Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation helper function

Patrick Bellasi
2018-03-21T12:39:21Z

On 20-Mar 09:43, Dietmar Eggemann wrote:
> From: Quentin Perret <quentin.perret@arm.com>

[...]

> +static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
> +{
> + unsigned long util, fdom_max_util;
> + struct capacity_state *cs;
> + unsigned long energy = 0;
> + struct freq_domain *fdom;
> + int cpu;
> +
> + for_each_freq_domain(fdom) {
> + fdom_max_util = 0;
> + for_each_cpu_and(cpu, &(fdom->span), cpu_online_mask) {
> + util = cpu_util_next(cpu, p, dst_cpu);

Would be nice to find a way to cache all these util and reuse them
below... even just to ensure data consistency between the "cs"
computation and its usage...

> + fdom_max_util = max(util, fdom_max_util);
> + }
> +
> + /*
> + * Here we assume that the capacity states of CPUs belonging to
> + * the same frequency domains are shared. Hence, we look at the
> + * capacity state of the first CPU and re-use it for all.
> + */
> + cpu = cpumask_first(&(fdom->span));
> + cs = find_cap_state(cpu, fdom_max_util);
^^^^

The above code could theoretically return NULL, although likely EAS is
completely disabled if em->nb_cap_states == 0, right?

If that's the case then, in the previous function, you can certainly
avoid the initialization of *cs and maybe also add an explicit:

BUG_ON(em->nb_cap_states == 0);

which helps even just as "in code documentation".

But, I'm not sure if maintainers like BUG_ON in scheduler code :)

> +
> + /*
> + * The energy consumed by each CPU is derived from the power
^

Should we make more explicit that this is just the "active" energy
consumed?

> + * it dissipates at the expected OPP and its percentage of
> + * busy time.
> + */
> + for_each_cpu_and(cpu, &(fdom->span), cpu_online_mask) {
> + util = cpu_util_next(cpu, p, dst_cpu);
> + energy += cs->power * util / cs->cap;
> + }
> + }

nit-pick: empty line before return?

> + return energy;
> +}
> +
> /*
> * 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,
> --
> 2.11.0
>

--
#include <best/regards.h>

Patrick Bellasi