On 25/02/2021 09:36, vincent.donnef...@arm.com wrote:
> From: Vincent Donnefort <vincent.donnef...@arm.com>
[...]
> cpu_util_next() estimates the CPU utilization that would happen if the
> task was placed on dst_cpu as follows:
>
> max(cpu_util + task_util, cpu_util_est + _task_util_est)
>
> The task contribution to the energy delta can then be either:
>
> (1) _task_util_est, on a mostly idle CPU, where cpu_util is close to 0
> and _task_util_est > cpu_util.
> (2) task_util, on a mostly busy CPU, where cpu_util > _task_util_est.
>
> (cpu_util_est doesn't appear here. It is 0 when a CPU is idle and
> otherwise must be small enough so that feec() takes the CPU as a
> potential target for the task placement)
I still don't quite get the reasoning for (2) why task_util is used as
task contribution.
So we use 'cpu_util + task_util' instead of 'cpu_util_est +
_task_util_est' in (2).
I.e. since _task_util_est is always >= task_util during wakeup, cpu_util
must be > cpu_util_est (by more than _task_util_est - task_util).
I can see it for a CPU whose cpu_util has a fair amount of contributions
from blocked tasks which cpu_util_est wouldn't have.
[...]
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7043bb0f2621..146ac9fec4b6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6573,8 +6573,24 @@ compute_energy(struct task_struct *p, int dst_cpu,
> struct perf_domain *pd)
> * its pd list and will not be accounted by compute_energy().
> */
> for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
> - unsigned long cpu_util, util_cfs = cpu_util_next(cpu, p,
> dst_cpu);
> - struct task_struct *tsk = cpu == dst_cpu ? p : NULL;
> + unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu);
> + unsigned long cpu_util, util_running = util_freq;
> + struct task_struct *tsk = NULL;
> +
> + /*
> + * When @p is placed on @cpu:
> + *
> + * util_running = max(cpu_util, cpu_util_est) +
> + * max(task_util, _task_util_est)
> + *
> + * while cpu_util_next is: max(cpu_util + task_util,
> + * cpu_util_est + _task_util_est)
> + */
Nit pick:
s/on @cpu/on @dst_cpu ?
s/while cpu_util_next is/while cpu_util_next(cpu, p, cpu) would be
If dst_cpu != cpu (including dst_cpu == -1) task_util and _task_util_est
are not added to util resp. util_est.
Not sure if this is clear from the source code here?
[...]
Reviewed-by: Dietmar Eggemann <dietmar.eggem...@arm.com>
