linux-kernel - Re: [PATCH] rcu: Clarify nocb kthreads naming in RCU_NOCB

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Message-ID: <20200924225959.GY29330@paulmck-ThinkPad-P72>
Date:   Thu, 24 Sep 2020 15:59:59 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [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@...eaurora.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@...eaurora.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@...eaurora.org>
    [ paulmck: While in the area, update description and advice. ]
    Signed-off-by: Paul E. McKenney <paulmck@...nel.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