dccp ccid-3: Runtime verification of timer resolution (f76fd327) · Commits · 戴 / test

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>

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);

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