[PATCH tip/core/rcu 04/14] rcu: Provide compile-time control for no-CBs CPUs

From: Paul E. McKenney
Date: Sat Jan 05 2013 - 12:52:20 EST

From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
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.

+ 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.
endmenu # "RCU Subsystem"

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 37750bc..eb9b473 100644
--- 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);
+ if (!have_rcu_nocb_mask) {
+ alloc_bootmem_cpumask_var(&rcu_nocb_mask);
+ have_rcu_nocb_mask = true;
+ }
+ pr_info("\tExperimental no-CBs CPU 0\n");
+ cpumask_set_cpu(0, rcu_nocb_mask);
+#else /* #if CONFIG_RCU_NOCB_CPU_DEFAULT == 1 */
+ pr_info("\tExperimental no-CBs for all CPUs\n");
+ cpumask_setall(rcu_nocb_mask);
+#endif /* #else #if CONFIG_RCU_NOCB_CPU_DEFAULT == 1 */
+#endif /* #if CONFIG_RCU_NOCB_CPU_DEFAULT != 0 */
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);

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