This prevents module loading when the timer resolution is too low
(e.g. when using jiffies as a clocksource or when disabling high
resolution timers on sparc64).
Rationale:
----------
The DCCP base time resolution is 10 microseconds (RFC 4340, 13.1...3). Using a
timer with a lower resolution than that was found to trigger the following bug
warnings/problems on high-speed networks (e.g. local loopback):
* small RTT samples are rounded down to 0
(in some cases, even negative RTT samples occurred);
* the CCID-3 feedback timer complains that the feedback interval is 0,
since the coarse-grained resolution rounds RTT-wise intervals down.
The following syslog messages were observed with a low resolution:
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
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
net/dccp/ccids/ccid3.c | 13 +++++++++++++
net/dccp/dccp.h | 5 ++++-
net/dccp/timer.c | 3 +--
3 files changed, 18 insertions(+), 3 deletions(-)
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -86,10 +86,13 @@ extern void dccp_time_wait(struct sock *sk, int state, int timeo);
*/
#define DCCP_RTO_MAX ((unsigned)(64 * HZ))
+/* DCCP base time resolution - 10 microseconds (RFC 4340, 13.1 ... 13.3) */
+#define DCCP_TIME_RESOLUTION 10
+
/*
* RTT sampling: sanity bounds and fallback RTT value from RFC 4340, section 3.4
*/
-#define DCCP_SANE_RTT_MIN 100
+#define DCCP_SANE_RTT_MIN (10 * DCCP_TIME_RESOLUTION)
#define DCCP_FALLBACK_RTT (USEC_PER_SEC / 5)
#define DCCP_SANE_RTT_MAX (3 * USEC_PER_SEC)
--- 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);
--- a/net/dccp/timer.c
+++ b/net/dccp/timer.c
@@ -281,8 +281,7 @@ u32 dccp_timestamp(void)
{
s64 delta = ktime_us_delta(ktime_get_real(), dccp_timestamp_seed);
- do_div(delta, 10);
- return delta;
+ return div_u64(delta, DCCP_TIME_RESOLUTION);
}
EXPORT_SYMBOL_GPL(dccp_timestamp);
--
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