Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763643Ab3DDRHq (ORCPT ); Thu, 4 Apr 2013 13:07:46 -0400 Received: from mail-lb0-f171.google.com ([209.85.217.171]:47192 "EHLO mail-lb0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763549Ab3DDRHo (ORCPT ); Thu, 4 Apr 2013 13:07:44 -0400 MIME-Version: 1.0 In-Reply-To: <1364996263-12198-1-git-send-email-vincent.guittot@linaro.org> References: <1364996263-12198-1-git-send-email-vincent.guittot@linaro.org> Date: Thu, 4 Apr 2013 19:07:40 +0200 Message-ID: Subject: Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag From: Frederic Weisbecker To: Vincent Guittot Cc: LKML , linaro-kernel@lists.linaro.org, Peter Zijlstra , Ingo Molnar , Steven Rostedt , Mike Galbraith Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6610 Lines: 171 2013/4/3 Vincent Guittot : > 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 is: > 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. > > More generally, the NOHZ_IDLE flag must be initialized when new sched_domains > are created in order to ensure that NOHZ_IDLE and nr_busy_cpus are aligned. > > This condition can be ensured by adding a synchronize_rcu between the > destruction of old sched_domains and the creation of new ones so the NOHZ_IDLE > flag will not be updated with old sched_domain once it has been initialized. > But this solution introduces a additionnal latency in the rebuild sequence > that is called during cpu hotplug. > > As suggested by Frederic Weisbecker, another solution is to have the same > rcu lifecycle for both NOHZ_IDLE and sched_domain struct. I have introduce > a new sched_domain_rq struct that is the entry point for both sched_domains > and objects that must follow the same lifecycle like NOHZ_IDLE flags. They > will share the same RCU lifecycle and will be always synchronized. > > The synchronization is done at the cost of : > - an additional indirection for accessing the first sched_domain level > - an additional indirection and a rcu_dereference before accessing to the > NOHZ_IDLE flag. > > Change since v4: > - link both sched_domain and NOHZ_IDLE flag in one RCU object so > their states are always synchronized. > > 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 > --- > include/linux/sched.h | 6 +++ > kernel/sched/core.c | 105 ++++++++++++++++++++++++++++++++++++++++++++----- > kernel/sched/fair.c | 35 +++++++++++------ > kernel/sched/sched.h | 24 +++++++++-- > 4 files changed, 145 insertions(+), 25 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index d35d2b6..2a52188 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -959,6 +959,12 @@ struct sched_domain { > unsigned long span[0]; > }; > > +struct sched_domain_rq { > + struct sched_domain *sd; > + unsigned long flags; > + struct rcu_head rcu; /* used during destruction */ > +}; > + > static inline struct cpumask *sched_domain_span(struct sched_domain *sd) > { > return to_cpumask(sd->span); > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 7f12624..69e2313 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5602,6 +5602,15 @@ static void destroy_sched_domains(struct sched_domain *sd, int cpu) > destroy_sched_domain(sd, cpu); > } > > +static void destroy_sched_domain_rq(struct sched_domain_rq *sd_rq, int cpu) > +{ > + if (!sd_rq) > + return; > + > + destroy_sched_domains(sd_rq->sd, cpu); > + kfree_rcu(sd_rq, rcu); > +} > + > /* > * Keep a special pointer to the highest sched_domain that has > * SD_SHARE_PKG_RESOURCE set (Last Level Cache Domain) for this > @@ -5632,10 +5641,23 @@ static void update_top_cache_domain(int cpu) > * hold the hotplug lock. > */ > static void > -cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu) > +cpu_attach_domain(struct sched_domain_rq *sd_rq, struct root_domain *rd, > + int cpu) > { > struct rq *rq = cpu_rq(cpu); > - struct sched_domain *tmp; > + struct sched_domain_rq *tmp_rq; > + struct sched_domain *tmp, *sd = NULL; > + > + /* > + * If we don't have any sched_domain and associated object, we can > + * directly jump to the attach sequence otherwise we try to degenerate > + * the sched_domain > + */ > + if (!sd_rq) > + goto attach; > + > + /* Get a pointer to the 1st sched_domain */ > + sd = sd_rq->sd; > > /* Remove the sched domains which do not contribute to scheduling. */ > for (tmp = sd; tmp; ) { > @@ -5658,14 +5680,17 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu) > destroy_sched_domain(tmp, cpu); > if (sd) > sd->child = NULL; > + /* update sched_domain_rq */ > + sd_rq->sd = sd; > } > > +attach: > sched_domain_debug(sd, cpu); > > rq_attach_root(rq, rd); > - tmp = rq->sd; > - rcu_assign_pointer(rq->sd, sd); > - destroy_sched_domains(tmp, cpu); > + tmp_rq = rq->sd_rq; > + rcu_assign_pointer(rq->sd_rq, sd_rq); > + destroy_sched_domain_rq(tmp_rq, cpu); > > update_top_cache_domain(cpu); > } > @@ -5695,12 +5720,14 @@ struct sd_data { > }; > > struct s_data { > + struct sched_domain_rq ** __percpu sd_rq; > struct sched_domain ** __percpu sd; > struct root_domain *rd; > }; > > enum s_alloc { > sa_rootdomain, > + sa_sd_rq, > sa_sd, > sa_sd_storage, > sa_none, > @@ -5935,7 +5962,7 @@ 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); Is it possible that we can be dealing here with a sched_group/sched_group_power that is used on another CPU (from that CPU's rq->rq_sd->sd) concurrently? When we call build_sched_groups(), we might reuse an exisiting struct sched_group used elsewhere right? If so, is there a race with the above initialization? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/