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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>
This commit is contained in:
Gerrit Renker 2008-09-04 07:30:19 +02:00
parent 7d1af6a8d9
commit f76fd327a8
3 changed files with 18 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
net/dccp/ccids/ccid3.c
View File

@ -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);

5
net/dccp/dccp.h
View File

@ -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)

3
net/dccp/timer.c
View File

@ -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);