From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934987AbaGPSZD (ORCPT ); Wed, 16 Jul 2014 14:25:03 -0400 Received: from mail-ig0-f180.google.com ([209.85.213.180]:48802 "EHLO mail-ig0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934742AbaGPSY7 (ORCPT ); Wed, 16 Jul 2014 14:24:59 -0400 Message-ID: <53C6C377.7020900@gmail.com> Date: Wed, 16 Jul 2014 14:24:55 -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> <53C6C156.5010207@gmail.com> In-Reply-To: <53C6C156.5010207@gmail.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 02:15 PM, Pranith Kumar wrote: > > 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: > The previous patch will fail if CONFIG_NO_HZ_FULL is not defined, this should work. Thoughts? diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 00dc411..83f3f58 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 defined(CONFIG_NO_HZ_FULL) 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) */ +#endif /* #if defined(CONFIG_NO_HZ_FULL) */ if (ls == -1) { ls = int_sqrt(nr_cpu_ids); rcu_nocb_leader_stride = ls; -- Pranith