rcu: Apply Dave Jones's NOCB Kconfig help feedback

The Kconfig help text for the RCU_NOCB_CPU_NONE, RCU_NOCB_CPU_ZERO,
and RCU_NOCB_CPU_ALL Kconfig options was unclear, so this commit
adds a bit more detail.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 25 insertions, 9 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 8df5116621ec..f54bc60f2afb 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -682,9 +682,10 @@ 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.
+          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"
@@ -692,25 +693,40 @@ config RCU_NOCB_CPU_NONE
         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.
+          no-CBs CPUs, whose RCU callbacks will be invoked by per-CPU
+          kthreads whose names begin with "rcuo".  All other CPUs will
+          invoke their own RCU callbacks in softirq context.
+
+          Select this option if you want to choose no-CBs CPUs at
+          boot time, for example, to allow testing of different no-CBs
+          configurations without having to rebuild the kernel each time.
 
 config RCU_NOCB_CPU_ZERO
         bool "CPU 0 is a build_forced no-CBs CPU"
         depends on RCU_NOCB_CPU && !NO_HZ_FULL
         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.
+          This option forces CPU 0 to be a no-CBs CPU, so that its RCU
+          callbacks are invoked by a per-CPU kthread whose name begins
+          with "rcuo".  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.
+          or energy-efficiency reasons, but the real reason it exists
+          is to ensure that randconfig testing covers mixed systems.
 
 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.
+          boot parameter will be ignored.  All CPUs' RCU callbacks will
+          be executed in the context of per-CPU rcuo kthreads created for
+          this purpose.  Assuming that the kthreads whose names start with
+          "rcuo" are bound to "housekeeping" CPUs, this reduces OS jitter
+          on the remaining CPUs, but might decrease memory locality during
+          RCU-callback invocation, thus potentially degrading throughput.
 
           Select this if all CPUs need to be no-CBs CPUs for real-time
           or energy-efficiency reasons.