Re: [PATCH v7 11/14] sched/fair: Introduce an energy estimation helper function

Peter Zijlstra
2018-10-04T08:34:57Z

On Wed, Sep 12, 2018 at 10:13:06AM +0100, Quentin Perret wrote:
> +static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
> +{
> + struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
> + unsigned long util_est, util = READ_ONCE(cfs_rq->avg.util_avg);
> +
> + /*
> + * If @p migrates from @cpu to another, remove its contribution. Or,
> + * if @p migrates from another CPU to @cpu, add its contribution. In
> + * the other cases, @cpu is not impacted by the migration, so the
> + * util_avg should already be correct.
> + */
> + if (task_cpu(p) == cpu && dst_cpu != cpu)
> + util = max_t(long, util - task_util(p), 0);

That's not quite right; what you want to check for is underflow, but the
above also results in 0 if util - task_util() > LONG_MAX without an
underflow.

You could write: sub_positive(&util, task_util(p));

> + else if (task_cpu(p) != cpu && dst_cpu == cpu)
> + util += task_util(p);
> +
> + if (sched_feat(UTIL_EST)) {
> + util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> +
> + /*
> + * During wake-up, the task isn't enqueued yet and doesn't
> + * appear in the cfs_rq->avg.util_est.enqueued of any rq,
> + * so just add it (if needed) to "simulate" what will be
> + * cpu_util() after the task has been enqueued.
> + */
> + if (dst_cpu == cpu)
> + util_est += _task_util_est(p);
> +
> + util = max(util, util_est);
> + }
> +
> + return min_t(unsigned long, util, capacity_orig_of(cpu));

AFAICT both @util and capacity_orig_of() have 'unsigned long' as type.

> +}
> +
> +/*
> + * compute_energy(): Estimates the energy that would be consumed if @p was
> + * migrated to @dst_cpu. compute_energy() predicts what will be the utilization
> + * landscape of the * CPUs after the task migration, and uses the Energy Model
> + * to compute what would be the energy if we decided to actually migrate that
> + * task.
> + */
> +static long compute_energy(struct task_struct *p, int dst_cpu,
> + struct perf_domain *pd)

static long
compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)

> +{
> + long util, max_util, sum_util, energy = 0;
> + int cpu;
> +
> + while (pd) {
> + max_util = sum_util = 0;
> + /*
> + * The capacity state of CPUs of the current rd can be driven by
> + * CPUs of another rd if they belong to the same performance
> + * domain. So, account for the utilization of these CPUs too
> + * by masking pd with cpu_online_mask instead of the rd span.
> + *
> + * If an entire performance domain is outside of the current rd,
> + * it will not appear in its pd list and will not be accounted
> + * by compute_energy().
> + */
> + for_each_cpu_and(cpu, perf_domain_span(pd), cpu_online_mask) {
> + util = cpu_util_next(cpu, p, dst_cpu);
> + util = schedutil_freq_util(cpu, util, ENERGY_UTIL);
> + max_util = max(util, max_util);
> + sum_util += util;
> + }
> +
> + energy += em_pd_energy(pd->obj, max_util, sum_util);
> + pd = pd->next;
> + }

No real strong preference, but you could write that like:

for (; pd; pd = pd->next) {
}

> +
> + return energy;
> +}