smp: Start up non-boot CPUs asynchronously

[Arjan van de Ven]

On 2/14/2012 11:57 AM, Srivatsa S. Bhat wrote:

>> In addition to this, the reality is that the whole "bring cpus up"
>> sequence needs to be changed; the current one is very messy and requires
>> the hotplug lock for the whole bring up of each individual cpu... which
>> is a very unfortunate design; a much better design would be to only take
>> the lock for the actual registration of the newly brought up CPU to the
>> kernel, while running the physical bringup without the global lock.
>> If/when that change gets made, we can do the physical bring up in
>> parallel (with each other, but also with the rest of the kernel boot),
>> and do the registration en-mass at some convenient time in the boot,
>> potentially late.
>>
> 
> 
> Sounds like a good idea, but how will we take care of CPU_UP_PREPARE and
> CPU_STARTING callbacks then? Because, CPU_UP_PREPARE callbacks are run
> before bringing up the cpu and CPU_STARTING is called from the cpu that is
> coming up. Also, CPU_UP_PREPARE callbacks can be failed, which can lead
> to that particular cpu boot getting aborted. With the "late commissioning
> of CPUs" idea you proposed above, retaining such semantics could become
> very challenging.

some of these callbacks may need to be redesigned as well; or at least,
we may need to decouple the "physical" state of the CPU that's getting
brought up from the "logical" OS visible one.