need_active_balance() gates the asymmetric packing based due to power
save logic, but for packing we don't care.

This marks the type of balanace we are attempting to do perform from
f_b_g() and stops need_active_balance() power save logic gating a
balance in the asymmetric packing case.

Signed-off-by: Michael Neuling <mikey@neuling.org>

---

 kernel/sched_fair.c |   24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

Index: linux-2.6-ozlabs/kernel/sched_fair.c
===================================================================
--- linux-2.6-ozlabs.orig/kernel/sched_fair.c
+++ linux-2.6-ozlabs/kernel/sched_fair.c
@@ -91,6 +91,13 @@ const_debug unsigned int sysctl_sched_mi
 
 static const struct sched_class fair_sched_class;
 
+enum balance_type {
+	BALANCE_NONE = 0,
+	BALANCE_LOAD,
+	BALANCE_POWER,
+	BALANCE_PACKING
+};
+
 /**************************************************************
  * CFS operations on generic schedulable entities:
  */
@@ -2783,7 +2790,8 @@ static inline void calculate_imbalance(s
 static struct sched_group *
 find_busiest_group(struct sched_domain *sd, int this_cpu,
 		   unsigned long *imbalance, enum cpu_idle_type idle,
-		   int *sd_idle, const struct cpumask *cpus, int *balance)
+		   int *sd_idle, const struct cpumask *cpus, int *balance,
+		   enum balance_type *bt)
 {
 	struct sd_lb_stats sds;
 
@@ -2808,6 +2816,7 @@ find_busiest_group(struct sched_domain *
 	if (!(*balance))
 		goto ret;
 
+	*bt = BALANCE_PACKING;
 	if ((idle == CPU_IDLE || idle == CPU_NEWLY_IDLE) &&
 	    check_asym_packing(sd, &sds, this_cpu, imbalance))
 		return sds.busiest;
@@ -2828,6 +2837,7 @@ find_busiest_group(struct sched_domain *
 
 	/* Looks like there is an imbalance. Compute it */
 	calculate_imbalance(&sds, this_cpu, imbalance);
+	*bt = BALANCE_LOAD;
 	return sds.busiest;
 
 out_balanced:
@@ -2835,10 +2845,12 @@ out_balanced:
 	 * There is no obvious imbalance. But check if we can do some balancing
 	 * to ...
[ message continues ]

This explanation lacks a how/why.

So the problem is that need_active_balance() ends up returning false and
prevents the active balance from pulling a task to a lower available SMT

At the very least this wants more comments in the code. I'm not really
charmed by having to add yet another variable to pass around that mess,

--

[ message continues ]

Yeah, the current case only ever reads the balance type in the !=
BALANCE_POWER so a full enum might be overkill, but I though it might
come in useful for someone else.

Updated patch below.

Mikey


[PATCH 4/5] sched: fix need_active_balance() from preventing asymmetric packing 

need_active_balance() prevents a task being pulled onto a newly idle
package in an attempt to completely free it so it can be powered down.
Hence it returns false to load_balance() and prevents the active
balance from occurring.

Unfortunately, when asymmetric packing is enabled at the sibling level
this power save logic is preventing the packing balance from moving a
task to a lower idle thread.  At the sibling level SD_SHARE_CPUPOWER
and parent(SD_POWERSAVINGS_BALANCE) are enabled and the domain is also
non-idle (since we have at least 1 task we are trying to move down).
Hence the following code, prevents the an active balance from
occurring:

		if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
		    !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
			return 0;

To fix this, this patch classifies the type of balance we are
attempting to perform into none, load, power and packing based on what
function finds busiest in f_b_g().  This classification is then used
by need_active_balance() to prevent the above power saving logic from
stopping a balance due to asymmetric packing.  This ensures tasks can
be correctly moved down to lower sibling threads.  

Signed-off-by: Michael Neuling <mikey@neuling.org>
---

 kernel/sched_fair.c |   35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

Index: linux-2.6-ozlabs/kernel/sched_fair.c
===================================================================
--- linux-2.6-ozlabs.orig/kernel/sched_fair.c
+++ linux-2.6-ozlabs/kernel/sched_fair.c
@@ -91,6 +91,14 @@ const_debug unsigned int sysctl_sched_mi
 
 static const struct sched_class fair_sched_class;
 
+/* Enum to classify the type of balance we are ...
[ message continues ]