Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait ornmi

[Arjan van de Ven]

On 6/6/2012 8:49 AM, Paul E. McKenney wrote:
> On Wed, Jun 06, 2012 at 08:23:54AM -0700, Arjan van de Ven wrote:
>> On 6/6/2012 7:41 AM, Paul E. McKenney wrote:
>>> On Wed, Jun 06, 2012 at 10:43:43AM +0200, Peter Zijlstra wrote:
>>>> On Tue, 2012-06-05 at 15:12 -0700, Paul E. McKenney wrote:
>>>>>> What I can't see is the isolated functional, aside from the above
>>>>>> mentioned things, that's not strictly a per-cpu property, we can have a
>>>>>> group that's isolated from the rest but not from each other.
>>>>>
>>>>> I suspect that Thomas is thinking that the CPU is so idle that it no
>>>>> longer has to participate in TLB invalidation or RCU.  (Thomas will
>>>>> correct me if I am confused.)  But Peter, is that the level of idle
>>>>> you are thinking of? 
>>>>
>>>> No, we're talking about isolated, so its very much running something.
>>>
>>> From what I can see, if the CPU is running something, this is Thomas's
>>> "Isolated functional" state rather than his "Isolated idle" state.
>>> The isolated-idle state should not need to participate in TLB invalidation
>>> or RCU, so that the CPU never ever needs to wake up while in the
>>> isolated-idle state.
>>
>> btw TLB invalidation I think is a red herring in this discussion
>> (other than "global PTEs" kind of kernel pte changes);
>> at least on x86 this is not happening for a long time; if a CPU is
>> really idle (which means the CPU internally flushes the tlbs anyway),
>> Linux also switches to the kernel PTE set so there's no need for a flush
>> later on.
> 
> Right, as I understand it, only unmappings in the kernel address space
> would need to IPI an idle CPU.  But this is still a source of IPIs that
> could wake up the CPU, correct?

this is correct.
in practice, they happen pretty much only when you load or unload a
kernel module...

frankly I wouldn't worry about optimizing for that case ;-)
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