linux-kernel - Re: rcu: Provide compile-time control for no-CBs CPUs

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Message-ID: <20130430184612.GA14568@redhat.com>
Date:	Tue, 30 Apr 2013 14:46:12 -0400
From:	Dave Jones <davej@...hat.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	paul.mckenney@...aro.org
Subject: Re: rcu: Provide compile-time control for no-CBs CPUs

On Tue, Apr 30, 2013 at 03:21:26PM +0000, Linux Kernel wrote:

 > +choice
 > +	prompt "Build-forced no-CBs CPUs"
 > +	default RCU_NOCB_CPU_NONE
 > +	help
 > +	  This option allows no-CBs CPUs to be specified at build time.
 > +	  Additional no-CBs CPUs may be specified by the rcu_nocbs=
 > +	  boot parameter.
 > +
 > +config RCU_NOCB_CPU_NONE
 > +	bool "No build_forced no-CBs CPUs"
 > +	depends on RCU_NOCB_CPU
 > +	help
 > +	  This option does not force any of the CPUs to be no-CBs CPUs.
 > +	  Only CPUs designated by the rcu_nocbs= boot parameter will be
 > +	  no-CBs CPUs.
 > +
 > +config RCU_NOCB_CPU_ZERO
 > +	bool "CPU 0 is a build_forced no-CBs CPU"
 > +	depends on RCU_NOCB_CPU
 > +	help
 > +	  This option forces CPU 0 to be a no-CBs CPU.  Additional CPUs
 > +	  may be designated as no-CBs CPUs using the rcu_nocbs= boot
 > +	  parameter will be no-CBs CPUs.
 > +
 > +	  Select this if CPU 0 needs to be a no-CBs CPU for real-time
 > +	  or energy-efficiency reasons.
 > +
 > +config RCU_NOCB_CPU_ALL
 > +	bool "All CPUs are build_forced no-CBs CPUs"
 > +	depends on RCU_NOCB_CPU
 > +	help
 > +	  This option forces all CPUs to be no-CBs CPUs.  The rcu_nocbs=
 > +	  boot parameter will be ignored.
 > +
 > +	  Select this if all CPUs need to be no-CBs CPUs for real-time
 > +	  or energy-efficiency reasons.

Note that entering '?' when prompted in oldconfig only outputs the help
parameter from the choice section.

Additionally, nowhere in any of this text does it say what a "no-CB CPU" is,
or why I would care, or even what the downsides are for each option.

I nominate this changeset for an award for most obtuse kconfig text in 3.10 (so far)

	Dave

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