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    SubjectRe: [PATCH tip/core/rcu 04/14] rcu: Provide compile-time control for no-CBs CPUs
    On Mon, Jan 07, 2013 at 08:50:23AM -0800, Josh Triplett wrote:
    > 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.

    My first reaction was that this couldn't possibly handle the dependency
    on CONFIG_RCU_NOCB_CPU, but it actually does work, suppressing all the
    choices when CONFIG_RCU_NOCB_CPU=n. Cute!

    > > --- 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.

    Indeed, much better! Thank you!

    Thanx, Paul



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