[DCCP] ccid3: Two optimisations for sending rate recomputation

This performs two optimisations for the recomputation of the sending rate.

1) Currently the target sending rate X_calc is recalculated whenever
	a) the nofeedback timer expires, or
	b) a feedback packet is received.
   In the (a) case, recomputing X_calc is redundant, since

    * the parameters p and RTT do not change in between the
      reception of feedback packets;

    * the parameter X_recv is either modified from received
      feedback or via the nofeedback timer;

    * a test (`p == 0') in the nofeedback timer avoids using
      a stale/undefined value of X_calc if p was previously 0.

2) The nofeedback timer now only recomputes a timestamp when p == 0.
   This is according to step (4) of [RFC 3448, 4.3] and avoids
   unnecessarily determining a timestamp.

A debug statement about not updating X is also removed - it helps very
little in debugging and just clutters the logs.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 16 insertions, 11 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index f1b745ee9cb6..92e893ee77c2 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -138,9 +138,6 @@ static void ccid3_hc_tx_update_x(struct sock *sk, struct timeval *now)
         const __u32 old_x = hctx->ccid3hctx_x;
 
         if (hctx->ccid3hctx_p > 0) {
-                hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
-                                                     hctx->ccid3hctx_rtt,
-                                                     hctx->ccid3hctx_p);
                 hctx->ccid3hctx_x = max_t(u32, min(hctx->ccid3hctx_x_calc,
                                                    hctx->ccid3hctx_x_recv * 2),
                                                hctx->ccid3hctx_s / TFRC_T_MBI);
@@ -152,8 +149,7 @@ static void ccid3_hc_tx_update_x(struct sock *sk, struct timeval *now)
                                         usecs_div(hctx->ccid3hctx_s,
                                                   hctx->ccid3hctx_rtt)   );
                 hctx->ccid3hctx_t_ld = *now;
-        } else
-                ccid3_pr_debug("Not changing X\n");
+        }
 
         if (hctx->ccid3hctx_x != old_x)
                 ccid3_update_send_time(hctx);
@@ -223,9 +219,11 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
                         ccid3_pr_debug("%s, sk=%p, state=%s, not idle\n",
                                        dccp_role(sk), sk,
                                        ccid3_tx_state_name(hctx->ccid3hctx_state));
-                        /* Halve sending rate */
 
-                        /*  If (p == 0 || X_calc > 2 * X_recv)
+                        /*
+                         *  Modify the cached value of X_recv [RFC 3448, 4.4]
+                         *
+                         *  If (p == 0 || X_calc > 2 * X_recv)
                          *    X_recv = max(X_recv / 2, s / (2 * t_mbi));
                          *  Else
                          *    X_recv = X_calc / 4;
@@ -233,14 +231,15 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
                         BUG_ON(hctx->ccid3hctx_p && !hctx->ccid3hctx_x_calc);
 
                         if (hctx->ccid3hctx_p  == 0 ||
-                            hctx->ccid3hctx_x_calc > 2 * hctx->ccid3hctx_x_recv)
+                            hctx->ccid3hctx_x_calc > 2 * hctx->ccid3hctx_x_recv)  {
                                 hctx->ccid3hctx_x_recv = max_t(u32, hctx->ccid3hctx_x_recv / 2,
                                                                     hctx->ccid3hctx_s / (2 * TFRC_T_MBI));
-                        else
+                                if (hctx->ccid3hctx_p == 0)
+                                        dccp_timestamp(sk, &now);
+                        } else
                                 hctx->ccid3hctx_x_recv = hctx->ccid3hctx_x_calc / 4;
 
-                        /* Update sending rate */
-                        dccp_timestamp(sk, &now);
+                        /* Now recalculate X [RFC 3448, 4.3, step (4)] */
                         ccid3_hc_tx_update_x(sk, &now);
                 }
                 /*
@@ -496,6 +495,12 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                         hctx->ccid3hctx_rtt = (9 * hctx->ccid3hctx_rtt +
                                                    (u32)r_sample        ) / 10;
 
+                        /* Update sending rate (step 4 of [RFC 3448, 4.3]) */
+                        if (hctx->ccid3hctx_p > 0)
+                                hctx->ccid3hctx_x_calc =
+                                        tfrc_calc_x(hctx->ccid3hctx_s,
+                                                    hctx->ccid3hctx_rtt,
+                                                    hctx->ccid3hctx_p);
                         ccid3_hc_tx_update_x(sk, &now);
 
                         ccid3_pr_debug("%s(%p), RTT=%uus (sample=%ldus), s=%u, "