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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jun 06, 2012 at 05:46:40PM +0200, Peter Zijlstra wrote:
> On Wed, 2012-06-06 at 07:44 -0700, Paul E. McKenney wrote:
> 
> > > I don't think the isolation means userspace while(1) applications is
> > > interesting. Sure, some people do this, and we should dtrt for them, but
> > > the far more interesting case is 'regular' applications that do use
> > > system calls.
> > 
> > OK, I will bite.  What are the semantics/properties for your isolated set?
> 
> The scheduler will not place tasks from outside the set in the set and
> vice versa. Applications outside the set should not affect those in the
> set, except there where there are shared resources across the set
> boundary.
> 
> So the example of 1 process with multiple threads, some inside some
> outside have the obvious shared resource of the address space, hence TLB
> invalidates etc. will come through.

So, for example, a way of nicely partitioning the system to allow multiple
real-time applications to run without needing to do cross-partition
global priority queuing of the real-time tasks.  Cool!

> Now the kernel as a whole is also a shared resource, and this is where
> it all gets tricky, since if something inside the set ends up doing a
> memory allocation, it will have to participate in mm/ locks etc.

Yep.

> Same with RCU, if you cannot stay in an extended grace period for some
> reason or other, you have to participate in the global RCU state
> machinery.

Yep.

> But it should be so that if you don't do any of these things, you should
> also not be affected by them.
> 
> Now I realize this is a 'weak' model, but the 'strong' model proposed by
> tglx would make it impossible to run anything but the while(1) stuff,
> and that's of limited utility.

Thomas's strong model also supports a strong form of idle, as well as the
while(1) stuff, both of which have their uses.

							Thanx, Paul