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Date: Thu, 18 Feb 2010 18:16:47 -0800
From: Suresh Siddha <suresh.b.siddha@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
"Ma, Ling" <ling.ma@...el.com>,
"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
"ego@...ibm.com" <ego@...ibm.com>,
"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>
Subject: Re: change in sched cpu_power causing regressions with SCHED_MC
On Sat, 2010-02-13 at 02:36 -0800, Peter Zijlstra wrote:
> On Fri, 2010-02-12 at 17:31 -0800, Suresh Siddha wrote:
> >
> > We have one more problem that Yanmin and Ling Ma reported. On a dual
> > socket quad-core platforms (for example platforms based on NHM-EP), we
> > are seeing scenarios where one socket is completely busy (with all the 4
> > cores running with 4 tasks) and another socket is 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 like. We will have all these
> > benefits if we move two of those tasks to the other socket. Now both the
> > sockets can potentially go to turbo etc and improve performance.
> >
> > In short, your recent change (shown below) broke this behavior. In the
> > kernel summit you mentioned you made this change with out affecting the
> > behavior of SMT/MC. And my testing immediately after kernel-summit also
> > didn't show the problem (perhaps my test didn't hit this specific
> > change). But apparently we are having performance issues with this patch
> > (Ling Ma's bisect pointed to this patch). I will look more detailed into
> > this after the long weekend (to see if we can catch this scenario in
> > fix_small_imbalance() etc). But wanted to give you a quick heads up.
> > Thanks.
>
> Right, so the behaviour we want should be provided by SD_PREFER_SIBLING,
> it provides the capacity==1 thing the cpu_power games used to provide.
>
> Not saying it's not broken, but that's where the we should be looking to
> fix it.
Peter, Some portions of code in fix_small_imbalance() and
calculate_imbalance() are comparing max_load and busiest_load_per_task.
Some of these comparisons are ok but some of them are broken. Broken
comparisons are assuming that the cpu_power is SCHED_LOAD_SCALE. Also
there is one check which still assumes that the world is balanced when
max_load <= busiest_load_per_task. This is wrong with the recent changes
(as cpu power no longer reflects the group capacity that is needed to
implement SCHED_MC/SCHED_SMT).
The appended patch works for me and fixes the SCHED_MC performance
behavior. I am sending this patch out for a quick review and I will do
bit more testing tomorrow and If you don't follow what I am doing in
this patch and why, then stay tuned for a patch with complete changelog
that I will send tomorrow. Good night. Thanks.
---
Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
diff --git a/kernel/sched.c b/kernel/sched.c
index 3a8fb30..2f4cac0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3423,6 +3423,7 @@ 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)
@@ -3880,6 +3881,7 @@ 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;
}
@@ -3902,6 +3904,7 @@ 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;
@@ -3912,8 +3915,12 @@ 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;
}
@@ -3964,7 +3971,7 @@ 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;
/*
* 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
@@ -3975,9 +3982,30 @@ 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, 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.
+ */
+ 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,
@@ -4069,19 +4097,6 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
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);
--
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