linux-kernel - Re: KVM guest lockup

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Message-ID: <20081221115538.GA25958@x61s.gondor.com>
Date:	Sun, 21 Dec 2008 12:55:39 +0100
From:	Jan Niehusmann <jan@...dor.com>
To:	Max Kellermann <max@...mpel.org>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: KVM guest lockup

On Fri, Dec 19, 2008 at 06:44:01PM +0100, Max Kellermann wrote:
> By starting kvm with "-s", I was able to attach gdb to KVM.  The
> following backtrace is with default options:
> 
>  #0  0xffffffff8023f3e0 in update_wall_time () at kernel/time/timekeeping.c:515
[...]

> The next one is with "noapic nolapic acpi=off":
>  
>  #0  0xffffffff8023f35a in update_wall_time () at kernel/time/timekeeping.c:501
> Both backtraces are from vanilla 2.6.28-rc8, .config attached.
[...]

Looking at the code, both these lines of code are inside a loop which
may run for quite a while if clocksource_read(clock) returns
non-monotonic values:

        offset = (clocksource_read(clock) - clock->cycle_last) & clock->mask;
	[...]
        while (offset >= clock->cycle_interval) {
                /* accumulate one interval */
                offset -= clock->cycle_interval;
	[...]
	}

As I observed such behaviour only when the host has frequency scaling
activated: Could it be that in this case, clocksource_read(clock) on the
guest is misbehaving?

Jan

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