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

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Message-ID: <20140711190816.GR16041@linux.vnet.ibm.com>
Date:	Fri, 11 Jul 2014 12:08:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 08:57:33PM +0200, Frederic Weisbecker wrote:
> 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.

Agreed, this is only a problem when people run workloads for which
NO_HZ_FULL is not well-suited.  Which is why I settled on designating
the non-nohz_full= CPUs as the housekeeping CPUs -- people wanting to
run general workloads not suited to NO_HZ_FULL probably won't specify
nohz_full=.  If they don't, then any CPU can be a housekeeping CPU.

> > > > 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.

Hmmm...  That probably means that we need boot-time parameters to
make sysidle detection really happen.  Otherwise, many users will
get a nasty surprise once CONFIG_NO_HZ_FULL_SYSIDLE=y is enabled on
systems that really aren't running HPC or RT workloads.

I suppose that I could confine SYSIDLE's attention to the nohz_full=
CPUs -- that might actually make things work nicely in all cases with
no configuration of any sort required.  I will need to give this some
thought.

> > 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.

Indeed!  ;-)

								Thanx, Paul

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