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>

diff --git a/kernel/sched.c b/kernel/sched.c
index 5f21658..9f8506d 100644 (file)
--- 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 || sds.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;