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Subject[tip:sched/balancing] sched: Add comments to find_busiest_group() function
Commit-ID:  7b6340ef884aff69a54f8a530c73ad9da0a7c388
Gitweb: http://git.kernel.org/tip/7b6340ef884aff69a54f8a530c73ad9da0a7c388
Author: Gautham R Shenoy <ego@in.ibm.com>
AuthorDate: Wed, 25 Mar 2009 14:44:27 +0530
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 25 Mar 2009 10:30:49 +0100

sched: Add comments to find_busiest_group() function

Impact: cleanup

Add /** style comments around find_busiest_group(). Also add a few
explanatory comments.
This concludes the find_busiest_group() cleanup. The function is
now down to 72 lines from the original 313 lines.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: "Balbir Singh" <balbir@in.ibm.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: "Dhaval Giani" <dhaval@linux.vnet.ibm.com>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: "Vaidyanathan Srinivasan" <svaidy@linux.vnet.ibm.com>
LKML-Reference: <20090325091427.13992.18933.stgit@sofia.in.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
kernel/sched.c | 50 ++++++++++++++++++++++++++++++++++++++++++--------
1 files changed, 42 insertions(+), 8 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 5f21658..b7723bd 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3676,10 +3676,30 @@ static inline void calculate_imbalance(struct sd_lb_stats *sds, int this_cpu,
}
/******* find_busiest_group() helpers end here *********************/

-/*
- * 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.
+/**
+ * find_busiest_group - Returns the busiest group within the sched_domain
+ * if there is an imbalance. If there isn't an imbalance, and
+ * the user has opted for power-savings, it returns a group whose
+ * CPUs can be put to idle by rebalancing those tasks elsewhere, if
+ * such a group exists.
+ *
+ * Also calculates the amount of weighted load which should be moved
+ * to restore balance.
+ *
+ * @sd: The sched_domain whose busiest group is to be returned.
+ * @this_cpu: The cpu for which load balancing is currently being performed.
+ * @imbalance: Variable which stores amount of weighted load which should
+ * be moved to restore balance/put a group to idle.
+ * @idle: The idle status of this_cpu.
+ * @sd_idle: The idleness of sd
+ * @cpus: The set of CPUs under consideration for load-balancing.
+ * @balance: Pointer to a variable indicating if this_cpu
+ * is the appropriate cpu to perform load balancing at this_level.
+ *
+ * Returns: - the busiest group if imbalance exists.
+ * - If no imbalance and user has opted for power-savings balance,
+ * return the least loaded group whose CPUs can be
+ * put to idle by rebalancing its tasks onto our group.
*/
static struct sched_group *
find_busiest_group(struct sched_domain *sd, int this_cpu,
@@ -3697,17 +3717,31 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
update_sd_lb_stats(sd, this_cpu, idle, sd_idle, cpus,
balance, &sds);

+ /* Cases where imbalance does not exist from POV of this_cpu */
+ /* 1) this_cpu is not the appropriate cpu to perform load balancing
+ * at this level.
+ * 2) There is no busy sibling group to pull from.
+ * 3) This group is the busiest group.
+ * 4) This group is more busy than the avg busieness at this
+ * sched_domain.
+ * 5) The imbalance is within the specified limit.
+ * 6) Any rebalance would lead to ping-pong
+ */
if (balance && !(*balance))
goto ret;

- if (!sds.busiest || sds.this_load >= sds.max_load
- || sds.busiest_nr_running == 0)
+ if (!sds.busiest || sd.busiest_nr_running == 0)
+ goto out_balanced;
+
+ if (sds.this_load >= sds.max_load)
goto out_balanced;

sds.avg_load = (SCHED_LOAD_SCALE * sds.total_load) / sds.total_pwr;

- if (sds.this_load >= sds.avg_load ||
- 100*sds.max_load <= sd->imbalance_pct * sds.this_load)
+ if (sds.this_load >= sds.avg_load)
+ goto out_balanced;
+
+ if (100 * sds.max_load <= sd->imbalance_pct * sds.this_load)
goto out_balanced;

sds.busiest_load_per_task /= sds.busiest_nr_running;

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