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dccp ccid-3: Bug fix for the inter-packet scheduling algorithm
This fixes a subtle bug in the calculation of the inter-packet gap and shows that t_delta, as it is currently used, is not needed. And hence replaced. The algorithm from RFC 3448, 4.6 below continually computes a send time t_nom, which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies the scheduling granularity, s the packet size, and X the sending rate: t_distance = t_nom - t_now; // in microseconds t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds if (t_distance >= t_delta) { reschedule after (t_distance / 1000) milliseconds; } else { t_ipi = s / X; // inter-packet interval in usec t_nom += t_ipi; // compute the next send time send packet now; } 1) Description of the bug ------------------------- Rescheduling requires a conversion into milliseconds, due to this call chain: * ccid3_hc_tx_send_packet() returns a timeout in milliseconds, * this value is converted by msecs_to_jiffies() in dccp_write_xmit(), * and finally used as jiffy-expires-value for sk_reset_timer(). The highest jiffy resolution with HZ=1000 is 1 millisecond, so using a higher granularity does not make much sense here. As a consequence, values of t_distance < 1000 are truncated to 0. This issue has so far been resolved by using instead if (t_distance >= t_delta + 1000) reschedule after (t_distance / 1000) milliseconds; The bug is in artificially inflating t_delta to t_delta' = t_delta + 1000. This is unnecessarily large, a more adequate value is t_delta' = max(t_delta, 1000). 2) Consequences of using the corrected t_delta' ----------------------------------------------- Since t_delta <= t_gran/2 = 10^6/(2*HZ), we have t_delta <= 1000 as long as HZ >= 500. This means that t_delta' = max(1000, t_delta) is constant at 1000. On the other hand, when using a coarse HZ value of HZ < 500, we have three sub-cases that can all be reduced to using another constant of t_gran/2. (a) The first case arises when t_ipi > t_gran. Here t_delta' is the constant t_delta' = max(1000, t_gran/2) = t_gran/2. (b) If t_ipi <= 2000 < t_gran = 10^6/HZ usec, then t_delta = t_ipi/2 <= 1000, so that t_delta' = max(1000, t_delta) = 1000 < t_gran/2. (c) If 2000 < t_ipi <= t_gran, we have t_delta' = max(t_delta, 1000) = t_ipi/2. In the second and third cases we have delay values less than t_gran/2, which is in the order of less than or equal to half a jiffy. How these are treated depends on how fractions of a jiffy are handled: they are either always rounded down to 0, or always rounded up to 1 jiffy (assuming non-zero values). In both cases the error is on average in the order of 50%. Thus we are not increasing the error when in the second/third case we replace a value less than t_gran/2 with 0, by setting t_delta' to the constant t_gran/2. 3) Summary ---------- Fixing (1) and considering (2), the patch replaces t_delta with a constant, whose value depends on CONFIG_HZ, changing the above algorithm to: if (t_distance >= t_delta') reschedule after (t_distance / 1000) milliseconds; where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise. Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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commit
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2 changed files with 21 additions and 15 deletions
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@ -93,19 +93,16 @@ static inline u64 rfc3390_initial_rate(struct sock *sk)
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return scaled_div(w_init << 6, hctx->rtt);
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}
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/*
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* Recalculate t_ipi and delta (should be called whenever X changes)
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/**
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* ccid3_update_send_interval - Calculate new t_ipi = s / X_inst
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* This respects the granularity of X_inst (64 * bytes/second).
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*/
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static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx)
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{
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/* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */
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hctx->t_ipi = scaled_div32(((u64)hctx->s) << 6, hctx->x);
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/* Calculate new delta by delta = min(t_ipi / 2, t_gran / 2) */
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hctx->delta = min_t(u32, hctx->t_ipi / 2, TFRC_OPSYS_HALF_TIME_GRAN);
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ccid3_pr_debug("t_ipi=%u, delta=%u, s=%u, X=%u\n", hctx->t_ipi,
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hctx->delta, hctx->s, (unsigned)(hctx->x >> 6));
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ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hctx->t_ipi,
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hctx->s, (unsigned)(hctx->x >> 6));
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}
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static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hctx, ktime_t now)
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@ -340,8 +337,8 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
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* else
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* // send the packet in (t_nom - t_now) milliseconds.
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*/
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if (delay - (s64)hctx->delta >= 1000)
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return (u32)delay / 1000L;
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if (delay >= TFRC_T_DELTA)
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return (u32)delay / USEC_PER_MSEC;
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ccid3_hc_tx_update_win_count(hctx, now);
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break;
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@ -47,12 +47,23 @@
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/* Two seconds as per RFC 3448 4.2 */
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#define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC)
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/* In usecs - half the scheduling granularity as per RFC3448 4.6 */
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#define TFRC_OPSYS_HALF_TIME_GRAN (USEC_PER_SEC / (2 * HZ))
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/* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
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#define TFRC_T_MBI 64
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/*
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* The t_delta parameter (RFC 3448, 4.6): delays of less than %USEC_PER_MSEC are
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* rounded down to 0, since sk_reset_timer() here uses millisecond granularity.
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* Hence we can use a constant t_delta = %USEC_PER_MSEC when HZ >= 500. A coarse
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* resolution of HZ < 500 means that the error is below one timer tick (t_gran)
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* when using the constant t_delta = t_gran / 2 = %USEC_PER_SEC / (2 * HZ).
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*/
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#if (HZ >= 500)
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# define TFRC_T_DELTA USEC_PER_MSEC
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#else
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# define TFRC_T_DELTA (USEC_PER_SEC / (2 * HZ))
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#warning Coarse CONFIG_HZ resolution -- higher value recommended for TFRC.
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#endif
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enum ccid3_options {
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TFRC_OPT_LOSS_EVENT_RATE = 192,
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TFRC_OPT_LOSS_INTERVALS = 193,
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@ -92,7 +103,6 @@ enum ccid3_hc_tx_states {
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* @no_feedback_timer - Handle to no feedback timer
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* @t_ld - Time last doubled during slow start
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* @t_nom - Nominal send time of next packet
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* @delta - Send timer delta (RFC 3448, 4.6) in usecs
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* @hist - Packet history
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* @options_received - Parsed set of retrieved options
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*/
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@ -111,7 +121,6 @@ struct ccid3_hc_tx_sock {
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struct timer_list no_feedback_timer;
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ktime_t t_ld;
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ktime_t t_nom;
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u32 delta;
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struct tfrc_tx_hist_entry *hist;
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struct ccid3_options_received options_received;
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};
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