_bw; /* recent bw, to estimate if pipe is full */
+
+ /* For tracking ACK aggregation: */
+ u64 ack_epoch_mstamp; /* start of ACK sampling epoch */
+ u16 extra_acked[2]; /* max excess data ACKed in epoch */
+ u32 ack_epoch_acked:20, /* packets (S)ACKed in sampling epoch */
+ extra_acked_win_rtts:5, /* age of extra_acked, in round trips */
+ extra_acked_win_idx:1, /* current index in extra_acked array */
+ unused_c:6;
};
#define CYCLE_LEN 8 /* number of phases in a pacing gain cycle */
@@ -128,6 +136,14 @@ static const u32 bbr_probe_rtt_mode_ms =
/* Skip TSO below the following bandwidth (bits/sec): */
static const int bbr_min_tso_rate = 1200000;
+/* Pace at ~1% below estimated bw, on average, to reduce queue at bottleneck.
+ * In order to help drive the network toward lower queues and low latency while
+ * maintaining high utilization, the average pacing rate aims to be slightly
+ * lower than the estimated bandwidth. This is an important aspect of the
+ * design.
+ */
+static const int bbr_pacing_margin_percent = 1;
+
/* We use a high_gain value of 2/ln(2) because it's the smallest pacing gain
* that will allow a smoothly increasing pacing rate that will double each RTT
* and send the same number of packets per RTT that an un-paced, slow-starting
@@ -174,6 +190,15 @@ static const u32 bbr_lt_bw_diff = 4000 /
/* If we estimate we're policed, use lt_bw for this many round trips: */
static const u32 bbr_lt_bw_max_rtts = 48;
+/* Gain factor for adding extra_acked to target cwnd: */
+static const int bbr_extra_acked_gain = BBR_UNIT;
+/* Window length of extra_acked window. */
+static const u32 bbr_extra_acked_win_rtts = 5;
+/* Max allowed val for ack_epoch_acked, after which sampling epoch is reset */
+static const u32 bbr_ack_epoch_acked_reset_thresh = 1U << 20;
+/* Time period for clamping cwnd increment due to ack aggregation */
+static const u32 bbr_extra_acked_max_us = 100 * 1000;
+
static void bbr_check_probe_rtt_done(struct sock *sk);
/* Do we estimate that STARTUP filled the pipe? */
@@ -200,21 +225,33 @@ static u32 bbr_bw(const struct sock *sk)
return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk);
}
+/* Return maximum extra acked in past k-2k round trips,
+ * where k = bbr_extra_acked_win_rtts.
+ */
+static u16 bbr_extra_acked(const struct sock *sk)
+{
+ struct bbr *bbr = inet_csk_ca(sk);
+
+ return max(bbr->extra_acked[0], bbr->extra_acked[1]);
+}
+
/* Return rate in bytes per second, optionally with a gain.
* The order here is chosen carefully to avoid overflow of u64. This should
* work for input rates of up to 2.9Tbit/sec and gain of 2.89x.
*/
static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain)
{
- rate *= tcp_mss_to_mtu(sk, tcp_sk(sk)->mss_cache);
+ unsigned int mss = tcp_sk(sk)->mss_cache;
+
+ rate *= mss;
rate *= gain;
rate >>= BBR_SCALE;
- rate *= USEC_PER_SEC;
+ rate *= USEC_PER_SEC / 100 * (100 - bbr_pacing_margin_percent);
return rate >> BW_SCALE;
}
/* Convert a BBR bw and gain factor to a pacing rate in bytes per second. */
-static u32 bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain)
+static unsigned long bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain)
{
u64 rate = bw;
@@ -242,18 +279,12 @@ static void bbr_init_pacing_rate_from_rt
sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain);
}
-/* Pace using current bw estimate and a gain factor. In order to help drive the
- * network toward lower queues while maintaining high utilization and low
- * latency, the average pacing rate aims to be slightly (~1%) lower than the
- * estimated bandwidth. This is an important aspect of the design. In this
- * implementation this slightly lower pacing rate is achieved implicitly by not
- * including link-layer headers in the packet size used for the pacing rate.
- */
+/* Pace using current bw estimate and a gain factor. */
static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
{
struct tcp_sock *tp = tcp_sk(sk);
struct bbr *bbr = inet_csk_ca(sk);
- u32 rate = bbr_bw_to_pacing_rate(sk, bw, gain);
+ unsigned long rate = bbr_bw_to_pacing_rate(sk, bw, gain);
if (unlikely(!bbr->has_seen_rtt && tp->srtt_us))
bbr_init_pacing_rate_from_rtt(sk);
@@ -275,7 +306,7 @@ static u32 bbr_tso_segs_goal(struct sock
/* Sort of tcp_tso_autosize() but ignoring
* driver provided sk_gso_max_size.
*/
- bytes = min_t(u32, sk->sk_pacing_rate >> sk->sk_pacing_shift,
+ bytes = min_t(unsigned long, sk->sk_pacing_rate >> sk->sk_pacing_shift,
GSO_MAX_SIZE - 1 - MAX_TCP_HEADER);
segs = max_t(u32, bytes / tp->mss_cache, bbr_min_tso_segs(sk));
@@ -301,6 +332,8 @@ static void bbr_cwnd_event(struct sock *
if (event == CA_EVENT_TX_START && tp->app_limited) {
bbr->idle_restart = 1;
+ bbr->ack_epoch_mstamp = tp->tcp_mstamp;
+ bbr->ack_epoch_acked = 0;
/* Avoid pointless buffer overflows: pace at est. bw if we don't
* need more speed (we're restarting from idle and app-limited).
*/
@@ -353,7 +386,7 @@ static u32 bbr_bdp(struct sock *sk, u32
* which allows 2 outstanding 2-packet sequences, to try to keep pipe
* full even with ACK-every-other-packet delayed ACKs.
*/
-static u32 bbr_quantization_budget(struct sock *sk, u32 cwnd)
+static u32 bbr_quantization_budget(struct sock *sk, u32 cwnd, int gain)
{
struct bbr *bbr = inet_csk_ca(sk);
@@ -364,12 +397,72 @@ static u32 bbr_quantization_budget(struc
cwnd = (cwnd + 1) & ~1U;
/* Ensure gain cycling gets inflight above BDP even for small BDPs. */
- if (bbr->mode == BBR_PROBE_BW && bbr->cycle_idx == 0)
+ if (bbr->mode == BBR_PROBE_BW && gain > BBR_UNIT)
cwnd += 2;
return cwnd;
}
+/* Find inflight based on min RTT and the estimated bottleneck bandwidth. */
+static u32 bbr_inflight(struct sock *sk, u32 bw, int gain)
+{
+ u32 inflight;
+
+ inflight = bbr_bdp(sk, bw, gain);
+ inflight = bbr_quantization_budget(sk, inflight, gain);
+
+ return inflight;
+}
+
+/* With pacing at lower layers, there's often less data "in the network" than
+ * "in flight". With TSQ and departure time pacing at lower layers (e.g. fq),
+ * we often have several skbs queued in the pacing layer with a pre-scheduled
+ * earliest departure time (EDT). BBR adapts its pacing rate based on the
+ * inflight level that it estimates has already been "baked in" by previous
+ * departure time decisions. We calculate a rough estimate of the number of our
+ * packets that might be in the network at the earliest departure time for the
+ * next skb scheduled:
+ * in_network_at_edt = inflight_at_edt - (EDT - now) * bw
+ * If we're increasing inflight, then we want to know if the transmit of the
+ * EDT skb will push inflight above the target, so inflight_at_edt includes
+ * bbr_tso_segs_goal() from the skb departing at EDT. If decreasing inflight,
+ * then estimate if inflight will sink too low just before the EDT transmit.
+ */
+static u32 bbr_packets_in_net_at_edt(struct sock *sk, u32 inflight_now)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ struct bbr *bbr = inet_csk_ca(sk);
+ u64 now_ns, edt_ns, interval_us;
+ u32 interval_delivered, inflight_at_edt;
+
+ now_ns = tp->tcp_clock_cache;
+ edt_ns = max(tp->tcp_wstamp_ns, now_ns);
+ interval_us = div_u64(edt_ns - now_ns, NSEC_PER_USEC);
+ interval_delivered = (u64)bbr_bw(sk) * interval_us >> BW_SCALE;
+ inflight_at_edt = inflight_now;
+ if (bbr->pacing_gain > BBR_UNIT) /* increasing inflight */
+ inflight_at_edt += bbr_tso_segs_goal(sk); /* include EDT skb */
+ if (interval_delivered >= inflight_at_edt)
+ return 0;
+ return inflight_at_edt - interval_delivered;
+}
+
+/* Find the cwnd increment based on estimate of ack aggregation */
+static u32 bbr_ack_aggregation_cwnd(struct sock *sk)
+{
+ u32 max_aggr_cwnd, aggr_cwnd = 0;
+
+ if (bbr_extra_acked_gain && bbr_full_bw_reached(sk)) {
+ max_aggr_cwnd = ((u64)bbr_bw(sk) * bbr_extra_acked_max_us)
+ / BW_UNIT;
+ aggr_cwnd = (bbr_extra_acked_gain * bbr_extra_acked(sk))
+ >> BBR_SCALE;
+ aggr_cwnd = min(aggr_cwnd, max_aggr_cwnd);
+ }
+
+ return aggr_cwnd;
+}
+
/* An optimization in BBR to reduce losses: On the first round of recovery, we
* follow the packet conservation principle: send P packets per P packets acked.
* After that, we slow-start and send at most 2*P packets per P packets acked.
@@ -414,17 +507,6 @@ static bool bbr_set_cwnd_to_recover_or_r
return false;
}
-/* Find inflight based on min RTT and the estimated bottleneck bandwidth. */
-static u32 bbr_inflight(struct sock *sk, u32 bw, int gain)
-{
- u32 inflight;
-
- inflight = bbr_bdp(sk, bw, gain);
- inflight = bbr_quantization_budget(sk, inflight);
-
- return inflight;
-}
-
/* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss
* has drawn us down below target), or snap down to target if we're above it.
*/
@@ -441,9 +523,15 @@ static void bbr_set_cwnd(struct sock *sk
if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
goto done;
- /* If we're below target cwnd, slow start cwnd toward target cwnd. */
target_cwnd = bbr_bdp(sk, bw, gain);
- target_cwnd = bbr_quantization_budget(sk, target_cwnd);
+
+ /* Increment the cwnd to account for excess ACKed data that seems
+ * due to aggregation (of data and/or ACKs) visible in the ACK stream.
+ */
+ target_cwnd += bbr_ack_aggregation_cwnd(sk);
+ target_cwnd = bbr_quantization_budget(sk, target_cwnd, gain);
+
+ /* If we're below target cwnd, slow start cwnd toward target cwnd. */
if (bbr_full_bw_reached(sk)) /* only cut cwnd if we filled the pipe */
cwnd = min(cwnd + acked, target_cwnd);
else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND)
@@ -473,7 +561,7 @@ static bool bbr_is_next_cycle_phase(stru
if (bbr->pacing_gain == BBR_UNIT)
return is_full_length; /* just use wall clock time */
- inflight = rs->prior_in_flight; /* what was in-flight before ACK? */
+ inflight = bbr_packets_in_net_at_edt(sk, rs->prior_in_flight);
bw = bbr_max_bw(sk);
/* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at
@@ -708,6 +796,67 @@ static void bbr_update_bw(struct sock *s
}
}
+/* Estimates the windowed max degree of ack aggregation.
+ * This is used to provision extra in-flight data to keep sending during
+ * inter-ACK silences.
+ *
+ * Degree of ack aggregation is estimated as extra data acked beyond expected.
+ *
+ * max_extra_acked = "maximum recent excess data ACKed beyond max_bw * interval"
+ * cwnd += max_extra_acked
+ *
+ * Max extra_acked is clamped by cwnd and bw * bbr_extra_acked_max_us (100 ms).
+ * Max filter is an approximate sliding window of 5-10 (packet timed) round
+ * trips.
+ */
+static void bbr_update_ack_aggregation(struct sock *sk,
+ const struct rate_sample *rs)
+{
+ u32 epoch_us, expected_acked, extra_acked;
+ struct bbr *bbr = inet_csk_ca(sk);
+ struct tcp_sock *tp = tcp_sk(sk);
+
+ if (!bbr_extra_acked_gain || rs->acked_sacked <= 0 ||
+ rs->delivered < 0 || rs->interval_us <= 0)
+ return;
+
+ if (bbr->round_start) {
+ bbr->extra_acked_win_rtts = min(0x1F,
+ bbr->extra_acked_win_rtts + 1);
+ if (bbr->extra_acked_win_rtts >= bbr_extra_acked_win_rtts) {
+ bbr->extra_acked_win_rtts = 0;
+ bbr->extra_acked_win_idx = bbr->extra_acked_win_idx ?
+ 0 : 1;
+ bbr->extra_acked[bbr->extra_acked_win_idx] = 0;
+ }
+ }
+
+ /* Compute how many packets we expected to be delivered over epoch. */
+ epoch_us = tcp_stamp_us_delta(tp->delivered_mstamp,
+ bbr->ack_epoch_mstamp);
+ expected_acked = ((u64)bbr_bw(sk) * epoch_us) / BW_UNIT;
+
+ /* Reset the aggregation epoch if ACK rate is below expected rate or
+ * significantly large no. of ack received since epoch (potentially
+ * quite old epoch).
+ */
+ if (bbr->ack_epoch_acked <= expected_acked ||
+ (bbr->ack_epoch_acked + rs->acked_sacked >=
+ bbr_ack_epoch_acked_reset_thresh)) {
+ bbr->ack_epoch_acked = 0;
+ bbr->ack_epoch_mstamp = tp->delivered_mstamp;
+ expected_acked = 0;
+ }
+
+ /* Compute excess data delivered, beyond what was expected. */
+ bbr->ack_epoch_acked = min_t(u32, 0xFFFFF,
+ bbr->ack_epoch_acked + rs->acked_sacked);
+ extra_acked = bbr->ack_epoch_acked - expected_acked;
+ extra_acked = min(extra_acked, tp->snd_cwnd);
+ if (extra_acked > bbr->extra_acked[bbr->extra_acked_win_idx])
+ bbr->extra_acked[bbr->extra_acked_win_idx] = extra_acked;
+}
+
/* Estimate when the pipe is full, using the change in delivery rate: BBR
* estimates that STARTUP filled the pipe if the estimated bw hasn't changed by
* at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited
@@ -746,7 +895,7 @@ static void bbr_check_drain(struct sock
bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT);
} /* fall through to check if in-flight is already small: */
if (bbr->mode == BBR_DRAIN &&
- tcp_packets_in_flight(tcp_sk(sk)) <=
+ bbr_packets_in_net_at_edt(sk, tcp_packets_in_flight(tcp_sk(sk))) <=
bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT))
bbr_reset_probe_bw_mode(sk); /* we estimate queue is drained */
}
@@ -862,6 +1011,7 @@ static void bbr_update_gains(struct sock
static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
{
bbr_update_bw(sk, rs);
+ bbr_update_ack_aggregation(sk, rs);
bbr_update_cycle_phase(sk, rs);
bbr_check_full_bw_reached(sk, rs);
bbr_check_drain(sk, rs);
@@ -913,6 +1063,13 @@ static void bbr_init(struct sock *sk)
bbr_reset_lt_bw_sampling(sk);
bbr_reset_startup_mode(sk);
+ bbr->ack_epoch_mstamp = tp->tcp_mstamp;
+ bbr->ack_epoch_acked = 0;
+ bbr->extra_acked_win_rtts = 0;
+ bbr->extra_acked_win_idx = 0;
+ bbr->extra_acked[0] = 0;
+ bbr->extra_acked[1] = 0;
+
cmpxchg(&sk->sk_pacing_status, SK_PACING_NONE, SK_PACING_NEEDED);
}
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