Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936023Ab3DKO43 (ORCPT ); Thu, 11 Apr 2013 10:56:29 -0400 Received: from mail-we0-f170.google.com ([74.125.82.170]:39972 "EHLO mail-we0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934437Ab3DKO4Z (ORCPT ); Thu, 11 Apr 2013 10:56:25 -0400 Date: Thu, 11 Apr 2013 16:56:21 +0200 From: Frederic Weisbecker To: Vincent Guittot Cc: linux-kernel@vger.kernel.org, linaro-kernel@lists.linaro.org, peterz@infradead.org, mingo@kernel.org, rostedt@goodmis.org, efault@gmx.de, Paul Turner Subject: Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag Message-ID: <20130411145618.GA15699@somewhere> References: <1364996263-12198-1-git-send-email-vincent.guittot@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1364996263-12198-1-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 Content-Length: 9655 Lines: 293 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. 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. > + > 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. > + 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. > +} > + > +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. 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 ? > + 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. 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() ? 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/