Re: [PATCH tip/core/rcu 04/14] rcu: Provide compile-time control for no-CBs CPUs

[Josh Triplett <josh@joshtriplett.org>]

On Sat, Jan 05, 2013 at 09:48:54AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> 
> Currently, the only way to specify no-CBs CPUs is via the rcu_nocbs
> kernel command-line parameter.  This is inconvenient in some cases,
> particularly for randconfig testing, so this commit adds a new
> RCU_NOCB_CPU_DEFAULT kernel configuration parameter.  Setting this
> new parameter to zero (the default) retains the old behavior, setting
> it to one offloads callback processing from CPU 0 (along with any
> other CPUs specified by the rcu_nocbs boot-time parameter), and setting
> it to two offloads callback processing from all CPUs.
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Comments below.

>  init/Kconfig            |   13 +++++++++++++
>  kernel/rcutree_plugin.h |   13 +++++++++++++
>  2 files changed, 26 insertions(+), 0 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index fc6a3ca..35dcedb 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -676,6 +676,19 @@ config RCU_NOCB_CPU
>  	  Say Y here if you want to help to debug reduced OS jitter.
>  	  Say N here if you are unsure.
>  
> +config RCU_NOCB_CPU_DEFAULT
> +	int "Offload RCU callback processing from compile-selected CPUs"
> +	depends on RCU_NOCB_CPU
> +	range 0 2
> +	default 0
> +	help
> +	  Set this option to zero to only offload RCU callback processing
> +	  from those CPUs specified by the boot-time rcu_nocbs kernel
> +	  parameter.  Set it to one to offload processing from CPU 0
> +	  in addition to any CPUs specified at boot time.  Set it to
> +	  two to offload processing from all CPUs, regardless of the
> +	  setting of the boot-time rcu_nocbs kernel parameter.
> +

Rather than making users translate the meanings of 0, 1, and 2, this
seems like something better done with a choice/endchoice; that would
then offer a menu of options that can each have their own description.
Take a look at the "Memory split" option in arch/x86/Kconfig for an
example.

> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -86,6 +86,19 @@ static void __init rcu_bootup_announce_oddness(void)
>  	if (nr_cpu_ids != NR_CPUS)
>  		printk(KERN_INFO "\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
>  #ifdef CONFIG_RCU_NOCB_CPU
> +#if CONFIG_RCU_NOCB_CPU_DEFAULT != 0

That would then make each of these conditionals self-documenting as
well.

- Josh Triplett