sched: fix init NOHZ_IDLE flag

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@linaro.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d35d2b6ddbfb..2a521883b91d 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);