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SubjectRe: [PATCH] rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config
On Thu, Sep 24, 2020 at 12:04:10PM +0530, Neeraj Upadhyay wrote:
> Clarify the "x" in rcuox/N naming in RCU_NOCB_CPU config
> description.
>
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>

Applied with a few additional updates as shown below. As always, please
let me know if I messed anything up.

Thanx, Paul

------------------------------------------------------------------------

commit 8d1d776b4998896a6f8f4608edb0b258bd37ec9f
Author: Neeraj Upadhyay <neeraju@codeaurora.org>
Date: Thu Sep 24 12:04:10 2020 +0530

rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config

This commit clarifies that the "p" and the "s" in the in the RCU_NOCB_CPU
config-option description refer to the "x" in the "rcuox/N" kthread name.

Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
[ paulmck: While in the area, update description and advice. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index b71e21f..cdc57b4 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -221,19 +221,23 @@ config RCU_NOCB_CPU
Use this option to reduce OS jitter for aggressive HPC or
real-time workloads. It can also be used to offload RCU
callback invocation to energy-efficient CPUs in battery-powered
- asymmetric multiprocessors.
+ asymmetric multiprocessors. The price of this reduced jitter
+ is that the overhead of call_rcu() increases and that some
+ 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 "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.
-
- Say Y here if you want to help to debug reduced OS jitter.
+ 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.
+
+ Say Y here if you need reduced OS jitter, despite added overhead.
Say N here if you are unsure.

config TASKS_TRACE_RCU_READ_MB
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