[DCCP] ccid3: Finer-grained resolution of sending rates

This patch
 * resolves a bug where packets smaller than 32/64 bytes resulted in sending rates of 0
 * supports all sending rates from 1/64 bytes/second up to 4Gbyte/second
 * simplifies the present overflow problems in calculations

Current sending rate X and the cached value X_recv of the receiver-estimated
sending rate are both scaled by 64 (2^6) in order to
 * cope with low sending rates (minimally 1 byte/second)
 * allow upgrading to use a packets-per-second implementation of CCID 3
 * avoid calculation errors due to integer arithmetic cut-off

The patch implements a revised strategy from
http://www.mail-archive.com/dccp@vger.kernel.org/msg01040.html

The only difference with regard to that strategy is that t_ipi is already
used in the calculation of the nofeedback timeout, which saves one division.

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>

3 files changed, 78 insertions, 39 deletions

diff --git a/include/linux/tfrc.h b/include/linux/tfrc.h
index 31a9b25276fe..8a8462b4a4dd 100644
--- a/include/linux/tfrc.h
+++ b/include/linux/tfrc.h
@@ -37,10 +37,14 @@ struct tfrc_rx_info {
  *      @tfrctx_p:      current loss event rate (5.4)
  *      @tfrctx_rto:    estimate of RTO, equals 4*RTT (4.3)
  *      @tfrctx_ipi:    inter-packet interval (4.6)
+ *
+ *  Note: X and X_recv are both maintained in units of 64 * bytes/second. This
+ *        enables a finer resolution of sending rates and avoids problems with
+ *        integer arithmetic; u32 is not sufficient as scaling consumes 6 bits.
  */
 struct tfrc_tx_info {
-        __u32 tfrctx_x;
-        __u32 tfrctx_x_recv;
+        __u64 tfrctx_x;
+        __u64 tfrctx_x_recv;
         __u32 tfrctx_x_calc;
         __u32 tfrctx_rtt;
         __u32 tfrctx_p;
diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index c54663f21fdd..aa355d4cfc8a 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -108,8 +108,9 @@ static inline void ccid3_update_send_time(struct ccid3_hc_tx_sock *hctx)
 {
         timeval_sub_usecs(&hctx->ccid3hctx_t_nom, hctx->ccid3hctx_t_ipi);
 
-        /* Calculate new t_ipi (inter packet interval) by t_ipi = s / X_inst */
-        hctx->ccid3hctx_t_ipi = usecs_div(hctx->ccid3hctx_s, hctx->ccid3hctx_x);
+        /* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */
+        hctx->ccid3hctx_t_ipi = scaled_div(hctx->ccid3hctx_s,
+                                           hctx->ccid3hctx_x >> 6);
 
         /* Update nominal send time with regard to the new t_ipi */
         timeval_add_usecs(&hctx->ccid3hctx_t_nom, hctx->ccid3hctx_t_ipi);
@@ -128,26 +129,33 @@ static inline void ccid3_update_send_time(struct ccid3_hc_tx_sock *hctx)
  *          X = max(min(2 * X, 2 * X_recv), s / R);
  *          tld = now;
  *
+ * Note: X and X_recv are both stored in units of 64 * bytes/second, to support
+ *       fine-grained resolution of sending rates. This requires scaling by 2^6
+ *       throughout the code. Only X_calc is unscaled (in bytes/second).
+ *
  * If X has changed, we also update the scheduled send time t_now,
  * the inter-packet interval t_ipi, and the delta value.
- */ 
+ */
 static void ccid3_hc_tx_update_x(struct sock *sk, struct timeval *now)
 
 {
         struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
-        const __u32 old_x = hctx->ccid3hctx_x;
+        const  __u64 old_x = hctx->ccid3hctx_x;
 
         if (hctx->ccid3hctx_p > 0) {
-                hctx->ccid3hctx_x = max_t(u32, min(hctx->ccid3hctx_x_calc,
-                                                   hctx->ccid3hctx_x_recv * 2),
-                                               hctx->ccid3hctx_s / TFRC_T_MBI);
+
+                hctx->ccid3hctx_x = min_t(u64, hctx->ccid3hctx_x_calc << 6,
+                                               hctx->ccid3hctx_x_recv * 2  );
+                hctx->ccid3hctx_x = max_t(u64, hctx->ccid3hctx_x,
+                                          (hctx->ccid3hctx_s << 6)/TFRC_T_MBI);
 
         } else if (timeval_delta(now, &hctx->ccid3hctx_t_ld) >=
                                                           hctx->ccid3hctx_rtt) {
-                hctx->ccid3hctx_x = max(min(hctx->ccid3hctx_x_recv,
-                                            hctx->ccid3hctx_x      ) * 2,
-                                        usecs_div(hctx->ccid3hctx_s,
-                                                  hctx->ccid3hctx_rtt)   );
+
+                hctx->ccid3hctx_x = max(2 * min(hctx->ccid3hctx_x,
+                                                hctx->ccid3hctx_x_recv),
+                                        scaled_div(hctx->ccid3hctx_s << 6,
+                                                   hctx->ccid3hctx_rtt    ));
                 hctx->ccid3hctx_t_ld = *now;
         }
 
@@ -194,13 +202,13 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
         case TFRC_SSTATE_NO_FBACK:
                 /* RFC 3448, 4.4: Halve send rate directly */
                 hctx->ccid3hctx_x = max_t(u32, hctx->ccid3hctx_x / 2,
-                                               hctx->ccid3hctx_s / TFRC_T_MBI);
+                                          (hctx->ccid3hctx_s << 6)/TFRC_T_MBI);
 
-                ccid3_pr_debug("%s, sk=%p, state=%s, updated tx rate to %d "
+                ccid3_pr_debug("%s, sk=%p, state=%s, updated tx rate to %u "
                                "bytes/s\n",
                                dccp_role(sk), sk,
                                ccid3_tx_state_name(hctx->ccid3hctx_state),
-                               hctx->ccid3hctx_x);
+                               (unsigned)(hctx->ccid3hctx_x >> 6));
                 /* The value of R is still undefined and so we can not recompute
                  * the timout value. Keep initial value as per [RFC 4342, 5]. */
                 t_nfb = TFRC_INITIAL_TIMEOUT;
@@ -209,11 +217,11 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
         case TFRC_SSTATE_FBACK:
                 /*
                  * Check if IDLE since last timeout and recv rate is less than
-                 * 4 packets per RTT
+                 * 4 packets (in units of 64*bytes/sec) per RTT
                  */
                 if (!hctx->ccid3hctx_idle ||
-                    (hctx->ccid3hctx_x_recv >=
-                     4 * usecs_div(hctx->ccid3hctx_s, hctx->ccid3hctx_rtt))) {
+                    (hctx->ccid3hctx_x_recv >= 4 *
+                     scaled_div(hctx->ccid3hctx_s << 6, hctx->ccid3hctx_rtt))) {
                         struct timeval now;
 
                         ccid3_pr_debug("%s, sk=%p, state=%s, not idle\n",
@@ -227,17 +235,23 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
                          *    X_recv = max(X_recv / 2, s / (2 * t_mbi));
                          *  Else
                          *    X_recv = X_calc / 4;
+                         *
+                         *  Note that X_recv is scaled by 2^6 while X_calc is not
                          */
                         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_recv = max_t(u32, hctx->ccid3hctx_x_recv / 2,
-                                                                    hctx->ccid3hctx_s / (2 * TFRC_T_MBI));
+                            hctx->ccid3hctx_x_calc > (hctx->ccid3hctx_x_recv >> 5))  {
+
+                                hctx->ccid3hctx_x_recv =
+                                        max_t(u64, hctx->ccid3hctx_x_recv / 2,
+                                                  (hctx->ccid3hctx_s << 6) /
+                                                                (2*TFRC_T_MBI));
+
                                 if (hctx->ccid3hctx_p == 0)
                                         dccp_timestamp(sk, &now);
                         } else
-                                hctx->ccid3hctx_x_recv = hctx->ccid3hctx_x_calc / 4;
+                                hctx->ccid3hctx_x_recv = hctx->ccid3hctx_x_calc << 4;
 
                         /* Now recalculate X [RFC 3448, 4.3, step (4)] */
                         ccid3_hc_tx_update_x(sk, &now);
@@ -315,9 +329,9 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
                 hctx->ccid3hctx_t_last_win_count = now;
                 ccid3_hc_tx_set_state(sk, TFRC_SSTATE_NO_FBACK);
 
-                /* Set initial sending rate to 1 packet per second */
+                /* Set initial sending rate X/s to 1pps (X is scaled by 2^6) */
                 ccid3_hc_tx_update_s(hctx, skb->len);
-                hctx->ccid3hctx_x     = hctx->ccid3hctx_s;
+                hctx->ccid3hctx_x = hctx->ccid3hctx_s << 6;
 
                 /* First timeout, according to [RFC 3448, 4.2], is 1 second */
                 hctx->ccid3hctx_t_ipi = USEC_PER_SEC;
@@ -438,8 +452,8 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                         return;
                 }
 
-                /* Update receive rate */
-                hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate;
+                /* Update receive rate in units of 64 * bytes/second */
+                hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate << 6;
 
                 /* Update loss event rate */
                 pinv = opt_recv->ccid3or_loss_event_rate;
@@ -475,12 +489,14 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                  * q is a constant, RFC 3448 recomments 0.9
                  */
                 if (hctx->ccid3hctx_state == TFRC_SSTATE_NO_FBACK) {
-                        /* Use Larger Initial Windows [RFC 4342, sec. 5]
-                         * We deviate in that we use `s' instead of `MSS'. */
+                        /*
+                         * Larger Initial Windows [RFC 4342, sec. 5]
+                         * We deviate in that we use `s' instead of `MSS'.
+                         */
                         u16 w_init = min(    4 * hctx->ccid3hctx_s,
                                          max(2 * hctx->ccid3hctx_s, 4380));
                         hctx->ccid3hctx_rtt  = r_sample;
-                        hctx->ccid3hctx_x    = usecs_div(w_init, r_sample);
+                        hctx->ccid3hctx_x    = scaled_div(w_init<< 6, r_sample);
                         hctx->ccid3hctx_t_ld = now;
 
                         ccid3_update_send_time(hctx);
@@ -488,7 +504,7 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                         ccid3_pr_debug("%s(%p), s=%u, w_init=%u, "
                                        "R_sample=%ldus, X=%u\n", dccp_role(sk),
                                        sk, hctx->ccid3hctx_s, w_init, r_sample,
-                                       hctx->ccid3hctx_x);
+                                       (unsigned)(hctx->ccid3hctx_x >> 6));
 
                         ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
                 } else {
@@ -508,7 +524,7 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                                        sk, hctx->ccid3hctx_rtt, r_sample,
                                        hctx->ccid3hctx_s, hctx->ccid3hctx_p,
                                        hctx->ccid3hctx_x_calc,
-                                       hctx->ccid3hctx_x);
+                                       (unsigned)(hctx->ccid3hctx_x >> 6));
                 }
 
                 /* unschedule no feedback timer */
diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h
index 07596d704ef9..cd4fc542f73e 100644
--- a/net/dccp/ccids/ccid3.h
+++ b/net/dccp/ccids/ccid3.h
@@ -75,14 +75,14 @@ enum ccid3_hc_tx_states {
 
 /** struct ccid3_hc_tx_sock - CCID3 sender half-connection socket
  *
- * @ccid3hctx_x - Current sending rate
- * @ccid3hctx_x_recv - Receive rate
- * @ccid3hctx_x_calc - Calculated send rate (RFC 3448, 3.1)
+ * @ccid3hctx_x - Current sending rate in 64 * bytes per second
+ * @ccid3hctx_x_recv - Receive rate    in 64 * bytes per second
+ * @ccid3hctx_x_calc - Calculated rate in bytes per second
  * @ccid3hctx_rtt - Estimate of current round trip time in usecs
  * @ccid3hctx_p - Current loss event rate (0-1) scaled by 1000000
- * @ccid3hctx_s - Packet size
- * @ccid3hctx_t_rto - Retransmission Timeout (RFC 3448, 3.1)
- * @ccid3hctx_t_ipi - Interpacket (send) interval (RFC 3448, 4.6)
+ * @ccid3hctx_s - Packet size in bytes
+ * @ccid3hctx_t_rto - Nofeedback Timer setting in usecs
+ * @ccid3hctx_t_ipi - Interpacket (send) interval (RFC 3448, 4.6) in usecs
  * @ccid3hctx_state - Sender state, one of %ccid3_hc_tx_states
  * @ccid3hctx_last_win_count - Last window counter sent
  * @ccid3hctx_t_last_win_count - Timestamp of earliest packet
@@ -91,7 +91,7 @@ enum ccid3_hc_tx_states {
  * @ccid3hctx_idle - Flag indicating that sender is idling
  * @ccid3hctx_t_ld - Time last doubled during slow start
  * @ccid3hctx_t_nom - Nominal send time of next packet
- * @ccid3hctx_delta - Send timer delta
+ * @ccid3hctx_delta - Send timer delta (RFC 3448, 4.6) in usecs
  * @ccid3hctx_hist - Packet history
  * @ccid3hctx_options_received - Parsed set of retrieved options
  */
@@ -171,4 +171,23 @@ static inline struct ccid3_hc_rx_sock *ccid3_hc_rx_sk(const struct sock *sk)
     return ccid_priv(dccp_sk(sk)->dccps_hc_rx_ccid);
 }
 
+static inline u64 scaled_div(u64 a, u32 b)
+{
+        BUG_ON(b==0);
+        a *= 1000000;
+        do_div(a, b);
+        return a;
+}
+
+static inline u32 scaled_div32(u64 a, u32 b)
+{
+        u64 result = scaled_div(a, b);
+
+        if (result > UINT_MAX) {
+                DCCP_CRIT("Overflow: a(%llu)/b(%u) > ~0U",
+                          (unsigned long long)a, b);
+                return UINT_MAX;
+        }
+        return result;
+}
 #endif /* _DCCP_CCID3_H_ */