Re: [uml-devel] actually useful backtrace from a CPU-chewing hang

[Jeff Dike <jdike@addtoit.com>]

On Fri, May 02, 2008 at 12:34:54AM +0100, Nix wrote:
> loki:/tmp# /usr/bin/ntpdate -bv hades
> 30 Apr 22:45:52 ntpdate[8833]: ntpdate 4.2.4p4@1.1520-o Fri Feb 22 18:37:11 UTC 2008 (1)
> 30 Apr 22:45:52 ntpdate[8833]: step time server 192.168.14.18 offset -5.728539 sec
> 
> so in effect yes :)
> 
> Hm. A five-second skip forwards via date --set works fine. A five-second
> skip *backwards*, and instant boom.

I tried both directions for the hell of it.

With your config, I'm seeing a hang until the system time catches up
to what UML thought it should have been in the first place.  But it's
only a few seconds, not forever.

However, stracing it did reveal a bogus interval trying to be set,
which the patch below fixes.  It doesn't cause any behavior change
here, so YMMV.

This includes the previous patch, which I think is a good idea anyway,
so back that out and drop this in its place.

			Jeff

Index: linux-2.6.22/arch/um/os-Linux/time.c
===================================================================
--- linux-2.6.22.orig/arch/um/os-Linux/time.c	2008-04-24 13:21:28.000000000 -0400
+++ linux-2.6.22/arch/um/os-Linux/time.c	2008-05-02 12:21:01.000000000 -0400
@@ -107,6 +107,10 @@ static void deliver_alarm(void)
 	unsigned long long this_tick = os_nsecs();
 	int one_tick = UM_NSEC_PER_SEC / UM_HZ;
 
+	/* Protection against the host's time going backwards */
+	if ((last_tick != 0) && (this_tick < last_tick))
+		this_tick = last_tick;
+
 	if (last_tick == 0)
 		last_tick = this_tick - one_tick;
 
@@ -149,6 +153,9 @@ static int after_sleep_interval(struct t
 		start_usecs = usec;
 
 	start_usecs -= skew / UM_NSEC_PER_USEC;
+	if (start_usecs < 0)
+		start_usecs = 0;
+
 	tv = ((struct timeval) { .tv_sec  = start_usecs / UM_USEC_PER_SEC,
 				 .tv_usec = start_usecs % UM_USEC_PER_SEC });
 	interval = ((struct itimerval) { { 0, usec }, tv });

-- 
Work email - jdike at linux dot intel dot com

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