Date:	Fri, 11 Jul 2014 20:57:33 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Christoph Lameter <cl@...two.org>, 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 11:45:28AM -0700, Paul E. McKenney wrote:
> 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.

But did he see these slowdowns with nohz_full= parameter passed? I doubt he
tested that. And I'm not sure that people who need full dynticks will run
the usecases that trigger slowdowns with grace period kthreads.

I also doubt that people will often omit other CPUs than CPU 0 nohz_full=
range.

> 
> > > 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.

That said I expect CONFIG_NO_HZ_FULL_SYSIDLE=y to be always enable for those
who run NO_HZ_FULL in the long run.

> 
> 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.

Yeah NO_HZ_FULL=y is likely to be enabled in many distros. But you know the
amount of nohz_full= users.
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