linux-kernel - Re: [PATCH 0/2] rcu: Extended quiescent state for adaptive nohz

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Message-ID: <20120607224508.GX19601@linux.vnet.ibm.com>
Date:	Thu, 7 Jun 2012 15:45:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] rcu: Extended quiescent state for adaptive nohz

On Thu, Jun 07, 2012 at 04:21:09PM +0200, Frederic Weisbecker wrote:
> On Tue, Jun 05, 2012 at 04:46:40PM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 05, 2012 at 12:31:00PM +0200, Frederic Weisbecker wrote:
> > > On Mon, Jun 04, 2012 at 02:07:09PM -0700, Paul E. McKenney wrote:
> > > > On Mon, Jun 04, 2012 at 09:06:22PM +0200, Frederic Weisbecker wrote:
> > > > > 2012/6/4 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > > > > > On Mon, Jun 04, 2012 at 02:08:26PM +0200, fweisbec@...il.com wrote:
> > > > > >> From: Frederic Weisbecker <fweisbec@...il.com>
> > > > > >>
> > > > > >> Paul, Ingo,
> > > > > >>
> > > > > >> This is a rebase of the nohz cpusets RCU APIs on top of Paul's latest
> > > > > >> -rcu (rcu/core) branch.
> > > > > >>
> > > > > >> I have only built tested it yet, I need to do a full rebase of my
> > > > > >> tree to test it in practice. But I wanted to show you how it looks
> > > > > >> like first.
> > > > > >>
> > > > > >> I also wonder if we can set that to a tree somewhere. Ingo suggested
> > > > > >> to set up a tree on -tip to apply the uncontroversial part of nohz
> > > > > >> cpusets patches and iterate from there. I think it would accelerate
> > > > > >> everything if we start doing that.
> > > > > >
> > > > > > It would probably be best to put these two in the -rcu set in order to
> > > > > > avoid conflicts with possible further RCU_FAST_NO_HZ work.  I could
> > > > > > push this to -tip early, if that would help.
> > > > > 
> > > > > But then these APIs are going to be upstream on 3.6
> > > > > Is that ok for you even if they don't have any upstream user?
> > > > > We can ifdef it.
> > > > 
> > > > I figured on maintaining a separate rcu/idle topic branch that I would
> > > > merge locally for building and testing, but which I would not push
> > > > to rcu/next.  If Ingo agrees, I can push separately to -tip so that it
> > > > does not go upstream until you are ready, at which point I would merge
> > > > it into rcu/next.
> > > > 
> > > > Seem reasonable, or would something else work better?
> > > 
> > > Sounds very good!
> > 
> > Here you go:
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/idle
> 
> Thanks!
> 
> I can see you've implemented a version for TinyRCU. Nohz cpusets only work on
> SMP right now because there must be at least one CPU running with the tick
> to maintain the timekeeping. I'm pretty confident that one day we'll remove
> the jiffies and we'll be able to do the whole timekeeping by using the TSC
> or so. There is quite a way before we reach that though.

In the meantime, would it make sense to slow the tick rate by a factor
of 10 or so on that one CPU when nothing else is going on?  Or does
timekeeping absolutely require running the tick at full speed?

							Thanx, Paul

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