From f76fd327a8b32d3ad5b51639faf6f54d18be0981 Mon Sep 17 00:00:00 2001 From: Gerrit Renker Date: Thu, 4 Sep 2008 07:30:19 +0200 Subject: 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 --- net/dccp/timer.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'net/dccp/timer.c') diff --git a/net/dccp/timer.c b/net/dccp/timer.c index e02d5a94f4c..16359e29e7f 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); -- cgit v1.2.3-70-g09d2