I noticed expensive divides done in try_to_wakeup() and find_busiest_group()
on a bi dual core Opteron machine (total of 4 cores), moderatly loaded (15.000
context switch per second)
oprofile numbers :
CPU: AMD64 processors, speed 2600.05 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit
mask of 0x00 (No unit mask) count 50000
samples % symbol name
...
613914 1.0498 try_to_wake_up
834 0.0013 :ffffffff80227ae1: div %rcx
77513 0.1191 :ffffffff80227ae4: mov %rax,%r11
608893 1.0413 find_busiest_group
1841 0.0031 :ffffffff802260bf: div %rdi
140109 0.2394 :ffffffff802260c2: test %sil,%sil
Some of these divides can use the reciprocal divides we introduced some time
ago (currently used in slab AFAIK)
We can assume a load will fit in a 32bits number, because with a
SCHED_LOAD_SCALE=128 value, its still a theorical limit of 33554432
When/if we reach this limit one day, probably cpus will have a fast hardware
divide and we can zap the reciprocal divide trick.
Ingo suggested to rename cpu_power to __cpu_power to make clear it should not
be modified without changing its reciprocal value too.
I did not convert the divide in cpu_avg_load_per_task(), because tracking
nr_running changes may be not worth it ? We could use a static table of 32
reciprocal values but it would add a conditional branch and table lookup.
Signed-off-by: Eric Dumazet <[email protected]>
--- linux-2.6.21-rc1/include/linux/sched.h 2007-02-21 21:08:32.000000000 +0100
+++ linux-2.6.21-rc1-ed/include/linux/sched.h 2007-02-22 10:12:00.000000000 +0100
@@ -668,8 +668,14 @@ struct sched_group {
/*
* CPU power of this group, SCHED_LOAD_SCALE being max power for a
* single CPU. This is read only (except for setup, hotplug CPU).
+ * Note : Never change cpu_power without recompute its reciprocal
*/
- unsigned long cpu_power;
+ unsigned int __cpu_power;
+ /*
+ * reciprocal value of cpu_power to avoid expensive divides
+ * (see include/linux/reciprocal_div.h)
+ */
+ u32 reciprocal_cpu_power;
};
struct sched_domain {
--- linux-2.6.21-rc1/kernel/sched.c 2007-02-21 21:10:54.000000000 +0100
+++ linux-2.6.21-rc1-ed/kernel/sched.c 2007-02-22 10:12:00.000000000 +0100
@@ -52,6 +52,7 @@
#include <linux/tsacct_kern.h>
#include <linux/kprobes.h>
#include <linux/delayacct.h>
+#include <linux/reciprocal_div.h>
#include <asm/tlb.h>
#include <asm/unistd.h>
@@ -182,6 +183,25 @@ static unsigned int static_prio_timeslic
}
/*
+ * Divide a load by a sched group cpu_power : (load / sg->__cpu_power)
+ * Since cpu_power is a 'constant', we can use a reciprocal divide.
+ */
+static inline u32 sg_div_cpu_power(const struct sched_group *sg, u32 load)
+{
+ return reciprocal_divide(load, sg->reciprocal_cpu_power);
+}
+/*
+ * Each time a sched group cpu_power is changed,
+ * we must compute its reciprocal value
+ */
+static inline void sg_inc_cpu_power(struct sched_group *sg, u32 val)
+{
+ sg->__cpu_power += val;
+ sg->reciprocal_cpu_power = reciprocal_value(sg->__cpu_power);
+}
+
+
+/*
* task_timeslice() scales user-nice values [ -20 ... 0 ... 19 ]
* to time slice values: [800ms ... 100ms ... 5ms]
*
@@ -1241,7 +1261,8 @@ find_idlest_group(struct sched_domain *s
}
/* Adjust by relative CPU power of the group */
- avg_load = (avg_load * SCHED_LOAD_SCALE) / group->cpu_power;
+ avg_load = sg_div_cpu_power(group,
+ avg_load * SCHED_LOAD_SCALE);
if (local_group) {
this_load = avg_load;
@@ -2352,12 +2373,13 @@ find_busiest_group(struct sched_domain *
}
total_load += avg_load;
- total_pwr += group->cpu_power;
+ total_pwr += group->__cpu_power;
/* Adjust by relative CPU power of the group */
- avg_load = (avg_load * SCHED_LOAD_SCALE) / group->cpu_power;
+ avg_load = sg_div_cpu_power(group,
+ avg_load * SCHED_LOAD_SCALE);
- group_capacity = group->cpu_power / SCHED_LOAD_SCALE;
+ group_capacity = group->__cpu_power / SCHED_LOAD_SCALE;
if (local_group) {
this_load = avg_load;
@@ -2468,8 +2490,8 @@ group_next:
max_pull = min(max_load - avg_load, max_load - busiest_load_per_task);
/* How much load to actually move to equalise the imbalance */
- *imbalance = min(max_pull * busiest->cpu_power,
- (avg_load - this_load) * this->cpu_power)
+ *imbalance = min(max_pull * busiest->__cpu_power,
+ (avg_load - this_load) * this->__cpu_power)
/ SCHED_LOAD_SCALE;
/*
@@ -2503,27 +2525,28 @@ small_imbalance:
* moving them.
*/
- pwr_now += busiest->cpu_power *
+ pwr_now += busiest->__cpu_power *
min(busiest_load_per_task, max_load);
- pwr_now += this->cpu_power *
+ pwr_now += this->__cpu_power *
min(this_load_per_task, this_load);
pwr_now /= SCHED_LOAD_SCALE;
/* Amount of load we'd subtract */
- tmp = busiest_load_per_task * SCHED_LOAD_SCALE /
- busiest->cpu_power;
+ tmp = sg_div_cpu_power(busiest,
+ busiest_load_per_task * SCHED_LOAD_SCALE);
if (max_load > tmp)
- pwr_move += busiest->cpu_power *
+ pwr_move += busiest->__cpu_power *
min(busiest_load_per_task, max_load - tmp);
/* Amount of load we'd add */
- if (max_load * busiest->cpu_power <
+ if (max_load * busiest->__cpu_power <
busiest_load_per_task * SCHED_LOAD_SCALE)
- tmp = max_load * busiest->cpu_power / this->cpu_power;
+ tmp = sg_div_cpu_power(this,
+ max_load * busiest->__cpu_power);
else
- tmp = busiest_load_per_task * SCHED_LOAD_SCALE /
- this->cpu_power;
- pwr_move += this->cpu_power *
+ tmp = sg_div_cpu_power(this,
+ busiest_load_per_task * SCHED_LOAD_SCALE);
+ pwr_move += this->__cpu_power *
min(this_load_per_task, this_load + tmp);
pwr_move /= SCHED_LOAD_SCALE;
@@ -5486,7 +5509,7 @@ static void sched_domain_debug(struct sc
break;
}
- if (!group->cpu_power) {
+ if (!group->__cpu_power) {
printk("\n");
printk(KERN_ERR "ERROR: domain->cpu_power not "
"set\n");
@@ -5663,7 +5686,7 @@ init_sched_build_groups(cpumask_t span,
continue;
sg->cpumask = CPU_MASK_NONE;
- sg->cpu_power = 0;
+ sg->__cpu_power = 0;
for_each_cpu_mask(j, span) {
if (group_fn(j, cpu_map, NULL) != group)
@@ -6352,7 +6375,7 @@ next_sg:
continue;
}
- sg->cpu_power += sd->groups->cpu_power;
+ sg_inc_cpu_power(sg, sd->groups->__cpu_power);
}
sg = sg->next;
if (sg != group_head)
@@ -6427,6 +6450,8 @@ static void init_sched_groups_power(int
child = sd->child;
+ sd->groups->__cpu_power = 0;
+
/*
* For perf policy, if the groups in child domain share resources
* (for example cores sharing some portions of the cache hierarchy
@@ -6437,18 +6462,16 @@ static void init_sched_groups_power(int
if (!child || (!(sd->flags & SD_POWERSAVINGS_BALANCE) &&
(child->flags &
(SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES)))) {
- sd->groups->cpu_power = SCHED_LOAD_SCALE;
+ sg_inc_cpu_power(sd->groups, SCHED_LOAD_SCALE);
return;
}
- sd->groups->cpu_power = 0;
-
/*
* add cpu_power of each child group to this groups cpu_power
*/
group = child->groups;
do {
- sd->groups->cpu_power += group->cpu_power;
+ sg_inc_cpu_power(sd->groups, group->__cpu_power);
group = group->next;
} while (group != child->groups);
}
@@ -6608,7 +6631,7 @@ static int build_sched_domains(const cpu
sd = &per_cpu(node_domains, j);
sd->groups = sg;
}
- sg->cpu_power = 0;
+ sg->__cpu_power = 0;
sg->cpumask = nodemask;
sg->next = sg;
cpus_or(covered, covered, nodemask);
@@ -6636,7 +6659,7 @@ static int build_sched_domains(const cpu
"Can not alloc domain group for node %d\n", j);
goto error;
}
- sg->cpu_power = 0;
+ sg->__cpu_power = 0;
sg->cpumask = tmp;
sg->next = prev->next;
cpus_or(covered, covered, tmp);
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