linux-kernel - [RFC PATCH 2 2/2] sched: Nominate a power-efficient ilb in select_nohz

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Date:	Tue, 14 Apr 2009 10:25:35 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	"Ingo Molnar" <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Vaidyanathan Srinivasan" <svaidy@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, "Balbir Singh" <balbir@...ibm.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Andi Kleen" <andi@...stfloor.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: [RFC PATCH 2 2/2] 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@...ibm.com>
---

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

diff --git a/kernel/sched.c b/kernel/sched.c
index d702be7..bbf367d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4402,8 +4402,24 @@ 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;

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