Re: [PATCH v6 4/6] sched: get CPU's usage statistic

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 23/09/14 17:08, Vincent Guittot wrote:
> Monitor the usage level of each group of each sched_domain level. The usage is
> the amount of cpu_capacity that is currently used on a CPU or group of CPUs.
> We use the utilization_load_avg to evaluate the usage level of each group.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2cf153d..4097e3f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4523,6 +4523,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  	return target;
>  }
>  
> +static int get_cpu_usage(int cpu)
> +{
> +	unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg;
> +	unsigned long capacity = capacity_orig_of(cpu);
> +
> +	if (usage >= SCHED_LOAD_SCALE)
> +		return capacity + 1;

Why you are returning rq->cpu_capacity_orig + 1 (1025) in case
utilization_load_avg is greater or equal than 1024 and not usage or
(usage * capacity) >> SCHED_LOAD_SHIFT too?

In case the weight of a sched group is greater than 1, you might loose
the information that the whole sched group is over-utilized too.

You add up the individual cpu usage values for a group by
sgs->group_usage += get_cpu_usage(i) in update_sg_lb_stats and later use
sgs->group_usage in group_is_overloaded to compare it against
sgs->group_capacity (taking imbalance_pct into consideration).

> +
> +	return (usage * capacity) >> SCHED_LOAD_SHIFT;

Nit-pick: Since you're multiplying by a capacity value
(rq->cpu_capacity_orig) you should shift by SCHED_CAPACITY_SHIFT.

Just to make sure: You do this scaling of usage by cpu_capacity_orig
here only to cater for the fact that cpu_capacity_orig might be uarch
scaled (by arch_scale_cpu_capacity, !SMT) in update_cpu_capacity while
utilization_load_avg is currently not.
We don't even uArch scale on ARM TC2 big.LITTLE platform in mainline
today due to the missing clock-frequency property in the device tree.

I think it's hard for people to grasp that your patch-set takes uArch
scaling of capacity into consideration but not frequency scaling of
capacity (via arch_scale_freq_capacity, not used at the moment).

> +}
> +
>  /*
>   * select_task_rq_fair: Select target runqueue for the waking task in domains
>   * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -5663,6 +5674,7 @@ struct sg_lb_stats {
>  	unsigned long sum_weighted_load; /* Weighted load of group's tasks */
>  	unsigned long load_per_task;
>  	unsigned long group_capacity;
> +	unsigned long group_usage; /* Total usage of the group */
>  	unsigned int sum_nr_running; /* Nr tasks running in the group */
>  	unsigned int group_capacity_factor;
>  	unsigned int idle_cpus;
> @@ -6037,6 +6049,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  			load = source_load(i, load_idx);
>  
>  		sgs->group_load += load;
> +		sgs->group_usage += get_cpu_usage(i);
>  		sgs->sum_nr_running += rq->cfs.h_nr_running;
>  
>  		if (rq->nr_running > 1)
>