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Date: Thu, 4 Apr 2013 19:30:42 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
"linaro-kernel@...ts.linaro.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
On 4 April 2013 19:07, Frederic Weisbecker <fweisbec@...il.com> wrote:
> 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?
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
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