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sched: Fix SCHED_MC regression caused by change in sched cpu_power

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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 f93e65c186
 |  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>
Cc: <stable@kernel.org> # [2.6.32.x, 2.6.33.x]
LKML-Reference: <1266970432.11588.22.camel@sbs-t61.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Suresh Siddha 2010-02-23 16:13:52 -08:00 committed by Ingo Molnar
parent 83ab0aa0d5
commit dd5feea14a
1 changed files with 43 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

76
kernel/sched_fair.c
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@ -2097,6 +2097,7 @@ 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)
@ -2416,14 +2417,12 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
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);
/* 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;
@ -2453,7 +2452,6 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
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);
}
/*
@ -2473,7 +2471,6 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
/* 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.
@ -2483,8 +2480,8 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
* 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;
@ -2553,6 +2550,7 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
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;
}
@ -2575,6 +2573,7 @@ static inline void fix_small_imbalance(struct sd_lb_stats *sds,
{
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;
@ -2585,8 +2584,12 @@ static inline void fix_small_imbalance(struct sd_lb_stats *sds,
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;
}
@ -2637,7 +2640,14 @@ static inline void fix_small_imbalance(struct sd_lb_stats *sds,
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
@ -2648,9 +2658,29 @@ static inline void calculate_imbalance(struct sd_lb_stats *sds, int this_cpu,
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,
@ -2718,7 +2748,6 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
* 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))
goto ret;
@ -2737,25 +2766,6 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
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;