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Date: Thu, 4 Apr 2013 19:07:40 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
linaro-kernel@...ts.linaro.org,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mike Galbraith <efault@....de>
Subject: Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag
2013/4/3 Vincent Guittot <vincent.guittot@...aro.org>:
> 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 <vincent.guittot@...aro.org>
> ---
> 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?
--
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