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Commit f76fd327 authored by Gerrit Renker's avatar Gerrit Renker
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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: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 7d1af6a8

net/dccp/ccids/ccid3.c

+13 −0
Original line number Diff line number Diff line
@@ -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);

net/dccp/dccp.h

+4 −1
Original line number Diff line number Diff line
@@ -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)

net/dccp/timer.c

+1 −2
Original line number Diff line number Diff line
@@ -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);