linux-kernel - Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to non-NO_HZ

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Date:	Fri, 11 Jul 2014 20:25:43 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to
 non-NO_HZ_FULL CPUs

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.
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