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

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

On Fri, Jul 11, 2014 at 08:25:43PM +0200, Frederic Weisbecker wrote:
> 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.

When CONFIG_NO_HZ_FULL_SYSIDLE=y, then housekeeping kthreads are bound to
CPU 0.  However, doing this causes significant slowdowns according to
Fengguang's testing, so when CONFIG_NO_HZ_FULL_SYSIDLE=n, I bind the
housekeeping kthreads to the set of non-nohz_full CPUs.

> > 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.

When we have CONFIG_NO_HZ_FULL_SYSIDLE=y, agreed.  In that case, having
housekeeping CPUs on CPUs other than CPU 0 means that you never reach
full-system-idle state.

But in other cases, we appear to need more than one housekeeping CPU.
This is especially the case when people run general workloads on systems
that have NO_HZ_FULL=y, which appears to be a significant fraction of
the systems these days.

							Thanx, Paul