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Date: Fri, 02 Aug 2013 09:52:51 +0530
From: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To: Joonsoo Kim <iamjoonsoo.kim@....com>
CC: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Morten Rasmussen <morten.rasmussen@....com>,
Namhyung Kim <namhyung@...nel.org>,
Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH v2 2/3] sched: factor out code to should_we_balance()
Hi Joonsoo,
On 08/02/2013 07:20 AM, Joonsoo Kim wrote:
> Now checking whether this cpu is appropriate to balance or not
> is embedded into update_sg_lb_stats() and this checking has no direct
> relationship to this function. There is not enough reason to place
> this checking at update_sg_lb_stats(), except saving one iteration
> for sched_group_cpus.
>
> In this patch, I factor out this checking to should_we_balance() function.
> And before doing actual work for load_balancing, check whether this cpu is
> appropriate to balance via should_we_balance(). If this cpu is not
> a candidate for balancing, it quit the work immediately.
>
> With this change, we can save two memset cost and can expect better
> compiler optimization.
>
> Below is result of this patch.
>
> * Vanilla *
> text data bss dec hex filename
> 34499 1136 116 35751 8ba7 kernel/sched/fair.o
>
> * Patched *
> text data bss dec hex filename
> 34243 1136 116 35495 8aa7 kernel/sched/fair.o
>
> In addition, rename @balance to @should_balance in order to represent
> its purpose more clearly.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index eaae77e..7f51b8c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4426,22 +4426,17 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group)
> * @group: sched_group whose statistics are to be updated.
> * @load_idx: Load index of sched_domain of this_cpu for load calc.
> * @local_group: Does group contain this_cpu.
> - * @balance: Should we balance.
> * @sgs: variable to hold the statistics for this group.
> */
> static inline void update_sg_lb_stats(struct lb_env *env,
> struct sched_group *group, int load_idx,
> - int local_group, int *balance, struct sg_lb_stats *sgs)
> + int local_group, struct sg_lb_stats *sgs)
> {
> unsigned long nr_running, max_nr_running, min_nr_running;
> unsigned long load, max_cpu_load, min_cpu_load;
> - unsigned int balance_cpu = -1, first_idle_cpu = 0;
> unsigned long avg_load_per_task = 0;
> int i;
>
> - if (local_group)
> - balance_cpu = group_balance_cpu(group);
> -
> /* Tally up the load of all CPUs in the group */
> max_cpu_load = 0;
> min_cpu_load = ~0UL;
> @@ -4454,15 +4449,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> nr_running = rq->nr_running;
>
> /* Bias balancing toward cpus of our domain */
> - if (local_group) {
> - if (idle_cpu(i) && !first_idle_cpu &&
> - cpumask_test_cpu(i, sched_group_mask(group))) {
> - first_idle_cpu = 1;
> - balance_cpu = i;
> - }
> -
> + if (local_group)
> load = target_load(i, load_idx);
> - } else {
> + else {
> load = source_load(i, load_idx);
> if (load > max_cpu_load)
> max_cpu_load = load;
> @@ -4482,22 +4471,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> sgs->idle_cpus++;
> }
>
> - /*
> - * First idle cpu or the first cpu(busiest) in this sched group
> - * is eligible for doing load balancing at this and above
> - * domains. In the newly idle case, we will allow all the cpu's
> - * to do the newly idle load balance.
> - */
> - if (local_group) {
> - if (env->idle != CPU_NEWLY_IDLE) {
> - if (balance_cpu != env->dst_cpu) {
> - *balance = 0;
> - return;
> - }
> - update_group_power(env->sd, env->dst_cpu);
> - } else if (time_after_eq(jiffies, group->sgp->next_update))
> - update_group_power(env->sd, env->dst_cpu);
> - }
Observe what is happening in the above code which checks if we should
balance on the env->dst_cpu.
Only if the env->dst_cpu "belongs" to the group considered in
update_sg_lb_stats(), which means local_group = 1,should the above
checks be carried out.
Meaning, if there is a better CPU in the same group to which
env->dst_cpu belongs, to carry out load balancing for the system (in the
above case, balance_cpu), cancel the current iteration of load balancing
on env->dst_cpu. Wait for the right cpu in this group to do the load
balancing.
Keeping this in mind see the below comments around should_we_balance().
> @@ -5001,13 +4964,47 @@ static int need_active_balance(struct lb_env *env)
>
> static int active_load_balance_cpu_stop(void *data);
>
> +static int should_we_balance(struct lb_env *env)
> +{
> + struct sched_group *sg = env->sd->groups;
> + struct cpumask *sg_cpus, *sg_mask;
> + int cpu, balance_cpu = -1;
> +
> + /*
> + * In the newly idle case, we will allow all the cpu's
> + * to do the newly idle load balance.
> + */
> + if (env->idle == CPU_NEWLY_IDLE)
> + return 1;
> +
> + sg_cpus = sched_group_cpus(sg);
> + sg_mask = sched_group_mask(sg);
> + /* Try to find first idle cpu */
> + for_each_cpu_and(cpu, sg_cpus, env->cpus) {
> + if (!cpumask_test_cpu(cpu, sg_mask) || idle_cpu(cpu))
> + continue;
> +
> + balance_cpu = cpu;
> + break;
> + }
You need to iterate over all the groups of the sched domain env->sd and
not just the first group of env->sd like you are doing above. This is to
check to which group the env->dst_cpu belongs to. Only for that group
should you do the above check and set of balance_cpu and the below check
of balance_cpu != env->dst_cpu.
> +
> + if (balance_cpu == -1)
> + balance_cpu = group_balance_cpu(sg);
> +
> + /*
> + * First idle cpu or the first cpu(busiest) in this sched group
> + * is eligible for doing load balancing at this and above domains.
> + */
> + return balance_cpu != env->dst_cpu;
Regards
Preeti U Murthy
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