rcu: Remove restrictions on no-CBs CPUs

Currently, CPU 0 is constrained to not be a no-CBs CPU, and furthermore
at least one no-CBs CPU must remain online at any given time.  These
restrictions are problematic in some situations, such as cases where
all CPUs must run a real-time workload that needs to be insulated from
OS jitter and latencies due to RCU callback invocation.  This commit
therefore provides no-CBs CPUs a (very crude and energy-inefficient)
way to start and to wait for grace periods independently of the normal
RCU callback mechanisms.  This approach allows any or all of the CPUs to
be designated as no-CBs CPUs, and allows any proper subset of the CPUs
(whether no-CBs CPUs or not) to be offlined.

This commit also provides a fix for a locking bug spotted by Xie
ChanglongX <changlongx.xie@intel.com>.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 22616cd434bc..c8bd349eb638 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -655,7 +655,7 @@ config RCU_BOOST_DELAY
           Accept the default if unsure.
 
 config RCU_NOCB_CPU
-        bool "Offload RCU callback processing from boot-selected CPUs"
+        bool "Offload RCU callback processing from boot-selected CPUs (EXPERIMENTAL"
         depends on TREE_RCU || TREE_PREEMPT_RCU
         default n
         help
@@ -673,7 +673,7 @@ config RCU_NOCB_CPU
           callback, and (2) affinity or cgroups can be used to force
           the kthreads to run on whatever set of CPUs is desired.
 
-          Say Y here if you want reduced OS jitter on selected CPUs.
+          Say Y here if you want to help to debug reduced OS jitter.
           Say N here if you are unsure.
 
 endmenu # "RCU Subsystem"