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Subject[tip:sched/core] sched: Ensure cpu_power periodic update
Commit-ID:  4ec4412e1e91f44a3dcb97b6c9172a13fc78bac9
Gitweb: http://git.kernel.org/tip/4ec4412e1e91f44a3dcb97b6c9172a13fc78bac9
Author: Vincent Guittot <vincent.guittot@linaro.org>
AuthorDate: Mon, 12 Dec 2011 20:21:08 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 27 Jan 2012 13:28:49 +0100

sched: Ensure cpu_power periodic update

With a lot of small tasks, the softirq sched is nearly never called
when no_hz is enabled. In this case load_balance() is mainly called
with the newly_idle mode which doesn't update the cpu_power.

Add a next_update field which ensure a maximum update period when
there is short activity.

Having stale cpu_power information can skew the load-balancing
decisions, this is cured by the guaranteed update.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1323717668-2143-1-git-send-email-vincent.guittot@linaro.org
---
include/linux/sched.h | 1 +
kernel/sched/fair.c | 24 ++++++++++++++++--------
2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0e19595..92313a3f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -905,6 +905,7 @@ struct sched_group_power {
* single CPU.
*/
unsigned int power, power_orig;
+ unsigned long next_update;
/*
* Number of busy cpus in this group.
*/
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7c6414f..8e77a6b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -215,6 +215,8 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,

const struct sched_class fair_sched_class;

+static unsigned long __read_mostly max_load_balance_interval = HZ/10;
+
/**************************************************************
* CFS operations on generic schedulable entities:
*/
@@ -3776,6 +3778,11 @@ void update_group_power(struct sched_domain *sd, int cpu)
struct sched_domain *child = sd->child;
struct sched_group *group, *sdg = sd->groups;
unsigned long power;
+ unsigned long interval;
+
+ interval = msecs_to_jiffies(sd->balance_interval);
+ interval = clamp(interval, 1UL, max_load_balance_interval);
+ sdg->sgp->next_update = jiffies + interval;

if (!child) {
update_cpu_power(sd, cpu);
@@ -3883,12 +3890,15 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
* domains. In the newly idle case, we will allow all the cpu's
* to do the newly idle load balance.
*/
- if (idle != CPU_NEWLY_IDLE && local_group) {
- if (balance_cpu != this_cpu) {
- *balance = 0;
- return;
- }
- update_group_power(sd, this_cpu);
+ if (local_group) {
+ if (idle != CPU_NEWLY_IDLE) {
+ if (balance_cpu != this_cpu) {
+ *balance = 0;
+ return;
+ }
+ update_group_power(sd, this_cpu);
+ } else if (time_after_eq(jiffies, group->sgp->next_update))
+ update_group_power(sd, this_cpu);
}

/* Adjust by relative CPU power of the group */
@@ -4945,8 +4955,6 @@ static int __cpuinit sched_ilb_notifier(struct notifier_block *nfb,

static DEFINE_SPINLOCK(balancing);

-static unsigned long __read_mostly max_load_balance_interval = HZ/10;
-
/*
* Scale the max load_balance interval with the number of CPUs in the system.
* This trades load-balance latency on larger machines for less cross talk.

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