Re: [Xenomai] shielding fully CPU or CORE

[Jan Kiszka <jan.kiszka@web.de>]

On 2013-07-20 10:31, Gilles Chanteperdrix wrote:
> On 07/20/2013 09:54 AM, Jan Kiszka wrote:
> 
>> On 2013-07-19 11:17, Skander Bahloul wrote:
>>> Dear Xenomai list members,
>>>
>>> On a successful instal (but without NVIDIA driver, cf my previous
>>> message), I am now starting using Xenomai.
>>>
>>> We need to :
>>> 1* shield fully a CPU or core, against the scheduler, interruption, etc
>>> 2* run a single thread on this isolated CPU or core
>>>
>>> Step 2 is OK with the Xenomai API.
>>> But how should I achieve step 1 ?
>>>
>>> I did not find advice in the Xenomai documentation.
>>> I guess it should be done with :
>>>     $ sudo shield -a 0-3 -m -1
>>>
>>> Do you agree ?
>>> When may I find detailed info on CPU shielding ?
>>
>> With "shield fully", do you mean 99,9...% or true 100%? The latter is
>> not possible with current Linux kernels, thus also not with Xenomai. For
>> the former, try kernel parameter isocpus (to keep Linux off certain
>> cores), possibly in combination with xeno_hal.rthal_supported_cpus (to
>> limit Xenomai to those cores).
> 
> 
> Maybe a stupid idea, but can we not shutdown linux cpus, using cpu
> hotplug, so as to have full isolation?

We can, but we will also have to
 - reboot those CPUs
 - handle switches to Linux on the isolated cores (e.g. by migrating
   the task to a Linux core)
 - handle changes to the mm that is in use on isolated cores (TLB
   shotdowns)
 - handle Linux signals sent to Xenomai tasks on isolated cores

And there might be more subtle traps when using a core under Xenomai
that is offline for Linux. Nothing is impossible, but this is surely not
straightforward.

Jan


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