linux-kernel - Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks range

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Message-ID: <20130411191714.GQ29861@linux.vnet.ibm.com>
Date:	Thu, 11 Apr 2013 12:17:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks
 range

On Thu, Apr 11, 2013 at 07:28:05PM +0200, Frederic Weisbecker wrote:
> On Thu, Apr 11, 2013 at 10:11:53AM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 11, 2013 at 07:04:23PM +0200, Frederic Weisbecker wrote:
> > > On Thu, Apr 11, 2013 at 04:41:07PM +0000, Christoph Lameter wrote:
> > > > On Thu, 11 Apr 2013, Frederic Weisbecker wrote:
> > > > 
> > > > > If there is no performance issue with that I'm all for it.
> > > > 
> > > > Or have a
> > > > 
> > > > CONFIG_LOWLATENCY
> > > > 
> > > > that boots up a kernel with the proper configuration?
> > > 
> > > It may be too general for a naming. But I don't mind just
> > > selecting CONFIG_RCU_NOCBS_ALL unconditionally. It's easily
> > > changed in the future if anybody complains.
> > > 
> > > Btw, if CONFIG_RCU_NOCBS_ALL is set, the rcu_nocbs= parameter
> > > is ignored, right? If you want to keep that direction and not
> > > override the Kconfig choice, may be warn the user about that
> > > if the boot parameter is passed?
> > 
> > Fair point.  Let me think about how I should approach Thanx.
> 
> An alternative is to treat the Kconfig choice as a default setting
> that gets overriden by rcu_nocbs=
> 
> This would require to refactor the three way Kconfig layout you've set,
> but that aligns with what Ingo is suggesting me to do with the full dynticks
> range: having CONFIG_NO_HZ_EXTENDED_ALL that is overriden by nohz_extended=
> if any (beware though, "extended" will soon be renamed to "full", lets hope
> it's our last take ;)

So the idea is that if CONFIG_NO_HZ_EXTENDED_ALL is specified, the list
passed via nohz_extended= is ANDed rather than ORed?  Interesting...

> That unifies both behaviours and it looks more flexible to me.
> 
> Of course that means selecting CONFIG_RCU_NOCBS_ALL won't be enough for
> me to ensure my nohz range is also nocb, I'm fine with the cpumask check
> on boot though.
> 
> Anyway, that's just a suggestion, the most important is that the
> user is informed of what's happening and how to deal with it.

I will hold off a bit and see how things settle out.

							Thanx, Paul

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