From de6f2b59e5cd15a8772adb732a1d80e141a77115 Mon Sep 17 00:00:00 2001 From: Gerrit Renker Date: Thu, 4 Sep 2008 07:30:19 +0200 Subject: [PATCH] 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 --- net/dccp/ccids/ccid3.c | 17 +++++++---------- net/dccp/ccids/ccid3.h | 19 ++++++++++++++----- 2 files changed, 21 insertions(+), 15 deletions(-) diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c index b4cc62e2e2bb..eb1bda08eeb2 100644 --- a/net/dccp/ccids/ccid3.c +++ b/net/dccp/ccids/ccid3.c @@ -93,19 +93,16 @@ static inline u64 rfc3390_initial_rate(struct sock *sk) return scaled_div(w_init << 6, hctx->rtt); } -/* - * Recalculate t_ipi and delta (should be called whenever X changes) +/** + * ccid3_update_send_interval - Calculate new t_ipi = s / X_inst + * This respects the granularity of X_inst (64 * bytes/second). */ static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hctx) { - /* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */ hctx->t_ipi = scaled_div32(((u64)hctx->s) << 6, hctx->x); - /* Calculate new delta by delta = min(t_ipi / 2, t_gran / 2) */ - hctx->delta = min_t(u32, hctx->t_ipi / 2, TFRC_OPSYS_HALF_TIME_GRAN); - - ccid3_pr_debug("t_ipi=%u, delta=%u, s=%u, X=%u\n", hctx->t_ipi, - hctx->delta, hctx->s, (unsigned)(hctx->x >> 6)); + ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hctx->t_ipi, + hctx->s, (unsigned)(hctx->x >> 6)); } static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hctx, ktime_t now) @@ -340,8 +337,8 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb) * else * // send the packet in (t_nom - t_now) milliseconds. */ - if (delay - (s64)hctx->delta >= 1000) - return (u32)delay / 1000L; + if (delay >= TFRC_T_DELTA) + return (u32)delay / USEC_PER_MSEC; ccid3_hc_tx_update_win_count(hctx, now); break; diff --git a/net/dccp/ccids/ccid3.h b/net/dccp/ccids/ccid3.h index 0cfcfff8f5fb..92a5e1688a07 100644 --- a/net/dccp/ccids/ccid3.h +++ b/net/dccp/ccids/ccid3.h @@ -47,12 +47,23 @@ /* Two seconds as per RFC 3448 4.2 */ #define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC) -/* In usecs - half the scheduling granularity as per RFC3448 4.6 */ -#define TFRC_OPSYS_HALF_TIME_GRAN (USEC_PER_SEC / (2 * HZ)) - /* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */ #define TFRC_T_MBI 64 +/* + * The t_delta parameter (RFC 3448, 4.6): delays of less than %USEC_PER_MSEC are + * rounded down to 0, since sk_reset_timer() here uses millisecond granularity. + * Hence we can use a constant t_delta = %USEC_PER_MSEC when HZ >= 500. A coarse + * resolution of HZ < 500 means that the error is below one timer tick (t_gran) + * when using the constant t_delta = t_gran / 2 = %USEC_PER_SEC / (2 * HZ). + */ +#if (HZ >= 500) +# define TFRC_T_DELTA USEC_PER_MSEC +#else +# define TFRC_T_DELTA (USEC_PER_SEC / (2 * HZ)) +#warning Coarse CONFIG_HZ resolution -- higher value recommended for TFRC. +#endif + enum ccid3_options { TFRC_OPT_LOSS_EVENT_RATE = 192, TFRC_OPT_LOSS_INTERVALS = 193, @@ -92,7 +103,6 @@ enum ccid3_hc_tx_states { * @no_feedback_timer - Handle to no feedback timer * @t_ld - Time last doubled during slow start * @t_nom - Nominal send time of next packet - * @delta - Send timer delta (RFC 3448, 4.6) in usecs * @hist - Packet history * @options_received - Parsed set of retrieved options */ @@ -111,7 +121,6 @@ struct ccid3_hc_tx_sock { struct timer_list no_feedback_timer; ktime_t t_ld; ktime_t t_nom; - u32 delta; struct tfrc_tx_hist_entry *hist; struct ccid3_options_received options_received; }; -- 2.34.1