Re: [RFC][PATCH 8/8] sched: remove reciprocal for cpu_power

[Andreas Herrmann]

On Tue, Sep 01, 2009 at 10:34:39AM +0200, Peter Zijlstra wrote:
> Its a source of fail, also, now that cpu_power is dynamical, its a
> waste of time.
> 
> before:
> <idle>-0   [000]   132.877936: find_busiest_group: avg_load: 0 group_load: 8241 power: 1 
> 
> after:
> bash-1689  [001]   137.862151: find_busiest_group: avg_load: 10636288 group_load: 10387 power: 1
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
>  include/linux/sched.h |   10 +----
>  kernel/sched.c        |  100 +++++++++++++++++---------------------------------
>  2 files changed, 36 insertions(+), 74 deletions(-)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -120,30 +120,8 @@
>   */
>  #define RUNTIME_INF	((u64)~0ULL)
>  
> -#ifdef CONFIG_SMP
> -
>  static void double_rq_lock(struct rq *rq1, struct rq *rq2);
>  
> -/*
> - * 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
> -
>  static inline int rt_policy(int policy)
>  {
>  	if (unlikely(policy == SCHED_FIFO || policy == SCHED_RR))
> @@ -2335,8 +2313,7 @@ find_idlest_group(struct sched_domain *s
>  		}
>  
>  		/* Adjust by relative CPU power of the group */
> -		avg_load = sg_div_cpu_power(group,
> -				avg_load * SCHED_LOAD_SCALE);
> +		avg_load = (avg_load * SCHED_LOAD_SCALE) / group->cpu_power;
>  
>  		if (local_group) {
>  			this_load = avg_load;
> @@ -3768,7 +3745,6 @@ static void update_cpu_power(struct sche
>  	unsigned long weight = cpumask_weight(sched_domain_span(sd));
>  	unsigned long power = SCHED_LOAD_SCALE;
>  	struct sched_group *sdg = sd->groups;
> -	unsigned long old = sdg->__cpu_power;
>  
>  	/* here we could scale based on cpufreq */
>  
> @@ -3783,33 +3759,26 @@ static void update_cpu_power(struct sche
>  	if (!power)
>  		power = 1;
>  
> -	if (power != old) {
> -		sdg->__cpu_power = power;
> -		sdg->reciprocal_cpu_power = reciprocal_value(power);
> -	}
> +	sdg->cpu_power = power;
>  }
>  
>  static void update_group_power(struct sched_domain *sd, int cpu)
>  {
>  	struct sched_domain *child = sd->child;
>  	struct sched_group *group, *sdg = sd->groups;
> -	unsigned long power = sdg->__cpu_power;
>  
>  	if (!child) {
>  		update_cpu_power(sd, cpu);
>  		return;
>  	}
>  
> -	sdg->__cpu_power = 0;
> +	sdg->cpu_power = 0;
>  
>  	group = child->groups;
>  	do {
> -		sdg->__cpu_power += group->__cpu_power;
> +		sdg->cpu_power += group->cpu_power;
>  		group = group->next;
>  	} while (group != child->groups);
> -
> -	if (power != sdg->__cpu_power)
> -		sdg->reciprocal_cpu_power = reciprocal_value(sdg->__cpu_power);
>  }
>  
>  /**
> @@ -3889,8 +3858,7 @@ static inline void update_sg_lb_stats(st
>  	}
>  
>  	/* Adjust by relative CPU power of the group */
> -	sgs->avg_load = sg_div_cpu_power(group,
> -			sgs->group_load * SCHED_LOAD_SCALE);
> +	sgs->avg_load = (sgs->group_load * SCHED_LOAD_SCALE) / group->cpu_power;
>  
>  
>  	/*
> @@ -3902,14 +3870,14 @@ static inline void update_sg_lb_stats(st
>  	 *      normalized nr_running number somewhere that negates
>  	 *      the hierarchy?
>  	 */
> -	avg_load_per_task = sg_div_cpu_power(group,
> -			sum_avg_load_per_task * SCHED_LOAD_SCALE);
> +	avg_load_per_task = (sum_avg_load_per_task * SCHED_LOAD_SCALE) /
> +		group->cpu_power;
>  
>  	if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task)
>  		sgs->group_imb = 1;
>  
>  	sgs->group_capacity =
> -		DIV_ROUND_CLOSEST(group->__cpu_power, SCHED_LOAD_SCALE);
> +		DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
>  }
>  
>  /**
> @@ -3951,7 +3919,7 @@ static inline void update_sd_lb_stats(st
>  			return;
>  
>  		sds->total_load += sgs.group_load;
> -		sds->total_pwr += group->__cpu_power;
> +		sds->total_pwr += group->cpu_power;
>  
>  		/*
>  		 * In case the child domain prefers tasks go to siblings
> @@ -4016,28 +3984,28 @@ static inline void fix_small_imbalance(s
>  	 * moving them.
>  	 */
>  
> -	pwr_now += sds->busiest->__cpu_power *
> +	pwr_now += sds->busiest->cpu_power *
>  			min(sds->busiest_load_per_task, sds->max_load);
> -	pwr_now += sds->this->__cpu_power *
> +	pwr_now += sds->this->cpu_power *
>  			min(sds->this_load_per_task, sds->this_load);
>  	pwr_now /= SCHED_LOAD_SCALE;
>  
>  	/* Amount of load we'd subtract */
> -	tmp = sg_div_cpu_power(sds->busiest,
> -			sds->busiest_load_per_task * SCHED_LOAD_SCALE);
> +	tmp = (sds->busiest_load_per_task * SCHED_LOAD_SCALE) /
> +		sds->busiest->cpu_power;
>  	if (sds->max_load > tmp)
> -		pwr_move += sds->busiest->__cpu_power *
> +		pwr_move += sds->busiest->cpu_power *
>  			min(sds->busiest_load_per_task, sds->max_load - tmp);
>  
>  	/* Amount of load we'd add */
> -	if (sds->max_load * sds->busiest->__cpu_power <
> +	if (sds->max_load * sds->busiest->cpu_power <
>  		sds->busiest_load_per_task * SCHED_LOAD_SCALE)
> -		tmp = sg_div_cpu_power(sds->this,
> -			sds->max_load * sds->busiest->__cpu_power);
> +		tmp = (sds->max_load * sds->busiest->cpu_power) /
> +			sds->this->cpu_power;
>  	else
> -		tmp = sg_div_cpu_power(sds->this,
> -			sds->busiest_load_per_task * SCHED_LOAD_SCALE);
> -	pwr_move += sds->this->__cpu_power *
> +		tmp = (sds->busiest_load_per_task * SCHED_LOAD_SCALE) /
> +			sds->this->cpu_power;
> +	pwr_move += sds->this->cpu_power *
>  			min(sds->this_load_per_task, sds->this_load + tmp);
>  	pwr_move /= SCHED_LOAD_SCALE;
>  
> @@ -4072,8 +4040,8 @@ static inline void calculate_imbalance(s
>  			sds->max_load - sds->busiest_load_per_task);
>  
>  	/* How much load to actually move to equalise the imbalance */
> -	*imbalance = min(max_pull * sds->busiest->__cpu_power,
> -		(sds->avg_load - sds->this_load) * sds->this->__cpu_power)
> +	*imbalance = min(max_pull * sds->busiest->cpu_power,
> +		(sds->avg_load - sds->this_load) * sds->this->cpu_power)
>  			/ SCHED_LOAD_SCALE;
>  
>  	/*
> @@ -4208,7 +4176,7 @@ static unsigned long power_of(int cpu)
>  	if (!group)
>  		return SCHED_LOAD_SCALE;
>  
> -	return group->__cpu_power;
> +	return group->cpu_power;
>  }
>  
>  /*
> @@ -7934,7 +7902,7 @@ static int sched_domain_debug_one(struct
>  			break;
>  		}
>  
> -		if (!group->__cpu_power) {
> +		if (!group->cpu_power) {
>  			printk(KERN_CONT "\n");
>  			printk(KERN_ERR "ERROR: domain->cpu_power not "
>  					"set\n");
> @@ -7958,9 +7926,9 @@ static int sched_domain_debug_one(struct
>  		cpulist_scnprintf(str, sizeof(str), sched_group_cpus(group));
>  
>  		printk(KERN_CONT " %s", str);
> -		if (group->__cpu_power != SCHED_LOAD_SCALE) {
> -			printk(KERN_CONT " (__cpu_power = %d)",
> -				group->__cpu_power);
> +		if (group->cpu_power != SCHED_LOAD_SCALE) {
> +			printk(KERN_CONT " (cpu_power = %d)",
> +				group->cpu_power);
>  		}
>  
>  		group = group->next;
> @@ -8245,7 +8213,7 @@ init_sched_build_groups(const struct cpu
>  			continue;
>  
>  		cpumask_clear(sched_group_cpus(sg));
> -		sg->__cpu_power = 0;
> +		sg->cpu_power = 0;
>  
>  		for_each_cpu(j, span) {
>  			if (group_fn(j, cpu_map, NULL, tmpmask) != group)
> @@ -8503,7 +8471,7 @@ static void init_numa_sched_groups_power
>  				continue;
>  			}
>  
> -			sg_inc_cpu_power(sg, sd->groups->__cpu_power);
> +			sg->cpu_power += sd->groups->cpu_power;
>  		}
>  		sg = sg->next;
>  	} while (sg != group_head);
> @@ -8540,7 +8508,7 @@ static int build_numa_sched_groups(struc
>  		sd->groups = sg;
>  	}
>  
> -	sg->__cpu_power = 0;
> +	sg->cpu_power = 0;
>  	cpumask_copy(sched_group_cpus(sg), d->nodemask);
>  	sg->next = sg;
>  	cpumask_or(d->covered, d->covered, d->nodemask);
> @@ -8563,7 +8531,7 @@ static int build_numa_sched_groups(struc
>  			       "Can not alloc domain group for node %d\n", j);
>  			return -ENOMEM;
>  		}
> -		sg->__cpu_power = 0;
> +		sg->cpu_power = 0;
>  		cpumask_copy(sched_group_cpus(sg), d->tmpmask);
>  		sg->next = prev->next;
>  		cpumask_or(d->covered, d->covered, d->tmpmask);
> @@ -8641,7 +8609,7 @@ static void init_sched_groups_power(int 
>  
>  	child = sd->child;
>  
> -	sd->groups->__cpu_power = 0;
> +	sd->groups->cpu_power = 0;
>  
>  	if (!child) {
>  		power = SCHED_LOAD_SCALE;
> @@ -8657,7 +8625,7 @@ static void init_sched_groups_power(int 
>  			power /= weight;
>  			power >>= SCHED_LOAD_SHIFT;
>  		}
> -		sg_inc_cpu_power(sd->groups, power);
> +		sd->groups->cpu_power += power;
>  		return;
>  	}
>  
> @@ -8666,7 +8634,7 @@ static void init_sched_groups_power(int 
>  	 */
>  	group = child->groups;
>  	do {
> -		sg_inc_cpu_power(sd->groups, group->__cpu_power);
> +		sd->groups->cpu_power += group->cpu_power;
>  		group = group->next;
>  	} while (group != child->groups);
>  }
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -870,15 +870,9 @@ 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
> +	 * single CPU.
>  	 */
> -	unsigned int __cpu_power;
> -	/*
> -	 * reciprocal value of cpu_power to avoid expensive divides
> -	 * (see include/linux/reciprocal_div.h)
> -	 */
> -	u32 reciprocal_cpu_power;
> +	unsigned int cpu_power;
>  
>  	/*
>  	 * The CPUs this group covers.
> 
> -- 
> 

Nice.
You might also want to remove the respective header file:

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -64,7 +64,6 @@
 #include <linux/tsacct_kern.h>
 #include <linux/kprobes.h>
 #include <linux/delayacct.h>
-#include <linux/reciprocal_div.h>
 #include <linux/unistd.h>
 #include <linux/pagemap.h>
 #include <linux/hrtimer.h>


Regards,
Andreas

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