Re: [PATCH] cpu hotplug issue

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-07-27 18:35, Vasilis Liaskovitis wrote:
> Hi,
> 
> On Mon, Jul 25, 2011 at 3:18 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>
>> OK, hacks below plus the following three patches make CPU hotplug work
>> again - with some exceptions. Here are the patches:
>>
>> 1. http://thread.gmane.org/gmane.comp.emulators.kvm.devel/76484
>> 2. http://thread.gmane.org/gmane.comp.emulators.qemu/110272
>> 3. http://thread.gmane.org/gmane.comp.emulators.qemu/110426
>>
>> And here are the hacks (well, the first hunk is clearly a fix, the last
>> one clearly a hack, /me still undecided about the rest):
> 
> thanks for these. Testing a bit late here...
> Applying these 3 patches + hacks/fix on master doesn't solve the
> initial "CPU: not responding" dmesg hotplug issue on my setup.
> The ACPI event is delivered correctly, but "echo 1 >
> /sys/devices/system/cpu/cpuX/online" still fails.
> e.g. tested with "-smp 1,maxcpus=2"
> 
> Could you share your qemu-kvm command line?

Nothing special:

qemu-system-x86_64 /my/image -m 1G -snapshot -smp 2,maxcpus=3 \
	-cpu host,-kvmclock

> I have been trying 2.6.39
> guest kernel (as opposed to 3.0.2) but
> I doubt that's the issue.
> Can someone else confirm whether this solves the issue for their setup or not?
> 
>> I see two remaining problems:
>>  - kvmclock is somehow broken, either in my guest kernel (OpenSUSE HEAD
>>    3.0.0-2) or the host, -cpu host,-kvmclock works around sporadic
>>    guest lockups on echo 1 > /sys...
> 
> could you explain this a bit further? specifying "-cpu host,-kvmclock"
> avoids the lockups? I have tried

Yes - apparently. I haven't checked if kvmclock is actually the problem
or just changing the boundary conditions so that the issue no longer
shows up here.

> adding this to my command line but didn't help (but then again my
> problem is not guest lockups: the onlining fails)
> 
>>  - Seabios tends to lock up once every few system_reset after some
>>    CPU has been hot-added - also in TCG mode. It seems to dislike any
>>    setup of #CPUs > smp_cpus (whatever that implies in details).
> 
> I always get a lockedup seabios when rebooting after a hotplug attempt
> with these patches (or with my initial vcpu_dirty workaround FWIW)

Yes, this problem is most likely independent of the above. I even got
those lockups when creating smp_cpus+1 vcpus in pc_cpus_init.

Jan

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