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authorGerrit Renker <gerrit@erg.abdn.ac.uk>2008-09-04 07:30:19 +0200
committerGerrit Renker <gerrit@erg.abdn.ac.uk>2008-09-04 07:45:40 +0200
commitf76fd327a8b32d3ad5b51639faf6f54d18be0981 (patch)
tree86de66d77206ff624cce7402f73f97ea3575a6c9 /net/dccp
parent7d1af6a8d935678248d057564e75e1452409a53c (diff)
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>
Diffstat (limited to 'net/dccp')
-rw-r--r--net/dccp/ccids/ccid3.c13
-rw-r--r--net/dccp/dccp.h5
-rw-r--r--net/dccp/timer.c3
3 files changed, 18 insertions, 3 deletions
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);
diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index 1585fa26488c..b63a82ccb2b2 100644
--- 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)
diff --git a/net/dccp/timer.c b/net/dccp/timer.c
index e02d5a94f4c0..16359e29e7f5 100644
--- 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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