From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757180Ab3BVMc2 (ORCPT ); Fri, 22 Feb 2013 07:32:28 -0500 Received: from mail-vc0-f178.google.com ([209.85.220.178]:49626 "EHLO mail-vc0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756863Ab3BVMc1 (ORCPT ); Fri, 22 Feb 2013 07:32:27 -0500 Date: Fri, 22 Feb 2013 13:32:15 +0100 From: Frederic Weisbecker To: Vincent Guittot Cc: linux-kernel@vger.kernel.org, linaro-dev@lists.linaro.org, peterz@infradead.org, mingo@kernel.org, rostedt@goodmis.org, efault@gmx.de Subject: Re: [PATCH v4] sched: fix init NOHZ_IDLE flag Message-ID: <20130222123213.GA17948@somewhere.redhat.com> References: <1361435356-28466-1-git-send-email-vincent.guittot@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1361435356-28466-1-git-send-email-vincent.guittot@linaro.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 21, 2013 at 09:29:16AM +0100, Vincent Guittot wrote: > On my smp platform which is made of 5 cores in 2 clusters, I have the > nr_busy_cpu field of sched_group_power struct that is not null when the > platform is fully idle. The root cause seems to be: > During the boot sequence, some CPUs reach the idle loop and set their > NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus > field is initialized later with the assumption that all CPUs are in the busy > state whereas some CPUs have already set their NOHZ_IDLE flag. > During the initialization of the sched_domain, we set the NOHZ_IDLE flag when > nr_busy_cpus is initialized to 0 in order to have a coherent configuration. > If a CPU enters idle and call set_cpu_sd_state_idle during the build of the > new sched_domain it will not corrupt the initial state > set_cpu_sd_state_busy is modified and clears the NOHZ_IDLE only if a non NULL > sched_domain is attached to the CPU (which is the case during the rebuild) > > Change since V3; > - NOHZ flag is not cleared if a NULL domain is attached to the CPU > - Remove patch 2/2 which becomes useless with latest modifications > > Change since V2: > - change the initialization to idle state instead of busy state so a CPU that > enters idle during the build of the sched_domain will not corrupt the > initialization state > > Change since V1: > - remove the patch for SCHED softirq on an idle core use case as it was > a side effect of the other use cases. > > Signed-off-by: Vincent Guittot > --- > kernel/sched/core.c | 4 +++- > kernel/sched/fair.c | 9 +++++++-- > 2 files changed, 10 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 26058d0..c730a4e 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5884,7 +5884,9 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd) > return; > > update_group_power(sd, cpu); > - atomic_set(&sg->sgp->nr_busy_cpus, sg->group_weight); > + atomic_set(&sg->sgp->nr_busy_cpus, 0); > + set_bit(NOHZ_IDLE, nohz_flags(cpu)); > + > } > > int __weak arch_sd_sibling_asym_packing(void) > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 81fa536..2701a92 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5403,15 +5403,20 @@ static inline void set_cpu_sd_state_busy(void) > { > struct sched_domain *sd; > int cpu = smp_processor_id(); > + int clear = 0; > > if (!test_bit(NOHZ_IDLE, nohz_flags(cpu))) > return; > - clear_bit(NOHZ_IDLE, nohz_flags(cpu)); > > rcu_read_lock(); > - for_each_domain(cpu, sd) > + for_each_domain(cpu, sd) { > atomic_inc(&sd->groups->sgp->nr_busy_cpus); > + clear = 1; > + } > rcu_read_unlock(); > + > + if (likely(clear)) > + clear_bit(NOHZ_IDLE, nohz_flags(cpu)); I fear there is still a race window: = CPU 0 = = CPU 1 = // NOHZ_IDLE is set set_cpu_sd_state_busy() { dom1 = rcu_dereference(dom1); inc(dom1->nr_busy_cpus) rcu_assign_pointer(dom 1, NULL) // create new domain init_sched_group_power() { atomic_set(&tmp->nr_busy_cpus, 0); set_bit(NOHZ_IDLE, nohz_flags(cpu 1)); rcu_assign_pointer(dom 1, tmp) clear_bit(NOHZ_IDLE, nohz_flags(cpu)); } I don't know if there is any sane way to deal with this issue other than having nr_busy_cpus and nohz_flags in the same object sharing the same lifecycle.