From f76fd327a8b32d3ad5b51639faf6f54d18be0981 Mon Sep 17 00:00:00 2001 From: Gerrit Renker Date: Thu, 4 Sep 2008 07:30:19 +0200 Subject: dccp ccid-3: Runtime verification of timer resolution The DCCP base time resolution is 10 microseconds (RFC 4340, 13.1 ... 13.3). Using a timer with a lower resolution was found to trigger the following bug warnings/problems on high-speed networks (e.g. local loopback): * RTT samples are rounded down to 0 if below resolution; * in some cases, negative RTT samples were observed; * the CCID-3 feedback timer complains that the feedback interval is 0, since the feedback interval is in the order of 1 RTT or less and RTT measurement rounded this down to 0; On an Intel computer this will for instance happen when using a boot-time parameter of "clocksource=jiffies". The following system log messages were observed: 11:24:00 kernel: BUG: delta (0) <= 0 at ccid3_hc_rx_send_feedback() 11:26:12 kernel: BUG: delta (0) <= 0 at ccid3_hc_rx_send_feedback() 11:26:30 kernel: dccp_sample_rtt: unusable RTT sample 0, using min 11:26:30 last message repeated 5 times This patch defines a global constant for the time resolution, adds this in timer.c, and checks the available clock resolution at CCID-3 module load time. When the resolution is worse than 10 microseconds, module loading exits with a message "socket type not supported". Signed-off-by: Gerrit Renker --- net/dccp/ccids/ccid3.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'net/dccp/ccids') diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c index 0d406f82e433..f566eb76aeb2 100644 --- a/net/dccp/ccids/ccid3.c +++ b/net/dccp/ccids/ccid3.c @@ -869,6 +869,19 @@ MODULE_PARM_DESC(ccid3_debug, "Enable debug messages"); static __init int ccid3_module_init(void) { + struct timespec tp; + + /* + * Without a fine-grained clock resolution, RTTs/X_recv are not sampled + * correctly and feedback is sent either too early or too late. + */ + hrtimer_get_res(CLOCK_MONOTONIC, &tp); + if (tp.tv_sec || tp.tv_nsec > DCCP_TIME_RESOLUTION * NSEC_PER_USEC) { + printk(KERN_ERR "%s: Timer too coarse (%ld usec), need %u-usec" + " resolution - check your clocksource.\n", __func__, + tp.tv_nsec/NSEC_PER_USEC, DCCP_TIME_RESOLUTION); + return -ESOCKTNOSUPPORT; + } return ccid_register(&ccid3); } module_init(ccid3_module_init); -- cgit v1.2.3-55-g7522