Re: [uml-devel] Lockups with the fixed timer code :/

[Jeff Dike <jdike@addtoit.com>]

On Sat, Apr 05, 2008 at 05:45:39PM +0100, Nix wrote:
> The fixed timer patch you posted a few weeks back has indeed fixed my
> select()-based timeout woes.
> 
> Unfortunately, both with the old kludgy approach and with the new
> remain-versus-max estimator code, I see intermittent tight lockups of
> the UML kernel-space ptrace thread, with that thread chewing all
> available CPU time and the virtual machine, unsurprisingly, going
> unresponsive. 

Below is another patch for you to try.  I spent most of last week
chasing this one.  The symptoms are somewhat similar to yours -
intermittent UML hangs, although not with UML spinning, and it still
pings.

The problem is NTP adjusting the multiplier part of the clock-provided
cycles-to-ns conversion function.  UML pretended to have a ns clock,
with a multiplier of 1.  When NTP adjusted that down in order to slow
down the clock, that became 0, and time stopped.

The fix below is to switch to a usec clock, with a multiplier of 1000,
which can be adjusted with much more granualarity.

      	     	      Jeff

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

Index: linux-2.6.22/arch/um/kernel/time.c
===================================================================
--- linux-2.6.22.orig/arch/um/kernel/time.c	2008-04-10 12:53:32.000000000 -0400
+++ linux-2.6.22/arch/um/kernel/time.c	2008-04-14 10:30:00.000000000 -0400
@@ -75,7 +75,7 @@ static irqreturn_t um_timer(int irq, voi
 
 static cycle_t itimer_read(void)
 {
-	return os_nsecs();
+	return os_nsecs() / 1000;
 }
 
 static struct clocksource itimer_clocksource = {
@@ -83,7 +83,7 @@ static struct clocksource itimer_clockso
 	.rating		= 300,
 	.read		= itimer_read,
 	.mask		= CLOCKSOURCE_MASK(64),
-	.mult		= 1,
+	.mult		= 1000,
 	.shift		= 0,
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
 };

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