[PATCH v4] sched: fix init NOHZ_IDLE flag

From: Vincent Guittot
Date: Thu Feb 21 2013 - 03:29:35 EST

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 seems to be:
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.
During the initialization of the sched_domain, we set the NOHZ_IDLE flag when
nr_busy_cpus is initialized to 0 in order to have a coherent configuration.
If a CPU enters idle and call set_cpu_sd_state_idle during the build of the
new sched_domain it will not corrupt the initial state
set_cpu_sd_state_busy is modified and clears the NOHZ_IDLE only if a non NULL
sched_domain is attached to the CPU (which is the case during the rebuild)

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@xxxxxxxxxx>
kernel/sched/core.c | 4 +++-
kernel/sched/fair.c | 9 +++++++--
2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 26058d0..c730a4e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5884,7 +5884,9 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd)

update_group_power(sd, cpu);
- atomic_set(&sg->sgp->nr_busy_cpus, sg->group_weight);
+ atomic_set(&sg->sgp->nr_busy_cpus, 0);
+ set_bit(NOHZ_IDLE, nohz_flags(cpu));

int __weak arch_sd_sibling_asym_packing(void)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 81fa536..2701a92 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5403,15 +5403,20 @@ static inline void set_cpu_sd_state_busy(void)
struct sched_domain *sd;
int cpu = smp_processor_id();
+ int clear = 0;

if (!test_bit(NOHZ_IDLE, nohz_flags(cpu)))
- clear_bit(NOHZ_IDLE, nohz_flags(cpu));

- for_each_domain(cpu, sd)
+ for_each_domain(cpu, sd) {
+ clear = 1;
+ }
+ if (likely(clear))
+ clear_bit(NOHZ_IDLE, nohz_flags(cpu));

void set_cpu_sd_state_idle(void)

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