On Tue, Apr 30, 2013 at 02:46:12PM -0400, Dave Jones wrote:
> 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.
I must defer to the Kconfig experts on this issue. I have added them on CC.
> 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.
In the absence of any Kconfig change, would the following be more helpful?
+choice
+ prompt "Build-forced no-CBs CPUs"
+ default RCU_NOCB_CPU_NONE
+ help
+ This option allows no-CBs CPUs (whose RCU callbacks are invoked
+ from kthreads rather than from softirq context) 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, whose RCU callbacks will be invoked by per-CPU
+ rcuo kthreads. All other CPUs will invoke their own RCU
+ callbacks in softirq context.
+
+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, so that its
+ RCU callbacks are invoked by a per-CPU rcuo kthread.
+ Additional CPUs may be designated as no-CBs CPUs using the
+ rcu_nocbs= boot parameter will be no-CBs CPUs. All other CPUs
+ will invoke their own RCU callbacks in softirq context.
+
+ 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. All CPUs' RCU callbacks will
+ be executed in the context of per-CPU rcuo kthreads created
+ for this purpose.
+
+ Select this if all CPUs need to be no-CBs CPUs for real-time
+ or energy-efficiency reasons.
> I nominate this changeset for an award for most obtuse kconfig text in 3.10
> (so far)
Woo-hoo!!! An award! ;-)
Thanx, Paul
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