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McKenney" To: Neeraj Upadhyay Cc: josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config Message-ID: <20200924225959.GY29330@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <1600929250-20344-1-git-send-email-neeraju@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1600929250-20344-1-git-send-email-neeraju@codeaurora.org> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 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 [ paulmck: While in the area, update description and advice. ] Signed-off-by: Paul E. McKenney 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