From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Friesen Subject: question about rcuc/X tasks Date: Mon, 12 Dec 2016 16:42:00 -0600 Message-ID: <584F27B8.2090406@windriver.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit To: Return-path: Received: from mail1.windriver.com ([147.11.146.13]:37326 "EHLO mail1.windriver.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751697AbcLLWmE (ORCPT ); Mon, 12 Dec 2016 17:42:04 -0500 Received: from ALA-HCA.corp.ad.wrs.com (ala-hca.corp.ad.wrs.com [147.11.189.40]) by mail1.windriver.com (8.15.2/8.15.1) with ESMTPS id uBCMg3qS029835 (version=TLSv1 cipher=AES128-SHA bits=128 verify=FAIL) for ; Mon, 12 Dec 2016 14:42:03 -0800 (PST) Sender: linux-rt-users-owner@vger.kernel.org List-ID: Hi, I'm testing the CentOS 7.3 RT kernel, 3.10.0-514.rt56.420 (I realize this is a heavily-patched kernel, but I'm hoping general information will still apply.) My kernel is booted with "isolcpus=1-15 rcu_nocbs=1-15" as kernel parameters, and /dev/cpu_dma_latency is set to 3. The relevent config options are set as follows: CONFIG_PREEMPT=y CONFIG_PREEMPT_RT_BASE=y CONFIG_HAVE_PREEMPT_LAZY=y CONFIG_PREEMPT_LAZY=y CONFIG_PREEMPT_RT_FULL=y CONFIG_RCU_BOOST=y CONFIG_RCU_KTHREAD_PRIO=2 CONFIG_RCU_BOOST_DELAY=500 CONFIG_RCU_NOCB_CPU=y CONFIG_RCU_NOCB_CPU_NONE=y The issue I'm seeing is that when I run cyclictest, ftrace shows rcuc/ running for 5-6 usec quite a few times per second. Based on the fact that Documentation/kernel-per-CPU-kthreads.txt describes CONFIG_RCU_NOCB_CPU_ALL=y as a solution by preventing the rcuc/%u kthreads from having any work to do, I had expected that the "rcu_nocbs=1-15" kernel parameter would have a similar effect. Does anyone have any idea what's going on here, and how I can prevent the rcuc/ tasks from waking up? Thanks, Chris