The RCU_NOCB_CPU help text currently fails to mention rcuog kthreads,
so this commit adds this information.
Reported-by: Olivier Langlois <oliv...@trillion01.com>
Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
kernel/rcu/Kconfig | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 3e079de0f5b43..b9b6bc55185db 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -249,16 +249,24 @@ config RCU_NOCB_CPU
workloads will incur significant increases in context-switch
rates.
- This option offloads callback invocation from the set of CPUs
- specified at boot time by the rcu_nocbs parameter. For each
- such CPU, a kthread ("rcuox/N") will be created to invoke
- callbacks, where the "N" is the CPU being offloaded, and where
- the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
- RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
- from running on the specified CPUs, but (1) the kthreads may be
- preempted between each callback, and (2) affinity or cgroups can
- be used to force the kthreads to run on whatever set of CPUs is
- desired.
+ This option offloads callback invocation from the set of
+ CPUs specified at boot time by the rcu_nocbs parameter.
+ For each such CPU, a kthread ("rcuox/N") will be created to
+ invoke callbacks, where the "N" is the CPU being offloaded,
+ and where the "x" is "p" for RCU-preempt (PREEMPTION kernels)
+ and "s" for RCU-sched (!PREEMPTION kernels). This option
+ also creates another kthread for each sqrt(nr_cpu_ids) CPUs
+ ("rcuog/N", where N is the first CPU in that group to come
+ online), which handles grace periods for its group. Nothing
+ prevents these kthreads from running on the specified CPUs,
+ but (1) the kthreads may be preempted between each callback,
+ and (2) affinity or cgroups can be used to force the kthreads
+ to run on whatever set of CPUs is desired.
+
+ The sqrt(nr_cpu_ids) grouping may be overridden using the
+ rcutree.rcu_nocb_gp_stride kernel boot parameter. This can
+ be especially helpful for smaller numbers of CPUs, where
+ sqrt(nr_cpu_ids) can be a bit of a blunt instrument.
Say Y here if you need reduced OS jitter, despite added overhead.
Say N here if you are unsure.
--
2.40.1
