On Mon, Apr 18, 2022 at 05:54:03PM +0000, Joel Fernandes (Google) wrote:
> From: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxx>
Much better, queued for review and testing, thank you!
I resolved a (trivial) conflict with -rcu's "dev" branch. Could you
please check the resolultion below in case my notion of "trivial" and/or
"resolved" is excessively optimistic?
Thanx, Paul
------------------------------------------------------------------------
commit 21b595e4233e6885bdea2819c206a470cc207ea5
Author: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Date: Mon Apr 18 17:54:03 2022 +0000
rcu/nocb: Add an option to offload all CPUs on boot
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.
[ paulmck: Forward-port to -rcu "dev" branch. ]
Signed-off-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
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 46694e13398a..ef29220c805f 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1155,11 +1155,21 @@ 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_nocb_is_setup) {
+ need_rcu_nocb_mask = true;
+ offload_all = true;
+ }
+#endif
+
#if defined(CONFIG_NO_HZ_FULL)
- if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+ 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) {
@@ -1180,6 +1190,9 @@ 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,
