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rcu/nocb: Add an option to offload all CPUs on boot 

Systems built with CONFIG_RCU_NOCB_CPU=y but booted without either
the rcu_nocbs= or rcu_nohz_full= kernel-boot parameters will not have
callback offloading on any of the CPUs, nor can any of the CPUs be
switched to enable callback offloading at runtime.  Although this is
intentional, it would be nice to have a way to offload all the CPUs
without having to make random bootloaders specify either the rcu_nocbs=
or the rcu_nohz_full= kernel-boot parameters.

This commit therefore provides a new CONFIG_RCU_NOCB_CPU_DEFAULT_ALL
Kconfig option that switches the default so as to offload callback
processing on all of the CPUs.  This default can still be overridden
using the rcu_nocbs= and rcu_nohz_full= kernel-boot parameters.

Reviewed-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
(In v4.1, fixed issues with CONFIG maze reported by kernel test robot).
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
This commit is contained in:
Joel Fernandes 2022-04-22 17:52:47 +00:00 committed by Paul E. McKenney
parent 3a5761dc02
commit b37a667c62
3 changed files with 34 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/admin-guide/kernel-parameters.txt
View File

@ -3659,6 +3659,9 @@
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
@ -4557,6 +4560,9 @@
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

13
kernel/rcu/Kconfig
View File

@ -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

17
kernel/rcu/tree_nocb.h
View File

@ -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_NO_HZ_FULL)
if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
#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,