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

[Arjan van de Ven]

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.
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