sched: Nominate a power-efficient ilb in select_nohz_balancer()

The CPU that first goes idle becomes the idle-load-balancer and remains
that until either it picks up a task or till all the CPUs of the system
goes idle.

Optimize this further to allow it to relinquish it's post
once all it's siblings in the power-aware sched_domain go idle, thereby
allowing the whole package-core to go idle. While relinquising the post,
nominate another an idle-load balancer from a semi-idle core/package.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090414045535.7645.31641.stgit@sofia.in.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index b0fefa3..36d213b 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4414,8 +4414,24 @@ int select_nohz_load_balancer(int stop_tick)
                        /* make me the ilb owner */
                        if (atomic_cmpxchg(&nohz.load_balancer, -1, cpu) == -1)
                                return 1;
-               } else if (atomic_read(&nohz.load_balancer) == cpu)
+               } else if (atomic_read(&nohz.load_balancer) == cpu) {
+                       int new_ilb;
+
+                       if (!(sched_smt_power_savings ||
+                                               sched_mc_power_savings))
+                               return 1;
+                       /*
+                        * Check to see if there is a more power-efficient
+                        * ilb.
+                        */
+                       new_ilb = find_new_ilb(cpu);
+                       if (new_ilb < nr_cpu_ids && new_ilb != cpu) {
+                               atomic_set(&nohz.load_balancer, -1);
+                               resched_cpu(new_ilb);
+                               return 0;
+                       }
                        return 1;
+               }
        } else {
                if (!cpumask_test_cpu(cpu, nohz.cpu_mask))
                        return 0;