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 <gerrit@erg.abdn.ac.uk>

1 files changed, 13 insertions, 0 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 0d406f82e43..f566eb76aeb 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);