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Subject[PATCH v4 rcu/dev] rcu/nocb: Add an option to offload all CPUs on boot
Date
From: Joel Fernandes <joel@joelfernandes.org>

On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
which ends up not offloading any CPU. This patch removes a dependency
from the bootloader having to know about RCU and about how to provide
the mask.

With the new option enabled, all CPUs will be offloaded on boot unless
rcu_nocbs= or rcu_nohz_full= kernel parameters provide a CPU list.

Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
---
v4: Rebased on rcu/dev, fixed conflict with Frederick's patch changing
location of nocb_is_setup variable.

Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
kernel/rcu/Kconfig | 13 +++++++++++++
kernel/rcu/tree_nocb.h | 13 +++++++++++++
3 files changed, 32 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 789ef586009b..1e82ecb7a649 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3572,6 +3572,9 @@
just as if they had also been called out in the
rcu_nocbs= boot parameter.

+ Note that this argument takes precedence over
+ the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
+
noiotrap [SH] Disables trapped I/O port accesses.

noirqdebug [X86-32] Disables the code which attempts to detect and
@@ -4475,6 +4478,9 @@
no-callback mode from boot but the mode may be
toggled at runtime via cpusets.

+ Note that this argument takes precedence over
+ the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
+
rcu_nocb_poll [KNL]
Rather than requiring that offloaded CPUs
(specified by rcu_nocbs= above) explicitly
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 1c630e573548..27aab870ae4c 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -262,6 +262,19 @@ config RCU_NOCB_CPU
Say Y here if you need reduced OS jitter, despite added overhead.
Say N here if you are unsure.

+config RCU_NOCB_CPU_DEFAULT_ALL
+ bool "Offload RCU callback processing from all CPUs by default"
+ depends on RCU_NOCB_CPU
+ default n
+ help
+ Use this option to offload callback processing from all CPUs
+ by default, in the absence of the rcu_nocbs or nohz_full boot
+ parameter. This also avoids the need to use any boot parameters
+ to achieve the effect of offloading all CPUs on boot.
+
+ Say Y here if you want offload all CPUs by default on boot.
+ Say N here if you are unsure.
+
config TASKS_TRACE_RCU_READ_MB
bool "Tasks Trace RCU readers use memory barriers in user and idle"
depends on RCU_EXPERT && TASKS_TRACE_RCU
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 4cf9a29bba79..9f29f25a0cbc 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1197,11 +1197,21 @@ void __init rcu_init_nohz(void)
{
int cpu;
bool need_rcu_nocb_mask = false;
+ bool offload_all = false;
struct rcu_data *rdp;

+#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
+ if (!rcu_state.nocb_is_setup) {
+ need_rcu_nocb_mask = true;
+ offload_all = true;
+ }
+#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
+
#if defined(CONFIG_NO_HZ_FULL)
if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
need_rcu_nocb_mask = true;
+ offload_all = false; /* NO_HZ_FULL has its own mask. */
+ }
#endif /* #if defined(CONFIG_NO_HZ_FULL) */

if (need_rcu_nocb_mask) {
@@ -1222,6 +1232,9 @@ void __init rcu_init_nohz(void)
cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
#endif /* #if defined(CONFIG_NO_HZ_FULL) */

+ if (offload_all)
+ cpumask_setall(rcu_nocb_mask);
+
if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
cpumask_and(rcu_nocb_mask, cpu_possible_mask,
--
2.36.0.rc0.470.gd361397f0d-goog
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