rcu: Provide compile-time control for no-CBs CPUs

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 set of
kernel configuration parameters.  CONFIG_RCU_NOCB_CPU_NONE (the default)
retains the old behavior, CONFIG_RCU_NOCB_CPU_ZERO offloads callback
processing from CPU 0 (along with any other CPUs specified by the
rcu_nocbs boot-time parameter), and CONFIG_RCU_NOCB_CPU_ALL 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>

2 files changed, 53 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index c8bd349eb638..307499704580 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -676,6 +676,45 @@ config RCU_NOCB_CPU
           Say Y here if you want to help to debug reduced OS jitter.
           Say N here if you are unsure.
 
+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.
+
+config RCU_NOCB_CPU_NONE
+        bool "No build_forced no-CBs CPUs"
+        depends on RCU_NOCB_CPU
+        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.
+
+config RCU_NOCB_CPU_ZERO
+        bool "CPU 0 is a build_forced no-CBs CPU"
+        depends on RCU_NOCB_CPU
+        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.
+
+          Select this if CPU 0 needs to be a no-CBs CPU for real-time
+          or energy-efficiency reasons.
+
+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.
+
+          Select this if all CPUs need to be no-CBs CPUs for real-time
+          or energy-efficiency reasons.
+
+endchoice
+
 endmenu # "RCU Subsystem"
 
 config IKCONFIG
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 44f958a88b21..3e33aefce0ea 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -85,6 +85,20 @@ 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
+#ifndef CONFIG_RCU_NOCB_CPU_NONE
+        if (!have_rcu_nocb_mask) {
+                alloc_bootmem_cpumask_var(&rcu_nocb_mask);
+                have_rcu_nocb_mask = true;
+        }
+#ifdef CONFIG_RCU_NOCB_CPU_ZERO
+        pr_info("\tExperimental no-CBs 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("\tExperimental no-CBs for all CPUs\n");
+        cpumask_setall(rcu_nocb_mask);
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
+#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
         if (have_rcu_nocb_mask) {
                 cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
                 pr_info("\tExperimental no-CBs CPUs: %s.\n", nocb_buf);