Commit 645f4c6f authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

tcp: switch rcv_rtt_est and rcvq_space to high resolution timestamps 



Some devices or distributions use HZ=100 or HZ=250

TCP receive buffer autotuning has poor behavior caused by this choice.
Since autotuning happens after 4 ms or 10 ms, short distance flows
get their receive buffer tuned to a very high value, but after an initial
period where it was frozen to (too small) initial value.

With tp->tcp_mstamp introduction, we can switch to high resolution
timestamps almost for free (at the expense of 8 additional bytes per
TCP structure)

Note that some TCP stacks use usec TCP timestamps where this
patch makes even more sense : Many TCP flows have < 500 usec RTT.
Hopefully this finer TS option can be standardized soon.

Tested:
 HZ=100 kernel
 ./netperf -H lpaa24 -t TCP_RR -l 1000 -- -r 10000,10000 &

 Peer without patch :
 lpaa24:~# ss -tmi dst lpaa23
 ...
 skmem:(r0,rb8388608,...)
 rcv_rtt:10 rcv_space:3210000 minrtt:0.017

 Peer with the patch :
 lpaa23:~# ss -tmi dst lpaa24
 ...
 skmem:(r0,rb428800,...)
 rcv_rtt:0.069 rcv_space:30000 minrtt:0.017

We can see saner RCVBUF, and more precise rcv_rtt information.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Acked-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent a6db50b8

include/linux/tcp.h

+6 −6
Original line number Original line Diff line number Diff line
@@ -333,16 +333,16 @@ struct tcp_sock { 




/* Receiver side RTT estimation */

/* Receiver side RTT estimation */

	struct {

	struct {

		u32	rtt;

		u32		rtt_us;

		u32		seq;

		u32		seq;

		u32	time;

		struct skb_mstamp time;

	} rcv_rtt_est;

	} rcv_rtt_est;





/* Receiver queue space */

/* Receiver queue space */

	struct {

	struct {

		int		space;

		int		space;

		u32		seq;

		u32		seq;

		u32	time;

		struct skb_mstamp time;

	} rcvq_space;

	} rcvq_space;





/* TCP-specific MTU probe information. */

/* TCP-specific MTU probe information. */

net/ipv4/tcp.c

+1 −1
Original line number Original line Diff line number Diff line
@@ -2853,7 +2853,7 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info) 
	info->tcpi_snd_ssthresh = tp->snd_ssthresh;

	info->tcpi_snd_ssthresh = tp->snd_ssthresh;

	info->tcpi_advmss = tp->advmss;

	info->tcpi_advmss = tp->advmss;





	info->tcpi_rcv_rtt = jiffies_to_usecs(tp->rcv_rtt_est.rtt)>>3;

	info->tcpi_rcv_rtt = tp->rcv_rtt_est.rtt_us >> 3;

	info->tcpi_rcv_space = tp->rcvq_space.space;

	info->tcpi_rcv_space = tp->rcvq_space.space;





	info->tcpi_total_retrans = tp->total_retrans;

	info->tcpi_total_retrans = tp->total_retrans;

net/ipv4/tcp_input.c

+17 −11
Original line number Original line Diff line number Diff line
@@ -442,7 +442,8 @@ void tcp_init_buffer_space(struct sock *sk) 
		tcp_sndbuf_expand(sk);

		tcp_sndbuf_expand(sk);





	tp->rcvq_space.space = tp->rcv_wnd;

	tp->rcvq_space.space = tp->rcv_wnd;

	tp->rcvq_space.time = tcp_time_stamp;

	skb_mstamp_get(&tp->tcp_mstamp);

	tp->rcvq_space.time = tp->tcp_mstamp;

	tp->rcvq_space.seq = tp->copied_seq;

	tp->rcvq_space.seq = tp->copied_seq;





	maxwin = tcp_full_space(sk);

	maxwin = tcp_full_space(sk);
@@ -518,7 +519,7 @@ EXPORT_SYMBOL(tcp_initialize_rcv_mss); 
 */

 */

static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)

static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)

{

{

	u32 new_sample = tp->rcv_rtt_est.rtt;

	u32 new_sample = tp->rcv_rtt_est.rtt_us;

	long m = sample;

	long m = sample;





	if (m == 0)

	if (m == 0)
@@ -548,21 +549,23 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) 
		new_sample = m << 3;

		new_sample = m << 3;

	}

	}





	if (tp->rcv_rtt_est.rtt != new_sample)

	tp->rcv_rtt_est.rtt_us = new_sample;

		tp->rcv_rtt_est.rtt = new_sample;

}

}





static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)

static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)

{

{

	if (tp->rcv_rtt_est.time == 0)

	u32 delta_us;



	if (tp->rcv_rtt_est.time.v64 == 0)

		goto new_measure;

		goto new_measure;

	if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))

	if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))

		return;

		return;

	tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rcv_rtt_est.time, 1);

	delta_us = skb_mstamp_us_delta(&tp->tcp_mstamp, &tp->rcv_rtt_est.time);

	tcp_rcv_rtt_update(tp, delta_us, 1);





new_measure:

new_measure:

	tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;

	tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;

	tp->rcv_rtt_est.time = tcp_time_stamp;

	tp->rcv_rtt_est.time = tp->tcp_mstamp;

}

}





static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,

static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
@@ -572,7 +575,10 @@ static inline void tcp_rcv_rtt_measure_ts(struct sock *sk, 
	if (tp->rx_opt.rcv_tsecr &&

	if (tp->rx_opt.rcv_tsecr &&

	    (TCP_SKB_CB(skb)->end_seq -

	    (TCP_SKB_CB(skb)->end_seq -

	     TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss))

	     TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss))

		tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rx_opt.rcv_tsecr, 0);

		tcp_rcv_rtt_update(tp,

				   jiffies_to_usecs(tcp_time_stamp -

						    tp->rx_opt.rcv_tsecr),

				   0);

}

}





/*

/*
@@ -585,8 +591,8 @@ void tcp_rcv_space_adjust(struct sock *sk) 
	int time;

	int time;

	int copied;

	int copied;





	time = tcp_time_stamp - tp->rcvq_space.time;

	time = skb_mstamp_us_delta(&tp->tcp_mstamp, &tp->rcvq_space.time);

	if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 0)

	if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)

		return;

		return;





	/* Number of bytes copied to user in last RTT */

	/* Number of bytes copied to user in last RTT */
@@ -642,7 +648,7 @@ void tcp_rcv_space_adjust(struct sock *sk) 




new_measure:

new_measure:

	tp->rcvq_space.seq = tp->copied_seq;

	tp->rcvq_space.seq = tp->copied_seq;

	tp->rcvq_space.time = tcp_time_stamp;

	tp->rcvq_space.time = tp->tcp_mstamp;

}

}





/* There is something which you must keep in mind when you analyze the

/* There is something which you must keep in mind when you analyze the

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