tcp: track data delivery rate for a TCP connection

This patch generates data delivery rate (throughput) samples on a
per-ACK basis. These rate samples can be used by congestion control
modules, and specifically will be used by TCP BBR in later patches in
this series.

Key state:

tp->delivered: Tracks the total number of data packets (original or not)
	       delivered so far. This is an already-existing field.

tp->delivered_mstamp: the last time tp->delivered was updated.

Algorithm:

A rate sample is calculated as (d1 - d0)/(t1 - t0) on a per-ACK basis:

  d1: the current tp->delivered after processing the ACK
  t1: the current time after processing the ACK

  d0: the prior tp->delivered when the acked skb was transmitted
  t0: the prior tp->delivered_mstamp when the acked skb was transmitted

When an skb is transmitted, we snapshot d0 and t0 in its control
block in tcp_rate_skb_sent().

When an ACK arrives, it may SACK and ACK some skbs. For each SACKed
or ACKed skb, tcp_rate_skb_delivered() updates the rate_sample struct
to reflect the latest (d0, t0).

Finally, tcp_rate_gen() generates a rate sample by storing
(d1 - d0) in rs->delivered and (t1 - t0) in rs->interval_us.

One caveat: if an skb was sent with no packets in flight, then
tp->delivered_mstamp may be either invalid (if the connection is
starting) or outdated (if the connection was idle). In that case,
we'll re-stamp tp->delivered_mstamp.

At first glance it seems t0 should always be the time when an skb was
transmitted, but actually this could over-estimate the rate due to
phase mismatch between transmit and ACK events. To track the delivery
rate, we ensure that if packets are in flight then t0 and and t1 are
times at which packets were marked delivered.

If the initial and final RTTs are different then one may be corrupted
by some sort of noise. The noise we see most often is sending gaps
caused by delayed, compressed, or stretched acks. This either affects
both RTTs equally or artificially reduces the final RTT. We approach
this by recording the info we need to compute the initial RTT
(duration of the "send phase" of the window) when we recorded the
associated inflight. Then, for a filter to avoid bandwidth
overestimates, we generalize the per-sample bandwidth computation
from:

    bw = delivered / ack_phase_rtt

to the following:

    bw = delivered / max(send_phase_rtt, ack_phase_rtt)

In large-scale experiments, this filtering approach incorporating
send_phase_rtt is effective at avoiding bandwidth overestimates due to
ACK compression or stretched ACKs.

Signed-off-by: Van Jacobson <vanj@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Nandita Dukkipati <nanditad@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 36 insertions, 1 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 38590fbc0ac5..c50e6aec005a 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -268,6 +268,8 @@ struct tcp_sock {
         u32     prr_out;        /* Total number of pkts sent during Recovery. */
         u32     delivered;      /* Total data packets delivered incl. rexmits */
         u32     lost;           /* Total data packets lost incl. rexmits */
+        struct skb_mstamp first_tx_mstamp;  /* start of window send phase */
+        struct skb_mstamp delivered_mstamp; /* time we reached "delivered" */
 
         u32     rcv_wnd;        /* Current receiver window              */
         u32     write_seq;      /* Tail(+1) of data held in tcp send buffer */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2f1648af4d12..b261c892605a 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -763,8 +763,14 @@ struct tcp_skb_cb {
         __u32           ack_seq;        /* Sequence number ACK'd        */
         union {
                 struct {
-                        /* There is space for up to 20 bytes */
+                        /* There is space for up to 24 bytes */
                         __u32 in_flight;/* Bytes in flight when packet sent */
+                        /* pkts S/ACKed so far upon tx of skb, incl retrans: */
+                        __u32 delivered;
+                        /* start of send pipeline phase */
+                        struct skb_mstamp first_tx_mstamp;
+                        /* when we reached the "delivered" count */
+                        struct skb_mstamp delivered_mstamp;
                 } tx;   /* only used for outgoing skbs */
                 union {
                         struct inet_skb_parm    h4;
@@ -860,6 +866,26 @@ struct ack_sample {
         u32 in_flight;
 };
 
+/* A rate sample measures the number of (original/retransmitted) data
+ * packets delivered "delivered" over an interval of time "interval_us".
+ * The tcp_rate.c code fills in the rate sample, and congestion
+ * control modules that define a cong_control function to run at the end
+ * of ACK processing can optionally chose to consult this sample when
+ * setting cwnd and pacing rate.
+ * A sample is invalid if "delivered" or "interval_us" is negative.
+ */
+struct rate_sample {
+        struct  skb_mstamp prior_mstamp; /* starting timestamp for interval */
+        u32  prior_delivered;   /* tp->delivered at "prior_mstamp" */
+        s32  delivered;         /* number of packets delivered over interval */
+        long interval_us;       /* time for tp->delivered to incr "delivered" */
+        long rtt_us;            /* RTT of last (S)ACKed packet (or -1) */
+        int  losses;            /* number of packets marked lost upon ACK */
+        u32  acked_sacked;      /* number of packets newly (S)ACKed upon ACK */
+        u32  prior_in_flight;   /* in flight before this ACK */
+        bool is_retrans;        /* is sample from retransmission? */
+};
+
 struct tcp_congestion_ops {
         struct list_head        list;
         u32 key;
@@ -946,6 +972,13 @@ static inline void tcp_ca_event(struct sock *sk, const enum tcp_ca_event event)
                 icsk->icsk_ca_ops->cwnd_event(sk, event);
 }
 
+/* From tcp_rate.c */
+void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb);
+void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
+                            struct rate_sample *rs);
+void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost,
+                  struct skb_mstamp *now, struct rate_sample *rs);
+
 /* These functions determine how the current flow behaves in respect of SACK
  * handling. SACK is negotiated with the peer, and therefore it can vary
  * between different flows.