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


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