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SubjectRe: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host
 On Thu, 01 Sep 2011 14:24:12 -0500, Anthony Liguori wrote:
> On 08/30/2011 07:26 AM, Marcelo Tosatti wrote:
>> On Mon, Aug 29, 2011 at 05:27:11PM -0600, Eric B Munson wrote:
>>> Currently, when qemu stops a guest kernel that guest will issue a
>>> soft lockup
>>> message when it resumes. This set provides the ability for qemu to
>>> comminucate
>>> to the guest that it has been stopped. When the guest hits the
>>> watchdog on
>>> resume it will check if it was suspended before issuing the
>>> warning.
>>>
>>> Eric B Munson (4):
>>> Add flag to indicate that a vm was stopped by the host
>>> Add functions to check if the host has stopped the vm
>>> Add generic stubs for kvm stop check functions
>>> Add check for suspended vm in softlockup detector
>>>
>>> arch/x86/include/asm/pvclock-abi.h | 1 +
>>> arch/x86/include/asm/pvclock.h | 2 ++
>>> arch/x86/kernel/kvmclock.c | 14 ++++++++++++++
>>> include/asm-generic/pvclock.h | 14 ++++++++++++++
>>> kernel/watchdog.c | 12 ++++++++++++
>>> 5 files changed, 43 insertions(+), 0 deletions(-)
>>> create mode 100644 include/asm-generic/pvclock.h
>>>
>>> --
>>> 1.7.4.1
>>
>> How is the host supposed to set this flag?
>>
>> As mentioned previously, if you save save/restore the offset added
>> to
>> kvmclock on stop/cont (and the TSC MSR, forgot to mention that), no
>> paravirt infrastructure is required. Which means the issue is also
>> fixed
>> for older guests.
>
> IIRC, the steal time patches have some logic that basically say:
>
> if there was steal time:
> kick soft lockup detector
>
> I wonder if that serves this purpose provided that time spent in stop
> is accounted as steal time. If it isn't, perhaps it should be?
>
> Regards,
>
> Anthony Liguori
>

I could be missing it, but I don't see anywhere in the steal time
patches that kicks the watchdog.

Accounting stopped time as stolen time opens a possible problem when
the accounting (CPU power modification) part is turned on. As a process
accumulates steal time its CPU power is increased. If we account
stopped time as stolen, it will do strange things with CPU power for
that process. I believe that it is for this reason that patch 4 of
Glauber's series explicitly states that halted time is not stolen time.
Stolen time is only accumulated when a vCPU actually has work to do.

Eric


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