From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934823AbaGPSP7 (ORCPT ); Wed, 16 Jul 2014 14:15:59 -0400 Received: from mail-ie0-f178.google.com ([209.85.223.178]:56187 "EHLO mail-ie0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752329AbaGPSP4 (ORCPT ); Wed, 16 Jul 2014 14:15:56 -0400 Message-ID: <53C6C156.5010207@gmail.com> Date: Wed, 16 Jul 2014 14:15:50 -0400 From: Pranith Kumar User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com CC: Ingo Molnar , linux-kernel@vger.kernel.org, fabf@skynet.be, davidshan@tencent.com, joe@perches.com, keescook@chromium.org, Peter Zijlstra Subject: Re: [GIT PULL rcu/next] RCU commits for 3.17 References: <20140713184237.GA30855@linux.vnet.ibm.com> <20140716115046.GC28434@gmail.com> <20140716131322.GA16396@gmail.com> <20140716141707.GU8690@linux.vnet.ibm.com> <20140716152629.GA9340@linux.vnet.ibm.com> <53C6A0E7.30006@gmail.com> <20140716172410.GV8690@linux.vnet.ibm.com> In-Reply-To: <20140716172410.GV8690@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/16/2014 01:24 PM, Paul E. McKenney wrote: >>> And of course if you don't actually specify a nohz_full= mask, then >>> tick_nohz_full_mask can be NULL at RCU initialization time, and if it >>> is also true that CONFIG_NO_HZ_FULL_ALL=n, this condition can persist >>> forever. >>> >> Hi Paul, >> >> The other location where tick_nohz_full_mask is being allocated is in >> tick_nohz_init_all(), called from tick_nohz_init(). rcu_init() is called before >> tick_nohz_init() in init/main.c. CONFIG_NO_HZ_FULL_ALL for allocation of the >> mask does not take effect when rcu_init() runs. >> >> So if nohz_full command line arg is not specified, tick_nohz_full_mask will >> always be NULL when rcu_init() runs. So just checking for tick_nohz_full_mask >> is NULL should be enough I guess. > Yep, that is indeed one of the conditions called out in the commit log > below. > > Thanx, Paul > Sorry, I was not clear. nohz_full= boot time parameter overrides the CONFIG_NO_HZ_FULL_ALL build time parameter. The check for !CONFIG_NO_HZ_FULL_ALL will disable masking out the cpus on which NOHZ is not enabled using the nohz_full boot time parameter, and hence you will enable callback offloading on all CPUs as was previously being done. So, if I have CONFIG_NO_HZ_FULL_ALL enabled at build time but pass nohz_cpu=1 at boot time, all the CPUs will have callback offloading enabled. I think the following looks like what is needed in our case: diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 00dc411..1123ed4 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2479,10 +2479,10 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp) if (rcu_nocb_mask == NULL) return; -#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) + if (tick_nohz_full_running) cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask); -#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */ + if (ls == -1) { ls = int_sqrt(nr_cpu_ids); rcu_nocb_leader_stride = ls;