Re: 2.7 thoughts

[Rusty Russell <rusty@rustcorp.com.au>]

On Fri, 10 Oct 2003 11:56:09 -0700
Tim Hockin <thockin@hockin.org> wrote:

> On Fri, Oct 10, 2003 at 11:29:09AM -0700, William Lee Irwin III wrote:
> > I think there is some generalized cpu hotplug stuff that's gone in that
> > direction already, though I don't know any details. The bits about non-
> > cooperative offlining were very interesting to hear, though.
> 
> I spoke with Rusty about it at OLS.  I haven't tracked the hotplug CPU
> projects.  I think that TASK_UNRUNNABLE is the sane way to handle said edge
> case, but at the time Rusty was leaning towards SIGPOWER.

Yeah, if a task has become unrunnable, I do a SIGPOWER (with a new cpu siginfo
field).  If hotplug CPUs had been introduced at the same time as affinity,
this would IMHO have been a valid approach, but may not be so now.

See my kernel.org page, and the sourceforge mailing list for details.

I haven't done anything fancy with cpu offlining, but the most painful bit
is all the callbacks for workqueue threads, migration threads etc.  Once
that's in place, doing the atomic-style switch should be quite possible.
There's a theoretical case where code would do:

	spin_lock
	foo[smp_processor_id()]++;
	...
	foo[smp_processor_id()]--;
	spin_unlock

So you might want to fake up the answer to smp_processor_id() for that
task/interrupt.  Other real per-cpu things might have problems (if you
were halfway through fiddling with the interrupt state on that CPU,
maybe MTRR), but that's what makes it fun.

Cheers,
Rusty.
-- 
   there are those who do and those who hang on and you don't see too
   many doers quoting their contemporaries.  -- Larry McVoy