Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Jul 11, 2014 at 01:10:41PM -0500, Christoph Lameter wrote:
> On Tue, 8 Jul 2014, Frederic Weisbecker wrote:
> 
> > > I was figuring that a fair number of the kthreads might eventually
> > > be using this, not just for the grace-period kthreads.
> >
> > Ok makes sense. But can we just rename the cpumask to housekeeping_mask?
> 
> That would imply that all no-nohz processors are housekeeping? So all
> processors with a tick are housekeeping?

Well, now that I think about it again, I would really like to keep housekeeping
to CPU 0 when nohz_full= is passed.

> 
> Could we make that set configurable? Ideally I'd like to have the ability
> restrict the housekeeping to one processor.

Ah, I'm curious about your usecase. But I think we can do that. And we should.

In fact I think that Paul could keep affining grace period kthread to CPU 0
for the sole case when we have nohz_full= parameter passed.

I think the performance issues reported to him refer to CONFIG_NO_HZ_FULL=y
config without nohz_full= parameter passed. That's the most important to address.

Optimizing the "nohz_full= passed" case is probably not very useful and worse
it complicate things a lot.

What do you think Paul? Can we simplify things that way? I'm pretty sure that
nobody cares about optimizing the nohz_full= case. That would really simplify
things to stick to CPU 0.