Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754775Ab3DLIXy (ORCPT ); Fri, 12 Apr 2013 04:23:54 -0400 Received: from mail-bk0-f51.google.com ([209.85.214.51]:56631 "EHLO mail-bk0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754028Ab3DLIXn (ORCPT ); Fri, 12 Apr 2013 04:23:43 -0400 MIME-Version: 1.0 In-Reply-To: <20130411145618.GA15699@somewhere> References: <1364996263-12198-1-git-send-email-vincent.guittot@linaro.org> <20130411145618.GA15699@somewhere> Date: Fri, 12 Apr 2013 10:23:41 +0200 Message-ID: Subject: Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag From: Vincent Guittot To: Frederic Weisbecker Cc: linux-kernel , "linaro-kernel@lists.linaro.org" , Peter Zijlstra , Ingo Molnar , Steven Rostedt , Mike Galbraith , Paul Turner 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: 10851 Lines: 319 On 11 April 2013 16:56, Frederic Weisbecker wrote: > On Wed, Apr 03, 2013 at 03:37:43PM +0200, 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 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 > > Ok, the lockless scheme involving nr_buzy_cpus and rq flags seem to be correct now. > We can hope somebody will come up with a less complicated solution. But for now that's > the best fix I've seen. Thanks > > I just have a few comments on details. > >> --- >> 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 */ >> +}; > > So the reason for this level of indirection won't be intuitive for those > who read that code. Please add some comments that explain why we need > that. ie: because we need the object lifecycle of sched_power and flags to be the > same for the lockless scheme to work. ok, i will add a comment > >> + >> 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; > > old_sd_rq would be a clearer name. ok > >> + 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); > > [...] >> +static void __sdrq_free(const struct cpumask *cpu_map, struct s_data *d) >> +{ >> + int j; >> + >> + for_each_cpu(j, cpu_map) >> + if (*per_cpu_ptr(d->sd_rq, j)) >> + kfree(*per_cpu_ptr(d->sd_rq, j)); > > kfree(NULL) works. thanks > >> +} >> + >> +static void build_sched_domain_rq(struct s_data *d, int cpu) >> +{ >> + struct sched_domain_rq *sd_rq; >> + struct sched_domain *sd; >> + >> + /* Attach sched_domain to sched_domain_rq */ >> + sd = *per_cpu_ptr(d->sd, cpu); >> + sd_rq = *per_cpu_ptr(d->sd_rq, cpu); >> + sd_rq->sd = sd; >> + /* Init flags */ >> + set_bit(NOHZ_IDLE, sched_rq_flags(sd_rq)); >> +} >> + >> struct sched_domain *build_sched_domain(struct sched_domain_topology_level *tl, >> struct s_data *d, const struct cpumask *cpu_map, >> struct sched_domain_attr *attr, struct sched_domain *child, >> @@ -6495,6 +6572,7 @@ static int build_sched_domains(const struct cpumask *cpu_map, >> struct sched_domain_attr *attr) >> { >> enum s_alloc alloc_state = sa_none; >> + struct sched_domain_rq *sd_rq; >> struct sched_domain *sd; >> struct s_data d; >> int i, ret = -ENOMEM; >> @@ -6547,11 +6625,18 @@ static int build_sched_domains(const struct cpumask *cpu_map, >> } >> } >> >> + /* Init objects that must follow the sched_domain lifecycle */ >> + for_each_cpu(i, cpu_map) { >> + build_sched_domain_rq(&d, i); >> + } > > I suggest you put the above domain_rq initialization before the domain > initialization. I can't because build_sched_domain will init struct s_data d that is used in build_sched_domain_rq > > So that we have that more intuitively ordered initialization: > > * set up domains_rq > * set up domains > * set up sched groups > * set up sched groups power > >> + >> /* Attach the domains */ >> rcu_read_lock(); >> for_each_cpu(i, cpu_map) { >> - sd = *per_cpu_ptr(d.sd, i); >> - cpu_attach_domain(sd, d.rd, i); >> + sd_rq = *per_cpu_ptr(d.sd_rq, i); >> + cpu_attach_domain(sd_rq, d.rd, i); >> + /* claim allocation of sched_domain_rq object */ >> + *per_cpu_ptr(d.sd_rq, i) = NULL; >> } >> rcu_read_unlock(); > [...] >> @@ -5673,7 +5681,12 @@ static void run_rebalance_domains(struct softirq_action *h) >> >> static inline int on_null_domain(int cpu) >> { >> - return !rcu_dereference_sched(cpu_rq(cpu)->sd); >> + struct sched_domain_rq *sd_rq = >> + rcu_dereference_sched(cpu_rq(cpu)->sd_rq); >> + struct sched_domain *sd = NULL; >> + if (sd_rq) >> + sd = sd_rq->sd; > > Is it possible to have sd_rq->sd == NULL ? I think so with the isolcpus as an example > >> + return !sd; >> } >> >> /* >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >> index cc03cfd..f589306 100644 >> --- a/kernel/sched/sched.h >> +++ b/kernel/sched/sched.h >> @@ -417,7 +417,7 @@ struct rq { >> >> #ifdef CONFIG_SMP >> struct root_domain *rd; >> - struct sched_domain *sd; >> + struct sched_domain_rq *sd_rq; >> >> unsigned long cpu_power; >> >> @@ -505,21 +505,37 @@ DECLARE_PER_CPU(struct rq, runqueues); >> >> #ifdef CONFIG_SMP >> >> -#define rcu_dereference_check_sched_domain(p) \ >> +#define rcu_dereference_check_sched_domain_rq(p) \ >> rcu_dereference_check((p), \ >> lockdep_is_held(&sched_domains_mutex)) >> >> +#define get_sched_domain_rq(cpu) \ >> + rcu_dereference_check_sched_domain_rq(cpu_rq(cpu)->sd_rq) > > How about rcu_dereference_domain_rq()? It seems important to me that > we keep the rcu_dereference_*() naming so that we don't hide what really > happens there behind a more opaque naming. you're right > > I mean RCU is tricky to deal with and it's important not to obfuscate its use. > >> + >> +#define rcu_dereference_check_sched_domain(cpu) ({ \ >> + struct sched_domain_rq *__sd_rq = get_sched_domain_rq(cpu); \ >> + struct sched_domain *__sd = NULL; \ >> + if (__sd_rq) \ >> + __sd = __sd_rq->sd; \ > > Same question for the NULL sd. > >> + __sd; \ >> +}) >> + >> +#define sched_rq_flags(sd_rq) (&sd_rq->flags) > > Hm, why this "sched_" prefix? Maybe rq_domain_flags() ? ok i'm going to change the name > > Thanks. -- 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/