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

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Message-ID: <20120610175456.GC2425@linux.vnet.ibm.com>
Date:	Sun, 10 Jun 2012 10:54:56 -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 Sun, Jun 10, 2012 at 12:58:56AM +0200, Frederic Weisbecker wrote:
> On Thu, Jun 07, 2012 at 03:45:08PM -0700, Paul E. McKenney wrote:
> > > 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?
> 
> I'm not sure of the possible consequences of that.

OK, so I will remove the TINY_RCU patches for the moment.  If reducing
tick speed on the sole remaining idle CPU ever becomes feasible and
useful, they are easy to add back in.

							Thanx, Paul

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