Re: CPU Hotplug rework

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2012-03-20 at 10:12 +1030, Rusty Russell wrote:
> On Mon, 19 Mar 2012 20:14:25 +0530, "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> wrote:
> > Hi,
> > 
> > There had been some discussion on CPU Hotplug redesign/rework
> > some time ago, but it was buried under a thread with a different
> > subject.
> > (http://thread.gmane.org/gmane.linux.kernel/1246208/focus=1246404)
> > 
> > So I am opening a new thread with an appropriate subject to discuss
> > what needs to be done and how to go about it, as part of the rework.
> > 
> > Peter Zijlstra and Paul McKenney had come up with TODO lists for the
> > rework, and here are their extracts from the previous discussion:
> 
> This is possible, but quite a lot of tricky auditing work.  There's an
> underlying assumption that stop_machine is the slow part, since it feels
> so heavy.

Depends on the machine and the needs. For the regular desktop with a
regular kernel, the stop_machine in hotplug isn't really a problem. For
_BIG_ machines stop_machine is a problem, for -RT stop_machine is a
problem.

So if we're going to re-architect hotplug anyway, it would be very good
to get rid of it, because I really don't see any hard reasons why we
would need it.

> Unfortunately, this doesn't seem to be the case in my testing.  The time
> for hotplug seems to be moving all the threads around.  So how about:

Agreed, the thread creation on online is the most expensive operation.

> (1) Let's not shutdown per-cpu kthreads, just leave them there to run
>     if the CPU comes back.

Wasn't as easy as it sounds, but should be doable.

> (2) Do something more efficient with userspace threads than migrating
>     them one at a time.

Sadly that can't really be done. We need to pick up every task
(userspace, but also running kernel threads) and update their state.

> Otherwise, we risk doing a great deal of work and gaining nothing
> (cleanups aside, of course).

I don't really think its possible to spend too much time cleaning up
hotplug at this point :-)