Re: HVM Migration of domU on Qemu-upstream DM causes stuck system clock with ACPI

[George Dunlap <george.dunlap@eu.citrix.com>]

On 31/05/13 11:59, George Dunlap wrote:
> [ 248.067326] CE: xen increased min_delta_ns to 225000 nsec
> [  248.067344] CE: xen increased min_delta_ns to 337500 nsec
> [  248.067361] CE: xen increased min_delta_ns to 506250 nsec
> [  248.067378] CE: xen increased min_delta_ns to 759375 nsec
> [  248.067396] CE: xen increased min_delta_ns to 1139062 nsec
> [  248.067413] CE: xen increased min_delta_ns to 1708593 nsec
> [  248.067428] CE: xen increased min_delta_ns to 2562889 nsec
> [  248.067441] CE: xen increased min_delta_ns to 3844333 nsec
> [  248.067453] CE: xen increased min_delta_ns to 4000000 nsec
> [  248.067466] CE: Reprogramming failure. Giving up
> [  248.068075] ata2.00: configured for MWDMA2
>
> Note the "CE: xen increased min_delta_ns to 150000nsec" at 231 for the 
> previous suspend, and now it's increasing it up to 4 milliseconds 
> before giving up for this suspend.
>
> Konrad, stefano, any idea what's going on here?

So it looks like those messages are coming from 
linux.git/kernel/time/clockevents.c.

clockevents_program_events() calls clockevents_program_min_delta(), 
which calls the Xen clock set_next_event (in 
linux.git/arch/x86/xen/time.c) , which calls VCPUOP_set_singleshot_timer 
(which is handled in xen.git/xen/common/domain.c).

If set_next_event() returns an error, it tries again a couple of times, 
then tries increasing the "min_delta" and trying again; eventually it 
wil give up.  So set_next_event() must be returning an error consistently.

The only time that VCPUOP_set_singleshot_timer should return an error is 
if the requrested time is in the past *and* the VCPU_SSHOTTMR_future 
flag is set (which it is apparently).

So it would appear that the VM is down over the period that some event 
wants to happen; and Linux does not contemplate the idea that we may 
have been unable to hit an event within 4ms.

Overall it looks like something we should fix in Linux.  Completely 
giving up on all timers seems much too extreme.  At worst it should just 
drop timers.  Probably what it should do is on each iteration, check to 
see if any events are currently in the past and just fire them 
immediately, taking them off the queue.

  -George