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 16:43, Stefan Bader wrote:
> 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).
> 

Is this needing something from my side to do? I could re-submit the whole patch
but it since it is Juergen's work it felt a little rude to do so.

-Stefan



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