[PATCH] sched: Nominate the idle load balancer from a semi-idle group.

[Gautham R Shenoy <ego@in.ibm.com>]

sched: Nominate the idle load balancer from a semi-idle group.

From: Gautham R Shenoy <ego@in.ibm.com>

Currently  the first cpu in the nohz cpu mask is nominated as
the idle load balancer.

However, this could be a cpu from an idle group,
thereby not yiedling the expected power savings.

Improve the logic to pick an idle cpu from a semi-idle group
for performing the task of idle load balancing, when
sched_mc/smt_powersavings is enabled.

This patch has no effect when sched_mc/smt_powersavings is turned off.

The patch is based on linux-2.6.27-rc5 and depends on the fix for
sched_mc_powersavings posted here--> http://lkml.org/lkml/2008/9/5/135

SPEC-Power Results: on a large multi-socket multi-core x86.
----------------------------------------------------------------
Summary with the patch:
         9% improvement in the overall score
         8% decrease in the Power consumption at 100% utilization.
	12% decrease in the Power consumption at idle.

----------------------------------------------------------------
kernel_version |Final Score |ssj ops | 100% Watts | Idle Watts |
---------------|------------|--------|-------------------------|
2.6.27-rc5     |   x        |   y    |    z       |    w       |
---------------|------------|--------|------------|------------|
2.6.27-rc5 +   |  1.05x     | 0.98y  |  0.94z     |  0.92w     |
sched_mc_fix   |            |        |            |            |
---------------|------------|--------|------------|------------|
2.6.27-rc5 +   |  1.09x     | 0.97y  |  0.92z     |  0.88w     |
sched_mc_fix + |            |        |            |            |
this_patch     |            |        |            |            |
----------------------------------------------------------------

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
---

 kernel/sched.c |   86 ++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 77 insertions(+), 9 deletions(-)


diff --git a/kernel/sched.c b/kernel/sched.c
index cc1f81b..c186719 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3960,6 +3960,82 @@ static void run_rebalance_domains(struct softirq_action *h)
 #endif
 }
 
+#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
+/**
+ * lowest_powersavings_sd: Returns the lowest level of power savings
+ * domain of the given cpu.
+ * @cpu: The cpu whose lowest level of power savings domain to
+ * be returned.
+ *
+ * Power savings domain is the sched_domain where in we perform
+ * load balancing for power savings. Such a domain would have
+ * SD_POWERSAVINGS_BALANCE bit set in it's flags.
+ */
+static inline struct sched_domain *lowest_powersavings_sd(int cpu)
+{
+	struct sched_domain *sd;
+
+	for_each_domain(cpu, sd)
+		if (sd && (sd->flags & SD_POWERSAVINGS_BALANCE))
+			break;
+
+	return sd;
+}
+
+/**
+ * for_each_powersavings_domain: Iterates over all the power savings
+ * scheduler domains (see comment for lowest_powersavings_sd) from
+ * the lowest to the highest.
+ * @cpu: The cpu whose powersavings_sd we're iterating over.
+ * @sd: variable holding the value of the power_savings_sd
+ * for cpu
+ */
+#define for_each_powersaving_domain(cpu, sd) \
+	for (sd = lowest_powersavings_sd(cpu); \
+		(sd && (sd->flags & SD_POWERSAVINGS_BALANCE)); sd = sd->parent)
+/**
+ * find_new_ilb: Finds or nominates a new idle load balancer.
+ * @cpu: The cpu which is nominating a new idle_load_balancer.
+ *
+ * This algorithm picks the idle load balancer such that it belongs to a
+ * semi-idle powersavings sched_domain. The idea is to try and avoid
+ * completely idle packages/cores just for the purpose of idle load balancing
+ * when there are other idle cpu's which are better suited for that job.
+ */
+static int find_new_ilb(int cpu)
+{
+	struct sched_domain *sd;
+	struct sched_group *ilb_group;
+	cpumask_t cpumask;
+
+	for_each_powersaving_domain(cpu, sd) {
+		ilb_group = sd->groups;
+
+		do {
+			cpus_and(cpumask, nohz.cpu_mask, ilb_group->cpumask);
+
+			/*
+			 * If there exists atleast one busy cpu within
+			 * this group, we should wake up the idle cpu from
+			 * here.
+			 */
+			if (!(cpus_empty(cpumask) ||
+				cpus_equal(cpumask, ilb_group->cpumask)))
+				return first_cpu(cpumask);
+
+			ilb_group = ilb_group->next;
+
+		} while (ilb_group != sd->groups);
+	}
+	return first_cpu(nohz.cpu_mask);
+}
+#else /*  (CONFIG_SCHED_MC || CONFIG_SCHED_SMT) */
+static inline int find_new_ilb(int call_cpu)
+{
+	return first_cpu(nohz.cpu_mask);
+}
+#endif
+
 /*
  * Trigger the SCHED_SOFTIRQ if it is time to do periodic load balancing.
  *
@@ -3984,15 +4060,7 @@ static inline void trigger_load_balance(struct rq *rq, int cpu)
 		}
 
 		if (atomic_read(&nohz.load_balancer) == -1) {
-			/*
-			 * simple selection for now: Nominate the
-			 * first cpu in the nohz list to be the next
-			 * ilb owner.
-			 *
-			 * TBD: Traverse the sched domains and nominate
-			 * the nearest cpu in the nohz.cpu_mask.
-			 */
-			int ilb = first_cpu(nohz.cpu_mask);
+			int ilb = find_new_ilb(cpu);
 
 			if (ilb < nr_cpu_ids)
 				resched_cpu(ilb);
-- 
Thanks and Regards
gautham