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Re: [Xen-devel] [PATCH v2] xen/timers: Fix memory leak with cpu unplug/plug


  • To: Julien Grall <julien.grall@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 8 Apr 2019 13:09:07 +0100
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>
  • Delivery-date: Mon, 08 Apr 2019 12:09:22 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 08/04/2019 12:38, Julien Grall wrote:
> Hi,
>
> On 4/8/19 11:47 AM, Andrew Cooper wrote:
>> On 08/04/2019 11:39, Julien Grall wrote:
>>> Hi,
>>>
>>> On 4/8/19 10:39 AM, Andrew Cooper wrote:
>>>> +    case CPU_RESUME_FAILED:
>>>> +        if ( !park_offline_cpus && system_state !=
>>>> SYS_STATE_suspend )
>>>
>>> This patch breaks compilation on arm32/arm64 because park_offline_cpus
>>> is not defined:
>>>
>>> timer.c: In function 'cpu_callback':
>>> timer.c:651:15: error: 'park_offline_cpus' undeclared (first use in
>>> this function)
>>>           if ( !park_offline_cpus && system_state !=
>>> SYS_STATE_suspend )
>>>                 ^~~~~~~~~~~~~~~~~
>>>
>>> What is the purpose of park_offline_cpus?
>>
>> Sorry.  I should have waited for a full build test first.
>>
>> park_offline_cpus is a workaround for Intel's MCE behaviour, where the
>> system will shut down rather than deliver an #MC if machine checking
>> isn't configured on all CPUs.
>>
>> As a result, we have to start all CPUs, even beyond maxcpus= and set up
>> machine check handling, and never ever free their stacks, even if we'd
>> prefer the CPUs to be offline.
>
> I am a bit confused, why this is necessary now for the timer and not
> in other places of the common code?
>
>>
>> Are you happy with a
>>
>> #define park_offline_cpus false >
>> in ARM?
>
> The name is fairly confusing if you don't know the background.
>
> But I have to admit that even with your explanation above, I still
> don't understand why you need to check park_offline_cpus in the timers.

It is all to do with how/when we free per-cpu data.

Technically speaking (with the memory leak fixed) the old arrangement
ought to function correctly, but the new arrangement is more efficient.

~Andrew

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