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 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.

-Stefan


> 
> Juergen
> 
>>
>>
>>> +            {
>>> +                ret = 0;
>>> +                break;
>>> +            }
>>> +        }
>>> +    }
>>> +    if ( !ret )
>>>           cpumask_set_cpu(cpu, &cpupool_locked_cpus);
>>>       spin_unlock(&cpupool_lock);
>>>
>>>
>>>
>>> Juergen
> 



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