tcp: higher throughput under reordering with adaptive RACK reordering wnd

Currently TCP RACK loss detection does not work well if packets are
being reordered beyond its static reordering window (min_rtt/4).Under
such reordering it may falsely trigger loss recoveries and reduce TCP
throughput significantly.

This patch improves that by increasing and reducing the reordering
window based on DSACK, which is now supported in major TCP implementations.
It makes RACK's reo_wnd adaptive based on DSACK and no. of recoveries.

- If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
  by srtt), since there is possibility that spurious retransmission was
  due to reordering delay longer than reo_wnd.

- Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
  no. of successful recoveries (accounts for full DSACK-based loss
  recovery undo). After that, reset it to default (min_rtt/4).

- At max, reo_wnd is incremented only once per rtt. So that the new
  DSACK on which we are reacting, is due to the spurious retx (approx)
  after the reo_wnd has been updated last time.

- reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
  absolute value to account for change in rtt.

In our internal testing, we observed significant increase in throughput,
in scenarios where reordering exceeds min_rtt/4 (previous static value).

Signed-off-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 2 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 8c431385b272..22f40c96a15b 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -210,8 +210,13 @@ struct tcp_sock {
 		u64 mstamp; /* (Re)sent time of the skb */
 		u32 rtt_us;  /* Associated RTT */
 		u32 end_seq; /* Ending TCP sequence of the skb */
-		u8 advanced; /* mstamp advanced since last lost marking */
-		u8 reord;    /* reordering detected */
+		u32 last_delivered; /* tp->delivered at last reo_wnd adj */
+		u8 reo_wnd_steps;   /* Allowed reordering window */
+#define TCP_RACK_RECOVERY_THRESH 16
+		u8 reo_wnd_persist:5, /* No. of recovery since last adj */
+		   dsack_seen:1, /* Whether DSACK seen after last adj */
+		   advanced:1,	 /* mstamp advanced since last lost marking */
+		   reord:1;	 /* reordering detected */
 	} rack;
 	u16	advmss;		/* Advertised MSS			*/
 	u32	chrono_start;	/* Start time in jiffies of a TCP chrono */