[Xen-devel] Re: [PATCH] Fix cpu online/offline bug: mce memory leak.

[Haitao Shan <maillists.shan@xxxxxxxxx>]

Ah, yes. I was misled by the patch name itself. :)

Smart fix, isn't it! I guess the issue is closed. Thanks, Jinsong.

Shan Haitao

2011/3/1 Keir Fraser <keir.xen@xxxxxxxxx>

On 01/03/2011 10:12, "Haitao Shan" <maillists.shan@xxxxxxxxx> wrote:

> Hi, Keir,
>
> Fixing memory leak would be always good. But I don't agree this could fix the
> problem I observed. 
> Yes, indeed, I observed the issue during stress test of cpu offline/online by
> offlining all cpus except CPU0 one by one and then bringing them back. In this
> case, with memory leak, after some time, xmalloc at onlining could hit a heap
> page that is formerly owned by domains, since usable Xen heap memory will be
> slowly used up. Without memory leak, xmalloc at onlining will be likely use
> the memory that is freed just by offlining. So it won't trigger this
> assertion.
>
> But let us think a typical usage model, you will offline all LPs on a socket
> so that it can be later removed physically. Some other time, you will bring
> them back. Between offlining and onlining, there could be a time interval as
> long as one week and activities such as creating and killing many guests. How
> can we ensure that we won't meet this assertion at that time?
>
> It is my understanding that this memory leak triggered the issue I raised but
> the issue itself can be triggered in many other ways. Please do correct me if
> I am wrong. Thanks!

The point is that the patch also moves the xmalloc() calls out of the new
CPU's bringup path, and into CPU_UP_PREPARE context. This means the
allocations will occur on a different CPU, before the new CPU begins
execution, and with irqs enabled. That's why it should fix your crash.

 -- Keir

> Shan Haitao
>
> 2011/3/1 Keir Fraser <keir.xen@xxxxxxxxx>
>> Some comments inline below. Overall a good patch to have, but needs some
>> minor adjustments!
>>
>>  -- Keir
>>
>> On 01/03/2011 09:09, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx> wrote:
>>
>>> Fix cpu online/offline bug: mce memory leak.
>>>
>>> Current Xen mce logic didn't free mcabanks. This would be a memory leak when
>>> cpu offline.
>>> When repeatly do cpu online/offline, this memory leak would make xenpool
>>> shrink, and at a time point,
>>> will call alloc_heap_pages --> flush_area_mask, which
>>> ASSERT(local_irq_is_enabled()).
>>> However, cpu online is irq disable, so it finally result in Xen crash.
>>>
>>> This patch fix the memory leak bug, and tested OK over 110,000 round cpu
>>> online/offline.
>>>
>>> Signed-off-by: Liu, Jinsong <jinsong.liu@xxxxxxxxx>
>>>
>>> diff -r 1a364b17d66a xen/arch/x86/cpu/mcheck/mce_intel.c
>>> --- a/xen/arch/x86/cpu/mcheck/mce_intel.c Fri Feb 25 01:26:01 2011 +0800
>>> +++ b/xen/arch/x86/cpu/mcheck/mce_intel.c Mon Feb 28 19:19:20 2011 +0800
>>> @@ -1227,9 +1227,24 @@ static void intel_init_mce(void)
>>>      mce_uhandler_num = sizeof(intel_mce_uhandlers)/sizeof(struct
>>> mca_error_handler);
>>>  }
>>>
>>> -static int intel_init_mca_banks(void)
>>> +static void cpu_mcabank_free(unsigned int cpu)
>>>  {
>>> -    struct mca_banks *mb1, *mb2, * mb3;
>>> +    struct mca_banks *mb1, *mb2, *mb3, *mb4;
>>> +
>>> +    mb1 = per_cpu(mce_clear_banks, cpu);
>>> +    mb2 = per_cpu(no_cmci_banks, cpu);
>>> +    mb3 = per_cpu(mce_banks_owned, cpu);
>>> +    mb4 = per_cpu(poll_bankmask, cpu);
>>> +
>>> +    mcabanks_free(mb1);
>>> +    mcabanks_free(mb2);
>>> +    mcabanks_free(mb3);
>>> +    mcabanks_free(mb4);
>>> +}
>>> +
>>> +static void cpu_mcabank_alloc(unsigned int cpu)
>>> +{
>>> +    struct mca_banks *mb1, *mb2, *mb3;
>>>
>>>      mb1 = mcabanks_alloc();
>>>      mb2 = mcabanks_alloc();
>>> @@ -1237,22 +1252,23 @@ static int intel_init_mca_banks(void)
>>>      if (!mb1 || !mb2 || !mb3)
>>>          goto out;
>>>
>>> -    __get_cpu_var(mce_clear_banks) = mb1;
>>> -    __get_cpu_var(no_cmci_banks) = mb2;
>>> -    __get_cpu_var(mce_banks_owned) = mb3;
>>> +    per_cpu(mce_clear_banks, cpu) = mb1;
>>> +    per_cpu(no_cmci_banks, cpu) = mb2;
>>> +    per_cpu(mce_banks_owned, cpu) = mb3;
>>> +    return;
>>>
>>> -    return 0;
>>>  out:
>>>      mcabanks_free(mb1);
>>>      mcabanks_free(mb2);
>>>      mcabanks_free(mb3);
>>> -    return -ENOMEM;
>>>  }
>>>
>>>  /* p4/p6 family have similar MCA initialization process */
>>>  enum mcheck_type intel_mcheck_init(struct cpuinfo_x86 *c)
>>>  {
>>> -    if (intel_init_mca_banks())
>>> +    if ( !this_cpu(mce_clear_banks) ||
>>> +         !this_cpu(no_cmci_banks)   ||
>>> +         !this_cpu(mce_banks_owned) )
>>>           return mcheck_none;
>>>
>>>      intel_init_mca(c);
>>> @@ -1301,13 +1317,19 @@ static int cpu_callback(
>>>  static int cpu_callback(
>>>      struct notifier_block *nfb, unsigned long action, void *hcpu)
>>>  {
>>> +    unsigned int cpu = (unsigned long)hcpu;
>>> +
>>>      switch ( action )
>>>      {
>>> +    case CPU_UP_PREPARE:
>>> +        cpu_mcabank_alloc(cpu);
>>> +        break;
>>>      case CPU_DYING:
>>>          cpu_mcheck_disable();
>>>          break;
>>>      case CPU_DEAD:
>>>          cpu_mcheck_distribute_cmci();
>>> +        cpu_mcabank_free(cpu);
>>>          break;
>>
>> You also need to cpu_mcabank_free(cpu) on CPU_UP_CANCELED. Else there's a
>> memory leak on failed CPU bringup.
>>
>>>      default:
>>>          break;
>>> @@ -1322,6 +1344,8 @@ static struct notifier_block cpu_nfb = {
>>>
>>>  static int __init intel_mce_initcall(void)
>>>  {
>>> +    void *hcpu = (void *)(long)smp_processor_id();
>>> +    cpu_callback(&cpu_nfb, CPU_UP_PREPARE, hcpu);
>>>      register_cpu_notifier(&cpu_nfb);
>>>      return 0;
>>>  }
>>> diff -r 1a364b17d66a xen/arch/x86/setup.c
>>> --- a/xen/arch/x86/setup.c Fri Feb 25 01:26:01 2011 +0800
>>> +++ b/xen/arch/x86/setup.c Mon Feb 28 19:19:20 2011 +0800
>>> @@ -1203,6 +1203,8 @@ void __init __start_xen(unsigned long mb
>>>
>>>      arch_init_memory();
>>>
>>> +    do_presmp_initcalls();
>>> +
>>>      identify_cpu(&boot_cpu_data);
>>>      if ( cpu_has_fxsr )
>>>          set_in_cr4(X86_CR4_OSFXSR);
>>> @@ -1235,8 +1237,6 @@ void __init __start_xen(unsigned long mb
>>>      initialize_keytable();
>>>
>>>      console_init_postirq();
>>> -
>>> -    do_presmp_initcalls();
>>
>> This looks risky, especially so close to 4.1 release. Instead of moving
>> do_presmp_initcalls(), please special-case cpu_mcabank_alloc() for CPU0 in
>> intel_mcheck_init(), and remove your direct call to
>> cpu_callback(...CPU_UP_PREPARE...) in intel_mce_initcall().
>>
>>>      for_each_present_cpu ( i )
>>>      {
>>
>>
>
>

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