Re: [ARM] Bash often segfaults in Dom0 with the latest Xen

[Julien Grall <julien.grall@linaro.org>]

On 06/05/2013 07:17 PM, Christoffer Dall wrote:

> On 5 June 2013 11:01, Stefano Stabellini
> <stefano.stabellini@eu.citrix.com> wrote:
>> On Wed, 5 Jun 2013, Christoffer Dall wrote:
>>> On 5 June 2013 10:53, Julien Grall <julien.grall@linaro.org> wrote:
>>>> On 06/05/2013 06:36 PM, Christoffer Dall wrote:
>>>>
>>>>>>
>>>>>> I'm using the linaro's branch ll_20130528.0, I have only few patches for
>>>>>> the dts and not yet in linaro tree patches.
>>>>>>
>>>>>> I have the same issue with linux 3.9-rc4 with multiple CPUs and I can't
>>>>>> really go before without carrying many xen patches to try it.
>>>>>>
>>>>>> I have tried different configuration with the number of CPUs in Xen
>>>>>> (pCPU) and linux (vCPU):
>>>>>>   - 2 pCPU 2 vCPU : segfaulting
>>>>>>   - 2 pCPU 1 vCPU : working
>>>>>>   - 1 pCPU 1 vCPU : working
>>>>>>   - 1 pCPU 2 vCPU : very slow but working
>>>>>>
>>>>> 2 pCPU 1 vCPU are you still compiling your dom0 as an SMP kernel, but
>>>>> only creating 1 vCPU or are you actually compiling the dom0 as UP?
>>>>
>>>>
>>>> Yes. It's same kernel with the same command line (ie without nosmp).
>>>> I have limited the number of dom0 vcpus with dom0_max_vcpus=1 on xen
>>>> command line.
>>>>
>>> It indicates a bug in Xen then. Curious that it only happens for user
>>> space in dom0, but perhaps you just haven't seen it in the kernel yet.
>>> Bash scripts are pretty intensive on page faults so perhaps there's a
>>> synchronization issue with some of your page fault handlers.
>>>
>>> You could try to touch all the memory inside dom0 (dd to a ramfs for
>>> example) and then run your bash script and see if the problem still
>>> occurs, that should point you to whether it's a stage-2 fault handling
>>> issue, but this is not a fool-proof approach. Maybe Xen can
>>> pre-allocate all the stage-2 entries?
>>
>> Xen pre-allocates all the memory for stage-2 entries (no overcommit or
>> populate on demand by default)
> 
> what was the conclusion when pinning the vcpu to dedicated pcpus - did
> the error still show up?


If I have 2 pCPU (CPU 0 and CPU 2) and 1 vCPU which is moving every 2
second between the pCPUs, Linux will freeze each time the vcpu is
running on CPU 1. I'm not sure why, perhaps another issue.

As Stefano advised me, I will setup the debugger on the arndale tomorrow
and see if I can find something.

-- 
Julien