linux-kernel - Re: vmstat: On demand vmstat workers V4

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Date:	Sat, 10 May 2014 18:12:34 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Tejun Heo <tj@...nel.org>, Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	hughd@...gle.com, viresh.kumar@...aro.org,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: vmstat: On demand vmstat workers V4

On Sat, May 10, 2014 at 02:20:36PM +0200, Thomas Gleixner wrote:
> On Fri, 9 May 2014, Paul E. McKenney wrote:
> 
> > On Sat, May 10, 2014 at 12:57:15AM +0200, Thomas Gleixner wrote:
> > > On Fri, 9 May 2014, Christoph Lameter wrote:
> > > > On Fri, 9 May 2014, Thomas Gleixner wrote:
> > > > > I understand why you want to get this done by a housekeeper, I just
> > > > > did not understand why we need this whole move it around business is
> > > > > required.
> > > > 
> > > > This came about because of another objection against having it simply
> > > > fixed to a processor. After all that processor may be disabled etc etc.
> > > 
> > > I really regret that I did not pay more attention (though my cycle
> > > constraints simply do not allow it).
> > 
> > As far as I can see, the NO_HZ_FULL timekeeping CPU is always zero.  If it
> > can change in NO_HZ_FULL kernels, RCU will do some very strange things!
> 
> Good. I seriously hope it stays that way.

Unless and until systems end up with so many CPUs that a single CPU
cannot keep up with all the housekeeping tasks.  But we should wait to
burn that bridge until after we drive off it.  ;-)

> > One possible issue here is that Christoph's patch is unconditional.
> > It takes effect for both NO_HZ_FULL and !NO_HZ_FULL.  If I recall
> > correctly, the timekeeping CPU -can- change in !NO_HZ_FULL kernels,
> > which might be what Christoph was trying to take into account.
> 
> Ok. Sorry, I was just in a lousy mood after wasting half a day in
> reviewing even lousier patches related to that NO_HZ* muck.

I can relate...

> So, right with NO_HZ_IDLE the time keeper can move around and
> housekeeping stuff might want to move around as well.
> 
> But it's not necessary a good idea to bundle that with the timekeeper,
> as under certain conditions the timekeeper duty can move around fast
> and left unassigned again when the system is fully idle.
> 
> And we really do not want a gazillion of sites which implement a
> metric ton of different ways to connect some random housekeeping jobs
> with the timekeeper.
> 
> So the proper solution to this is to have either a thread or a
> dedicated housekeeping worker, which is placed by the scheduler
> depending on the system configuration and workload.
> 
> That way it can be kept at cpu0 for the nohz=off and the nohz_full
> case. In the nohz_idle case we can have different placement
> algorithms. On a big/little ARM machine you probably want to keep it
> on the first cpu of one or the other cluster. And there might be other
> constraints on servers.
> 
> So we are way better of with a generic facility, where the various
> housekeeping jobs can be queued.
> 
> Does that make sense?

It might well.

Here is what I currently do for RCU:

1.	If !NO_HZ_FULL, I let the grace-period kthreads run wherever
	the scheduler wants them to.

2.	If NO_HZ_FULL, I bind the grace-period kthreads to the
	timekeeping CPU.

But if I could just mark it as a housekeeping kthread and have something
take care of it.

So let's see...

Your nohz=off case recognizes a real-time setup, correct?  In which
case it does make sense to get the housekeeping out of the way of the
worker CPUs.  I would look pretty silly arguing against the nohz_full
case, since that is what RCU does.  Right now I just pay attention to
the Kconfig parameter, but perhaps it would make sense to also look at
the boot parameters.  Especially since some distros seem to be setting
NO_HZ_FULL by default.  ;-)

							Thanx, Paul

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