[RFC PATCH 1/1] rcu: Allow user to ovveride RCU_NOCB_CPU_ALL at boot time

[Pranith Kumar <bobby.prani@gmail.com>]

A kernel built with RCU_NOCB_CPU_ALL build time option will offload callbacks
from all CPUs. The user cannot override this behavior without recompiling the
kernel with the RCU_NOCB_CPU_ALL option turned off. 

This commit allows the user to override the build-time option by using the
rcu_nocbs= boot time option without needing to recompile the kernel.

Please note that this is how NO_HZ_FULL_ALL build time option works and this
commit makes it work similar to that.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 init/Kconfig             | 14 +++++++-------
 kernel/rcu/tree_plugin.h |  8 ++++----
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 41066e4..7d363a4 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -768,13 +768,13 @@ 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.  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.
+	  If the user doesn't pass the rcu_nocbs= boot option, force all CPUs
+	  to be no-CBs CPUs.  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.
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a2113f6..b97a939 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -92,15 +92,15 @@ static void __init rcu_bootup_announce_oddness(void)
 	if (!have_rcu_nocb_mask) {
 		zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
 		have_rcu_nocb_mask = true;
+#ifdef CONFIG_RCU_NOCB_CPU_ALL
+		pr_info("\tOffload RCU callbacks from all CPUs\n");
+		cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
 	}
 #ifdef CONFIG_RCU_NOCB_CPU_ZERO
 	pr_info("\tOffload RCU callbacks from CPU 0\n");
 	cpumask_set_cpu(0, rcu_nocb_mask);
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
-#ifdef CONFIG_RCU_NOCB_CPU_ALL
-	pr_info("\tOffload RCU callbacks from all CPUs\n");
-	cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
 #endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
 	if (have_rcu_nocb_mask) {
 		if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
-- 
1.9.1