Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764007Ab3DDRar (ORCPT ); Thu, 4 Apr 2013 13:30:47 -0400 Received: from mail-bk0-f45.google.com ([209.85.214.45]:49332 "EHLO mail-bk0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763774Ab3DDRao (ORCPT ); Thu, 4 Apr 2013 13:30:44 -0400 MIME-Version: 1.0 In-Reply-To: References: <1364996263-12198-1-git-send-email-vincent.guittot@linaro.org> Date: Thu, 4 Apr 2013 19:30:42 +0200 Message-ID: Subject: Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag From: Vincent Guittot To: Frederic Weisbecker 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: 7115 Lines: 179 On 4 April 2013 19:07, Frederic Weisbecker wrote: > 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? No we are not reusing an existing struct, the sched_group/sched_group_power that is initialized here, has just been created by __visit_domain_allocation_hell in build_sched_domains. The sched_group/sched_group_power is not already attached to any CPU Vincent -- 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/