Re: Shutdown panic in disable_nonboot_cpus after cpupool-numa-split

[Stefan Bader <stefan.bader@canonical.com>]

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On 07.07.2014 16:28, Juergen Gross wrote:
> On 07/07/2014 04:08 PM, Stefan Bader wrote:
>> On 07.07.2014 15:03, Jürgen Groß wrote:
>>> On 07/07/2014 02:49 PM, Stefan Bader wrote:
>>>> On 07.07.2014 14:38, Jürgen Groß wrote:
>>>>> On 07/07/2014 02:00 PM, Andrew Cooper wrote:
>>>>>> On 07/07/14 12:33, Stefan Bader wrote:
>>>>>>> I recently noticed that I get a  panic (rebooting the system) on shutdown in
>>>>>>> some
>>>>>>    > cases. This happened only on my AMD system and also not all the time.
>>>>>> Finally
>>>>>>    > realized that it is related to the use of using cpupool-numa-split
>>>>>>    > (libxl with xen-4.4 maybe, but not 100% sure 4.3 as well).
>>>>>>    >
>>>>>>    > What happens is that on shutdown the hypervisor runs
>>>>>> disable_nonboot_cpus which
>>>>>>    > call cpu_down for each online cpu. There is a BUG_ON in the code for
>>>>>> the case of
>>>>>>    > cpu_down returning -EBUSY. This happens in my case as soon as the
>>>>>> first cpu that
>>>>>>    > has been moved to pool-1 by cpupool-numa-split is attempted. The
>>>>>> error is
>>>>>>    > returned by running the notifier_call_chain and I suspect that ends
>>>>>> up calling
>>>>>>    > cpupool_cpu_remove which always returns EBUSY for cpus not in pool0.
>>>>>>    >
>>>>>>    > I am not sure which end needs to be fixed but looping over all online
>>>>>> cpus in
>>>>>>    > disable_nonboot_cpus sounds plausible. So maybe the check for pool-0 in
>>>>>>    > cpupool_cpu_remove is wrong...?
>>>>>>    >
>>>>>>    > -Stefan
>>>>>>
>>>>>> Hmm yes - this looks completely broken.
>>>>>>
>>>>>> cpupool_cpu_remove() only has a single caller which is from cpu_down(),
>>>>>> and will unconditionally fail for cpus outside of the default pool.
>>>>>>
>>>>>> It is not obvious at all how this is supposed to work, and the comment
>>>>>> beside cpupool_cpu_remove() doesn't help.
>>>>>>
>>>>>> Can you try the following (only compile tested) patch, which looks
>>>>>> plausibly like it might DTRT.  The for_each_cpupool() is a little nastly
>>>>>> but there appears to be no cpu_to_cpupool mapping available.
>>>>>
>>>>> Your patch has the disadvantage to support hot-unplug of the last cpu in
>>>>> a cpupool. The following should work, however:
>>>>
>>>> Disadvantage and support sounded a bit confusing. But I think it means
>>>> hot-unplugging the last cpu of a pool is bad and should not be working.
>>>
>>> Correct.
>>>
>>>>
>>>>>
>>>>> diff --git a/xen/common/cpupool.c b/xen/common/cpupool.c
>>>>> index 4a0e569..73249d3 100644
>>>>> --- a/xen/common/cpupool.c
>>>>> +++ b/xen/common/cpupool.c
>>>>> @@ -471,12 +471,24 @@ static void cpupool_cpu_add(unsigned int cpu)
>>>>>     */
>>>>>    static int cpupool_cpu_remove(unsigned int cpu)
>>>>>    {
>>>>> -    int ret = 0;
>>>>> +    int ret = -EBUSY;
>>>>> +    struct cpupool **c;
>>>>>
>>>>>        spin_lock(&cpupool_lock);
>>>>> -    if ( !cpumask_test_cpu(cpu, cpupool0->cpu_valid))
>>>>> -        ret = -EBUSY;
>>>>> +    if ( cpumask_test_cpu(cpu, cpupool0->cpu_valid) )
>>>>> +        ret = 0;
>>>>>        else
>>>>> +    {
>>>>> +        for_each_cpupool(c)
>>>>> +        {
>>>>> +            if ( cpumask_test_cpu(cpu, (*c)->cpu_suspended ) )
>>>>
>>>> The rest seems to keep the semantics the same as before (though does that mean
>>>> unplugging the last cpu of pool-0 is ok?) But why testing for suspended here to
>>>> succeed (and not valid)?
>>>
>>> Testing valid would again enable to remove the last cpu of a cpupool in
>>> case of hotplugging. cpu_suspended is set if all cpus are to be removed
>>> due to shutdown, suspend to ram/disk, ...
>>
>> Ah, ok. Thanks for the detail explanation. So I was trying this change in
>> parallel and can confirm that it gets rid of the panic on shutdown. But when I
>> try to offline any cpu in pool1 (if echoing 0 into /sys/devices/xen_cpu/xen_cpu?
>> is the correct method) I always get EBUSY. IOW I cannot hot-unplug any cpu that
>> is in a pool other than 0. It does only work after removing it from pool1, then
>> add it to pool0 and then echo 0 into online.
> 
> That's how it was designed some years ago. I don't want to change the
> behavior in the hypervisor. Adding some tool support could make sense,
> however.

Ok, so in that case everything works as expected and the change fixes the
currently broken shutdown and could be properly submitted for inclusion (with my
tested-by).

-Stefan
> 
> Juergen
> 



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