linux-kernel - Re: [GIT PULL] nohz: Full dynticks base interface

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Message-ID: <CAFTL4hxobFT_X_HMVOwB5WDHNR-YVkBjj0c2DMECPKy9goB3LQ@mail.gmail.com>
Date:	Wed, 27 Mar 2013 00:48:20 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.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: [GIT PULL] nohz: Full dynticks base interface

2013/3/25 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> On Mon, Mar 25, 2013 at 06:12:12PM +0100, Frederic Weisbecker wrote:
>> 2013/3/25 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
>> > On Sun, Mar 24, 2013 at 03:46:40PM +0100, Frederic Weisbecker wrote:
>> >> 2013/3/24 Ingo Molnar <mingo@...nel.org>:
>> >> >
>> >> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
>> >> >
>> >> >> Hi Ingo,
>> >> >>
>> >> >> This settles the initial ground to start a special full dynticks tree in -tip
>> >> >> that we can iterate incrementally to accelerate the development.
>> >> >> It is based on tip:sched/core.
>> >> >>
>> >> >> I tried to rearrange a bit the naming. We are probably not yet done with
>> >> >> that but I guess we can fix it along with the rest.
>> >> >>
>> >> >> Please pull from:
>> >> >>
>> >> >> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
>> >> >>       full-dynticks-for-mingo
>> >> >>
>> >> >> Changes on these commits since they were part of 3.9-rc1-nohz1:
>> >> >>
>> >> >> * Force a timekeeping CPU over the full dynticks range
>> >> >> * Rename CONFIG_NO_HZ_FULL to CONFIG_NO_HZ_EXTENDED
>> >> >> * Following *_nohz_extended_* APIs renames
>> >> >> * Handle CPU hotplug for timekeeping
>> >> >> * Rename full_nohz= kernel parameter to nohz_extended=
>> >> >
>> >> > Note that boot parameters suck for pretty much any purpose but quirks -
>> >> > please also add a (default off!) Kconfig option to easily enable
>> >> > nohz_extended for all CPUs.
>> >> >
>> >> > That way I will be able to test it automatically via randconfig and such.
>> >>
>> >> Sure, I'm adding such an option.
>> >
>> > Hmmm...  This would be an option to make all but one CPU an adaptive-ticks
>> > CPU, right?  If so, this leads to the question of whether I should add a
>> > matching no-CBs Kconfig option.  My guess is "no", because the existing
>> > CONFIG_RCU_NOCB_CPU_ALL should work just fine -- there would be a CPU that
>> > was not an adaptive-ticks CPU, but does have its RCU callbacks offloaded.
>> >
>> > Or am I missing something here?
>>
>> No that looks right. Now I wonder if I should select
>> CONFIG_RCU_NOCB_CPU_ALL at the same time. Probably.
>
> Sounds like a good initial position to me.  If it somehow causes problems,
> we can always change it later.

Ah "rcu: Provide compile-time control for no-CBs CPUs" is not yet in
-tip so I can't do that yet. Ok for now I'm going to add
CONFIG_NO_HZ_EXTENDED_ALL and will select the matching RCU config once
it's visible upstream.
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