linux-kernel - [PATCH 3 3/6] sched: Add Comments at the beginning of find_busiest

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Date:	Wed, 18 Mar 2009 14:52:33 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	"Vaidyanathan Srinivasan" <svaidy@...ux.vnet.ibm.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Ingo Molnar" <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	"Suresh Siddha" <suresh.b.siddha@...el.com>,
	"Balbir Singh" <balbir@...ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: [PATCH 3 3/6] sched: Add Comments at the beginning of
	find_busiest_group.

Currently there are no comments pertaining to power-savings balance in
the function find_busiest_group. Add appropriate comments.

Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Ingo Molnar <mingo@...e.hu>
---

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

diff --git a/kernel/sched.c b/kernel/sched.c
index 407ee03..864c6ca 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3090,6 +3090,23 @@ static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
  * find_busiest_group finds and returns the busiest CPU group within the
  * domain. It calculates and returns the amount of weighted load which
  * should be moved to restore balance via the imbalance parameter.
+ *
+ * Power-savings-balance: Through the sysfs tunables sched_mc/smt_power_savings
+ * he user can opt for aggressive task consolidation as a means to save power.
+ * When this is activated, we would have the SD_POWERSAVINGS_BALANCE flag
+ * set for appropriate sched_domains,
+ *
+ * Within such sched_domains, find_busiest_group would try to identify
+ * a sched_group which can be freed-up and whose tasks can be migrated to
+ * a sibling group which has the capacity to accomodate the former's tasks.
+ * If such a "can-go-idle" sched_group does exist, then the sibling group
+ * which can accomodate it's tasks is returned as the busiest group.
+ *
+ * Furthermore, if the user opts for more aggressive power-aware load
+ * balancing through sched_smt/mc_power_savings = 2, i.e when the
+ * active_power_savings_level greater or equal to POWERSAVINGS_BALANCE_WAKEUP,
+ * find_busiest_group will also nominate the preferred CPU, on which the tasks
+ * should hence forth be woken up on, instead of bothering an idle-cpu.
  */
 static struct sched_group *
 find_busiest_group(struct sched_domain *sd, int this_cpu,

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