[CCID3]: Ignore trivial amounts of elapsed time

This patch fixes a previously undiscovered bug; the problem is in computing
the elapsed time as the time between `receiving' the packet (i.e. skb enters
CCID module) and sending feedback:

     - there is no layer-processing, queueing, or delay involved,
     - hence the elapsed time is in the order of 1 function call
     - this is in the dimension of maximally 50..100usec
     - which renders the use of elapsed time almost entirely useless.

The fix is simply to ignore such trivial amounts of elapsed time.

As a further advantage, the now useless elapsed_time field can be removed from
the socket, which reduces the socket structure by another four bytes.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ccids/ccid3.c

@@ -744,11 +744,6 @@ static void ccid3_hc_rx_send_feedback(struct sock *sk)
 	hcrx->ccid3hcrx_ccval_last_counter   = packet->dccphrx_ccval;
 	hcrx->ccid3hcrx_bytes_recv	     = 0;
 
-	/* Elapsed time information [RFC 4340, 13.2] in units of 10 * usecs */
-	delta = ktime_us_delta(now, packet->dccphrx_tstamp);
-	DCCP_BUG_ON(delta < 0);
-	hcrx->ccid3hcrx_elapsed_time = delta / 10;
-
 	if (hcrx->ccid3hcrx_p == 0)
 		hcrx->ccid3hcrx_pinv = ~0U;	/* see RFC 4342, 8.5 */
 	else if (hcrx->ccid3hcrx_p > 1000000) {
@@ -778,10 +773,7 @@ static int ccid3_hc_rx_insert_options(struct sock *sk, struct sk_buff *skb)
 	x_recv = htonl(hcrx->ccid3hcrx_x_recv);
 	pinv   = htonl(hcrx->ccid3hcrx_pinv);
 
-	if ((hcrx->ccid3hcrx_elapsed_time != 0 &&
-	     dccp_insert_option_elapsed_time(sk, skb,
-					     hcrx->ccid3hcrx_elapsed_time)) ||
-	    dccp_insert_option_timestamp(sk, skb) ||
+	if (dccp_insert_option_timestamp(sk, skb) ||
 	    dccp_insert_option(sk, skb, TFRC_OPT_LOSS_EVENT_RATE,
 			       &pinv, sizeof(pinv)) ||
 	    dccp_insert_option(sk, skb, TFRC_OPT_RECEIVE_RATE,

net/dccp/ccids/ccid3.h

@@ -147,7 +147,6 @@ enum ccid3_hc_rx_states {
  *  @ccid3hcrx_li_hist  -  Loss Interval History
  *  @ccid3hcrx_s  -  Received packet size in bytes
  *  @ccid3hcrx_pinv  -  Inverse of Loss Event Rate (RFC 4342, sec. 8.5)
- *  @ccid3hcrx_elapsed_time  -  Time since packet reception
  */
 struct ccid3_hc_rx_sock {
 	struct tfrc_rx_info		ccid3hcrx_tfrc;
@@ -165,7 +164,6 @@ struct ccid3_hc_rx_sock {
 	struct list_head		ccid3hcrx_li_hist;
 	u16				ccid3hcrx_s;
 	u32				ccid3hcrx_pinv;
-	u32				ccid3hcrx_elapsed_time;
 };
 
 static inline struct ccid3_hc_rx_sock *ccid3_hc_rx_sk(const struct sock *sk)