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Subject[RFC PATCH v5 4/7] sched: bias task wakeups to preferred semi-idle packages
Preferred wakeup cpu (from a semi idle package) has been
nominated in find_busiest_group() in the previous patch. Use
this information in sched_mc_preferred_wakeup_cpu in function
wake_idle() to bias task wakeups if the following conditions
are satisfied:
- The present cpu that is trying to wakeup the process is
idle and waking the target process on this cpu will
potentially wakeup a completely idle package
- The previous cpu on which the target process ran is
also idle and hence selecting the previous cpu may
wakeup a semi idle cpu package
- The task being woken up is allowed to run in the
nominated cpu (cpu affinity and restrictions)

Basically if both the current cpu and the previous cpu on
which the task ran is idle, select the nominated cpu from semi
idle cpu package for running the new task that is waking up.

Cache hotness is considered since the actual biasing happens
in wake_idle() only if the application is cache cold.

This technique will effectively move short running bursty jobs in
a mostly idle system.

Wakeup biasing for power savings gets automatically disabled if
system utilisation increases due to the fact that the probability
of finding both this_cpu and prev_cpu idle decreases.

Signed-off-by: Vaidyanathan Srinivasan <>

kernel/sched_fair.c | 17 +++++++++++++++++
1 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 98345e4..939f2a1 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1027,6 +1027,23 @@ static int wake_idle(int cpu, struct task_struct *p)
cpumask_t tmp;
struct sched_domain *sd;
int i;
+ unsigned int chosen_wakeup_cpu;
+ int this_cpu;
+ /*
+ * At POWERSAVINGS_BALANCE_WAKEUP level, if both this_cpu and prev_cpu
+ * are idle and this is not a kernel thread and this task's affinity
+ * allows it to be moved to preferred cpu, then just move!
+ */
+ this_cpu = smp_processor_id();
+ chosen_wakeup_cpu =
+ cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu;
+ if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP &&
+ idle_cpu(cpu) && idle_cpu(this_cpu) && p->mm &&
+ cpu_isset(chosen_wakeup_cpu, p->cpus_allowed))
+ return chosen_wakeup_cpu;

* If it is idle, then it is the best cpu to run this task.

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