linux-kernel - [tip: core/rcu] rcu: Clarify nocb kthreads naming in RCU_NOCB

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Date:   Sun, 13 Dec 2020 19:01:01 -0000
From:   "tip-bot2 for Neeraj Upadhyay" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Neeraj Upadhyay <neeraju@...eaurora.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     a3941517fcd6625adc540aef5ec3f717c8fa71e8
Gitweb:        https://git.kernel.org/tip/a3941517fcd6625adc540aef5ec3f717c8fa71e8
Author:        Neeraj Upadhyay <neeraju@...eaurora.org>
AuthorDate:    Thu, 24 Sep 2020 12:04:10 +05:30
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Thu, 19 Nov 2020 19:37:16 -08:00

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@...eaurora.org>
[ paulmck: While in the area, update description and advice. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/Kconfig | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

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