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Subject[041/116] sched: Fix SCHED_MC regression caused by change in sched cpu_power
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Suresh Siddha <suresh.b.siddha@intel.com>

commit dd5feea14a7de4edbd9f36db1a2db785de91b88d upstream

On platforms like dual socket quad-core platform, the scheduler load
balancer is not detecting the load imbalances in certain scenarios. This
is leading to scenarios like where one socket is completely busy (with
all the 4 cores running with 4 tasks) and leaving another socket
completely idle. This causes performance issues as those 4 tasks share
the memory controller, last-level cache bandwidth etc. Also we won't be
taking advantage of turbo-mode as much as we would like, etc.

Some of the comparisons in the scheduler load balancing code are
comparing the "weighted cpu load that is scaled wrt sched_group's
cpu_power" with the "weighted average load per task that is not scaled
wrt sched_group's cpu_power". While this has probably been broken for a
longer time (for multi socket numa nodes etc), the problem got aggrevated
via this recent change:

|
| commit f93e65c186ab3c05ce2068733ca10e34fd00125e
| Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
| Date: Tue Sep 1 10:34:32 2009 +0200
|
| sched: Restore __cpu_power to a straight sum of power
|

Also with this change, the sched group cpu power alone no longer reflects
the group capacity that is needed to implement MC, MT performance
(default) and power-savings (user-selectable) policies.

We need to use the computed group capacity (sgs.group_capacity, that is
computed using the SD_PREFER_SIBLING logic in update_sd_lb_stats()) to
find out if the group with the max load is above its capacity and how
much load to move etc.

Reported-by: Ma Ling <ling.ma@intel.com>
Initial-Analysis-by: Zhang, Yanmin <yanmin_zhang@linux.intel.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
[ -v2: build fix ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1266970432.11588.22.camel@sbs-t61.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

---
kernel/sched.c | 76 ++++++++++++++++++++++++++++++++-------------------------
1 file changed, 43 insertions(+), 33 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3402,6 +3402,7 @@ struct sd_lb_stats {
unsigned long max_load;
unsigned long busiest_load_per_task;
unsigned long busiest_nr_running;
+ unsigned long busiest_group_capacity;

int group_imb; /* Is there imbalance in this sd */
#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
@@ -3721,8 +3722,7 @@ static inline void update_sg_lb_stats(st
unsigned long load, max_cpu_load, min_cpu_load;
int i;
unsigned int balance_cpu = -1, first_idle_cpu = 0;
- unsigned long sum_avg_load_per_task;
- unsigned long avg_load_per_task;
+ unsigned long avg_load_per_task = 0;

if (local_group) {
balance_cpu = group_first_cpu(group);
@@ -3731,7 +3731,6 @@ static inline void update_sg_lb_stats(st
}

/* Tally up the load of all CPUs in the group */
- sum_avg_load_per_task = avg_load_per_task = 0;
max_cpu_load = 0;
min_cpu_load = ~0UL;

@@ -3761,7 +3760,6 @@ static inline void update_sg_lb_stats(st
sgs->sum_nr_running += rq->nr_running;
sgs->sum_weighted_load += weighted_cpuload(i);

- sum_avg_load_per_task += cpu_avg_load_per_task(i);
}

/*
@@ -3779,7 +3777,6 @@ static inline void update_sg_lb_stats(st
/* Adjust by relative CPU power of the group */
sgs->avg_load = (sgs->group_load * SCHED_LOAD_SCALE) / group->cpu_power;

-
/*
* Consider the group unbalanced when the imbalance is larger
* than the average weight of two tasks.
@@ -3789,8 +3786,8 @@ static inline void update_sg_lb_stats(st
* normalized nr_running number somewhere that negates
* the hierarchy?
*/
- avg_load_per_task = (sum_avg_load_per_task * SCHED_LOAD_SCALE) /
- group->cpu_power;
+ if (sgs->sum_nr_running)
+ avg_load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;

if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task)
sgs->group_imb = 1;
@@ -3859,6 +3856,7 @@ static inline void update_sd_lb_stats(st
sds->max_load = sgs.avg_load;
sds->busiest = group;
sds->busiest_nr_running = sgs.sum_nr_running;
+ sds->busiest_group_capacity = sgs.group_capacity;
sds->busiest_load_per_task = sgs.sum_weighted_load;
sds->group_imb = sgs.group_imb;
}
@@ -3881,6 +3879,7 @@ static inline void fix_small_imbalance(s
{
unsigned long tmp, pwr_now = 0, pwr_move = 0;
unsigned int imbn = 2;
+ unsigned long scaled_busy_load_per_task;

if (sds->this_nr_running) {
sds->this_load_per_task /= sds->this_nr_running;
@@ -3891,8 +3890,12 @@ static inline void fix_small_imbalance(s
sds->this_load_per_task =
cpu_avg_load_per_task(this_cpu);

- if (sds->max_load - sds->this_load + sds->busiest_load_per_task >=
- sds->busiest_load_per_task * imbn) {
+ scaled_busy_load_per_task = sds->busiest_load_per_task
+ * SCHED_LOAD_SCALE;
+ scaled_busy_load_per_task /= sds->busiest->cpu_power;
+
+ if (sds->max_load - sds->this_load + scaled_busy_load_per_task >=
+ (scaled_busy_load_per_task * imbn)) {
*imbalance = sds->busiest_load_per_task;
return;
}
@@ -3943,7 +3946,14 @@ static inline void fix_small_imbalance(s
static inline void calculate_imbalance(struct sd_lb_stats *sds, int this_cpu,
unsigned long *imbalance)
{
- unsigned long max_pull;
+ unsigned long max_pull, load_above_capacity = ~0UL;
+
+ sds->busiest_load_per_task /= sds->busiest_nr_running;
+ if (sds->group_imb) {
+ sds->busiest_load_per_task =
+ min(sds->busiest_load_per_task, sds->avg_load);
+ }
+
/*
* In the presence of smp nice balancing, certain scenarios can have
* max load less than avg load(as we skip the groups at or below
@@ -3954,9 +3964,29 @@ static inline void calculate_imbalance(s
return fix_small_imbalance(sds, this_cpu, imbalance);
}

- /* Don't want to pull so many tasks that a group would go idle */
- max_pull = min(sds->max_load - sds->avg_load,
- sds->max_load - sds->busiest_load_per_task);
+ if (!sds->group_imb) {
+ /*
+ * Don't want to pull so many tasks that a group would go idle.
+ */
+ load_above_capacity = (sds->busiest_nr_running -
+ sds->busiest_group_capacity);
+
+ load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_LOAD_SCALE);
+
+ load_above_capacity /= sds->busiest->cpu_power;
+ }
+
+ /*
+ * We're trying to get all the cpus to the average_load, so we don't
+ * want to push ourselves above the average load, nor do we wish to
+ * reduce the max loaded cpu below the average load. At the same time,
+ * we also don't want to reduce the group load below the group capacity
+ * (so that we can implement power-savings policies etc). Thus we look
+ * for the minimum possible imbalance.
+ * Be careful of negative numbers as they'll appear as very large values
+ * with unsigned longs.
+ */
+ max_pull = min(sds->max_load - sds->avg_load, load_above_capacity);

/* How much load to actually move to equalise the imbalance */
*imbalance = min(max_pull * sds->busiest->cpu_power,
@@ -4024,7 +4054,6 @@ find_busiest_group(struct sched_domain *
* 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;
@@ -4043,25 +4072,6 @@ find_busiest_group(struct sched_domain *
if (100 * sds.max_load <= sd->imbalance_pct * sds.this_load)
goto out_balanced;

- sds.busiest_load_per_task /= sds.busiest_nr_running;
- if (sds.group_imb)
- sds.busiest_load_per_task =
- min(sds.busiest_load_per_task, sds.avg_load);
-
- /*
- * We're trying to get all the cpus to the average_load, so we don't
- * want to push ourselves above the average load, nor do we wish to
- * reduce the max loaded cpu below the average load, as either of these
- * actions would just result in more rebalancing later, and ping-pong
- * tasks around. Thus we look for the minimum possible imbalance.
- * Negative imbalances (*we* are more loaded than anyone else) will
- * be counted as no imbalance for these purposes -- we can't fix that
- * by pulling tasks to us. Be careful of negative numbers as they'll
- * appear as very large values with unsigned longs.
- */
- if (sds.max_load <= sds.busiest_load_per_task)
- goto out_balanced;
-
/* Looks like there is an imbalance. Compute it */
calculate_imbalance(&sds, this_cpu, imbalance);
return sds.busiest;



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