If a user wants to boot without any CPU in offloaded mode initially but
with the possibility to offload them later using cpusets, provide a way
to simply pass an empty "rcu_nocbs" kernel parameter which will enforce
the creation of dormant nocb kthreads.
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
---
.../admin-guide/kernel-parameters.txt | 26 ++++++++++++-------
kernel/rcu/tree_nocb.h | 10 ++++---
2 files changed, 23 insertions(+), 13 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cd860dc7c60b..6ff1a5f06383 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4351,20 +4351,28 @@
Disable the Correctable Errors Collector,
see CONFIG_RAS_CEC help text.
- rcu_nocbs= [KNL]
- The argument is a cpu list, as described above.
+ rcu_nocbs[=cpu-list]
+ [KNL] The optional argument is a cpu list,
+ as described above.
- In kernels built with CONFIG_RCU_NOCB_CPU=y, set
- the specified list of CPUs to be no-callback CPUs.
- Invocation of these CPUs' RCU callbacks will be
- offloaded to "rcuox/N" kthreads created for that
- purpose, where "x" is "p" for RCU-preempt, and
- "s" for RCU-sched, and "N" is the CPU number.
- This reduces OS jitter on the offloaded CPUs,
+ In kernels built with CONFIG_RCU_NOCB_CPU=y, enable the
+ no-callback CPU mode. Invocation of such CPUs' RCU
+ callbacks will be offloaded to "rcuox/N" kthreads
+ created for that purpose, where "x" is "p" for
+ RCU-preempt, and "s" for RCU-sched, and "N" is the CPU
+ number. This reduces OS jitter on the offloaded CPUs,
which can be useful for HPC and real-time
workloads. It can also improve energy efficiency
for asymmetric multiprocessors.
+ If a cpulist is passed as an argument, the specified
+ list of CPUs is set to no-callback mode from boot.
+
+ If otherwise the '=' sign and the cpulist arguments are
+ omitted, no CPU will be set to no-callback mode from
+ boot but cpuset will allow for toggling that mode at
+ runtime.
+
rcu_nocb_poll [KNL]
Rather than requiring that offloaded CPUs
(specified by rcu_nocbs= above) explicitly
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 9d37916278d4..d915780d40c8 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -66,14 +66,16 @@ static bool rcu_nocb_is_setup;
static int __init rcu_nocb_setup(char *str)
{
alloc_bootmem_cpumask_var(&rcu_nocb_mask);
- if (cpulist_parse(str, rcu_nocb_mask)) {
- pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
- cpumask_setall(rcu_nocb_mask);
+ if (*str == '=') {
+ if (cpulist_parse(++str, rcu_nocb_mask)) {
+ pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
+ cpumask_setall(rcu_nocb_mask);
+ }
}
rcu_nocb_is_setup = true;
return 1;
}
-__setup("rcu_nocbs=", rcu_nocb_setup);
+__setup("rcu_nocbs", rcu_nocb_setup);
static int __init parse_rcu_nocb_poll(char *arg)
{
--
2.25.1
