linux-kernel - [PATCH tip/core/rcu 07/16] rcu: Clarify nocb kthreads naming in RCU_NOCB

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Message-Id: <20201105230921.19017-7-paulmck@kernel.org>
Date:   Thu,  5 Nov 2020 15:09:12 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 07/16] rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config

From: Neeraj Upadhyay <neeraju@...eaurora.org>

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
-- 
2.9.5