Commit 1f255691 authored by Priyaranjan Jha's avatar Priyaranjan Jha Committed by David S. Miller
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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: default avatarPriyaranjan Jha <priyarjha@google.com>
Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 6c49b5e2

Documentation/networking/ip-sysctl.txt

+1 −0
Original line number Diff line number Diff line
@@ -454,6 +454,7 @@ tcp_recovery - INTEGER 


	RACK: 0x1 enables the RACK loss detection for fast detection of lost

	      retransmissions and tail drops.

	RACK: 0x2 makes RACK's reordering window static (min_rtt/4).



	Default: 0x1

include/linux/tcp.h

+7 −2
Original line number Diff line number Diff line
@@ -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 */

include/net/tcp.h

+2 −0
Original line number Diff line number Diff line
@@ -246,6 +246,7 @@ extern int sysctl_tcp_wmem[3]; 
extern int sysctl_tcp_rmem[3];



#define TCP_RACK_LOSS_DETECTION  0x1 /* Use RACK to detect losses */

#define TCP_RACK_STATIC_REO_WND  0x2 /* Use static RACK reo wnd */



extern atomic_long_t tcp_memory_allocated;

extern struct percpu_counter tcp_sockets_allocated;
@@ -1901,6 +1902,7 @@ extern void tcp_rack_mark_lost(struct sock *sk); 
extern void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,

			     u64 xmit_time);

extern void tcp_rack_reo_timeout(struct sock *sk);

extern void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs);



/* At how many usecs into the future should the RTO fire? */

static inline s64 tcp_rto_delta_us(const struct sock *sk)

net/ipv4/tcp.c

+1 −0
Original line number Diff line number Diff line
@@ -447,6 +447,7 @@ void tcp_init_sock(struct sock *sk) 
	tcp_assign_congestion_control(sk);



	tp->tsoffset = 0;

	tp->rack.reo_wnd_steps = 1;



	sk->sk_state = TCP_CLOSE;

net/ipv4/tcp_input.c

+7 −0
Original line number Diff line number Diff line
@@ -856,6 +856,7 @@ void tcp_disable_fack(struct tcp_sock *tp) 
static void tcp_dsack_seen(struct tcp_sock *tp)

{

	tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;

	tp->rack.dsack_seen = 1;

}



static void tcp_update_reordering(struct sock *sk, const int metric,
@@ -2408,6 +2409,8 @@ static bool tcp_try_undo_recovery(struct sock *sk) 
			mib_idx = LINUX_MIB_TCPFULLUNDO;



		NET_INC_STATS(sock_net(sk), mib_idx);

	} else if (tp->rack.reo_wnd_persist) {

		tp->rack.reo_wnd_persist--;

	}

	if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {

		/* Hold old state until something *above* high_seq
@@ -2427,6 +2430,8 @@ static bool tcp_try_undo_dsack(struct sock *sk) 
	struct tcp_sock *tp = tcp_sk(sk);



	if (tp->undo_marker && !tp->undo_retrans) {

		tp->rack.reo_wnd_persist = min(TCP_RACK_RECOVERY_THRESH,

					       tp->rack.reo_wnd_persist + 1);

		DBGUNDO(sk, "D-SACK");

		tcp_undo_cwnd_reduction(sk, false);

		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
@@ -3644,6 +3649,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) 
	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una, &acked,

				    &sack_state);



	tcp_rack_update_reo_wnd(sk, &rs);



	if (tp->tlp_high_seq)

		tcp_process_tlp_ack(sk, ack, flag);

	/* If needed, reset TLP/RTO timer; RACK may later override this. */

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