Messages in this thread Patch in this message | | | Date | Thu, 22 Feb 2007 08:31:49 +0100 | From | Eric Dumazet <> | Subject | [PATCH] Speedup divides by cpu_power in scheduler |
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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.
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 <dada1@cosmosbay.com> --- 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 08:53:26.000000000 +0100 @@ -669,7 +669,12 @@ 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). */ - 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.orig 2007-02-21 21:10:54.000000000 +0100 +++ linux-2.6.21-rc1-ed/kernel/sched.c 2007-02-22 08:46:56.000000000 +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 +183,26 @@ 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; + BUG_ON(sg->cpu_power == 0); + 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 +1262,8 @@ 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; @@ -2355,7 +2377,8 @@ find_busiest_group(struct sched_domain * 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; @@ -2510,8 +2533,8 @@ small_imbalance: 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 * min(busiest_load_per_task, max_load - tmp); @@ -2519,10 +2542,11 @@ small_imbalance: /* Amount of load we'd add */ 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; + 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; @@ -6352,7 +6376,7 @@ 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 +6451,8 @@ 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 +6463,16 @@ 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); } | |