From: Eric Dumazet Date: Tue, 8 May 2007 07:32:57 +0000 (-0700) Subject: Speed up divides by cpu_power in scheduler X-Git-Tag: v2.6.22-rc1~578 X-Git-Url: http://git.rot13.org/?p=powerpc.git;a=commitdiff_plain;h=5517d86bea237c1d7078840182d9ebc0fe4c1afc Speed up divides by cpu_power in scheduler 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. [akpm@linux-foundation.org: !SMP build fix] Signed-off-by: Eric Dumazet Acked-by: Ingo Molnar Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/include/linux/sched.h b/include/linux/sched.h index 15ab3e0395..3d95c480f5 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -680,8 +680,14 @@ 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 { diff --git a/kernel/sched.c b/kernel/sched.c index 7459928623..e4a5888549 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -52,8 +52,9 @@ #include #include #include -#include +#include +#include #include /* @@ -181,6 +182,27 @@ static unsigned int static_prio_timeslice(int static_prio) return SCALE_PRIO(DEF_TIMESLICE, static_prio); } +#ifdef CONFIG_SMP +/* + * 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); +} +#endif + /* * task_timeslice() scales user-nice values [ -20 ... 0 ... 19 ] * to time slice values: [800ms ... 100ms ... 5ms] @@ -1256,7 +1278,8 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) } /* 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; @@ -2367,12 +2390,13 @@ find_busiest_group(struct sched_domain *sd, int this_cpu, } 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; @@ -2483,8 +2507,8 @@ 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; /* @@ -2518,28 +2542,29 @@ small_imbalance: * moving them. */ - pwr_now += busiest->cpu_power * - min(busiest_load_per_task, max_load); - pwr_now += this->cpu_power * - min(this_load_per_task, this_load); + pwr_now += busiest->__cpu_power * + min(busiest_load_per_task, max_load); + 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 * - min(this_load_per_task, this_load + tmp); + 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; /* Move if we gain throughput */ @@ -5501,7 +5526,7 @@ static void sched_domain_debug(struct sched_domain *sd, int cpu) break; } - if (!group->cpu_power) { + if (!group->__cpu_power) { printk("\n"); printk(KERN_ERR "ERROR: domain->cpu_power not " "set\n"); @@ -5678,7 +5703,7 @@ init_sched_build_groups(cpumask_t span, const cpumask_t *cpu_map, 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) @@ -6367,7 +6392,7 @@ 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) @@ -6442,6 +6467,8 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd) 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 @@ -6452,18 +6479,16 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd) 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); } @@ -6623,7 +6648,7 @@ static int build_sched_domains(const cpumask_t *cpu_map) 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); @@ -6651,7 +6676,7 @@ static int build_sched_domains(const cpumask_t *cpu_map) "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);