[tip:sched/core] sched: Nominate a power-efficient ilb in select_nohz_balancer()

[tip-bot for Gautham R Shenoy <ego@in.ibm.com>]

Commit-ID:  e790fb0ba64bfec158e1219d899cb588275d12ab
Gitweb:     http://git.kernel.org/tip/e790fb0ba64bfec158e1219d899cb588275d12ab
Author:     Gautham R Shenoy <ego@in.ibm.com>
AuthorDate: Tue, 14 Apr 2009 10:25:35 +0530
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 14 Apr 2009 11:49:19 +0200

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>


---
 kernel/sched.c |   18 +++++++++++++++++-
 1 files changed, 17 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index b0fefa3..36d213b 100644
--- 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;