[RFC 2/3] sched: fix init NOHZ_IDLE flag

[Vincent Guittot]

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.
We clear the NOHZ_IDLE flag when nr_busy_cpus is initialized in order to
have a coherent configuration.

Signed-off-by: Vincent Guittot <vincent...@linaro.org>
---
kernel/sched/core.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5dae0d2..05058e8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5817,6 +5817,7 @@ 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);
+ clear_bit(NOHZ_IDLE, nohz_flags(cpu));
}

int __weak arch_sd_sibling_asym_packing(void)
--
1.7.10

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[Vincent Guittot]

With the coupled cpuidle driver (but probably also with other drivers),
a CPU loops in a temporary safe state while waiting for other CPUs of its
cluster to be ready to enter the coupled C-state. If an IRQ or a softirq
occurs, the CPU will stay in this internal loop if there is no need
to resched. The SCHED softirq clears the NOHZ and increases
nr_busy_cpus. If there is no need to resched, we will not call
set_cpu_sd_state_idle because of this internal loop in a cpuidle state.
We have to call set_cpu_sd_state_idle in tick_nohz_irq_exit which is used
to handle such situation.

Signed-off-by: Vincent Guittot <vincent...@linaro.org>
---

kernel/time/tick-sched.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a402608..e19bbc9 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -526,6 +526,8 @@ void tick_nohz_irq_exit(void)
if (!ts->inidle)
return;

+ set_cpu_sd_state_idle();
+
__tick_nohz_idle_enter(ts);

[Vincent Guittot]

The function nohz_kick_needed modifies NOHZ_IDLE flag that is used to update
the nr_busy_cpus of the sched_group.
When the sched_domain are updated (because of the unplug of a CPUs as an
example) a null_domain is attached to CPUs. We have to test
likely(!on_null_domain(cpu) first in order to detect such intialization step
and to not modify the NOHZ_IDLE flag

Signed-off-by: Vincent Guittot <vincent...@linaro.org>
---

kernel/sched/fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3d0686c..1bf7c87 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5490,7 +5490,7 @@ void trigger_load_balance(struct rq *rq, int cpu)
likely(!on_null_domain(cpu)))
raise_softirq(SCHED_SOFTIRQ);
#ifdef CONFIG_NO_HZ
- if (nohz_kick_needed(rq, cpu) && likely(!on_null_domain(cpu)))
+ if (likely(!on_null_domain(cpu)) && nohz_kick_needed(rq, cpu))
nohz_balancer_kick(cpu);
#endif

[Vincent Guittot]

The nr_busy_cpus field of the sched_group_power is sometime different from 0
whereas the platform is fully idle. This serie fixes 3 use cases:
- when the SCHED softirq is raised on an idle core for idle load balance but
the platform doesn't go out of the cpuidle state
- when some CPUs enter idle state while booting all CPUs
- when a CPU is unplug and/or replug

Vincent Guittot (3):
sched: fix nr_busy_cpus with coupled cpuidle
sched: fix init NOHZ_IDLE flag
sched: fix update NOHZ_IDLE flag

kernel/sched/core.c | 1 +
kernel/sched/fair.c | 2 +-
kernel/time/tick-sched.c | 2 ++
3 files changed, 4 insertions(+), 1 deletion(-)