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Subject[PATCH v7] sched: fix init NOHZ_IDLE flag
Date
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.
A new nohz_flags has been added to sched_domain so both flags and sched_domain
will share the same RCU lifecycle and will be always synchronized. This
solution is prefered to the creation of a new struct with an extra pointer
indirection.

The synchronization is done at the cost of :
- An additional indirection and a rcu_dereference for accessing the NOHZ_IDLE
flag.
- We use only the nohz_flags field of the top sched_domain.

Change since v6:
- Add the flags in struct sched_domain instead of creating a sched_domain_rq.

Change since v5:
- minor variable and function name change.
- remove a useless null check before kfree
- fix a compilation error when NO_HZ is not set.

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>
---
include/linux/sched.h | 1 +
kernel/sched/fair.c | 34 ++++++++++++++++++++++++----------
2 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d35d2b6..cde4f7f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -899,6 +899,7 @@ struct sched_domain {
unsigned int wake_idx;
unsigned int forkexec_idx;
unsigned int smt_gain;
+ unsigned long nohz_flags; /* NOHZ_IDLE flag status */
int flags; /* See SD_* */
int level;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a33e59..09e440f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5394,14 +5394,21 @@ static inline void set_cpu_sd_state_busy(void)
{
struct sched_domain *sd;
int cpu = smp_processor_id();
-
- if (!test_bit(NOHZ_IDLE, nohz_flags(cpu)))
- return;
- clear_bit(NOHZ_IDLE, nohz_flags(cpu));
+ int first_nohz_idle = 1;

rcu_read_lock();
- for_each_domain(cpu, sd)
+ for_each_domain(cpu, sd) {
+ if (first_nohz_idle) {
+ if (!test_bit(NOHZ_IDLE, &sd->nohz_flags))
+ goto unlock;
+
+ clear_bit(NOHZ_IDLE, &sd->nohz_flags);
+ first_nohz_idle = 0;
+ }
+
atomic_inc(&sd->groups->sgp->nr_busy_cpus);
+ }
+unlock:
rcu_read_unlock();
}

@@ -5409,14 +5416,21 @@ void set_cpu_sd_state_idle(void)
{
struct sched_domain *sd;
int cpu = smp_processor_id();
-
- if (test_bit(NOHZ_IDLE, nohz_flags(cpu)))
- return;
- set_bit(NOHZ_IDLE, nohz_flags(cpu));
+ int first_nohz_idle = 1;

rcu_read_lock();
- for_each_domain(cpu, sd)
+ for_each_domain(cpu, sd) {
+ if (first_nohz_idle) {
+ if (test_bit(NOHZ_IDLE, &sd->nohz_flags))
+ goto unlock;
+
+ set_bit(NOHZ_IDLE, &sd->nohz_flags);
+ first_nohz_idle = 0;
+ }
+
atomic_dec(&sd->groups->sgp->nr_busy_cpus);
+ }
+unlock:
rcu_read_unlock();
}

--
1.7.9.5


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