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SubjectRe: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy
On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@arm.com> wrote:
>
> The Energy Aware Scheduler (EAS) tries to find best CPU for a waking up
> task. It probes many possibilities and compares the estimated energy values
> for different scenarios. For calculating those energy values it relies on
> Energy Model (EM) data and em_cpu_energy(). The precision which is used in
> EM data is in milli-Watts (or abstract scale), which sometimes is not
> sufficient. In some cases it might happen that two CPUs from different
> Performance Domains (PDs) get the same calculated value for a given task
> placement, but in more precised scale, they might differ. This rounding
> error has to be addressed. This patch prepares EAS code for better
> precision in the coming EM improvements.

Could you explain why 32bits results are not enough and you need to
move to 64bits ?

Right now the result is in the range [0..2^32[ mW. If you need more
precision and you want to return uW instead, you will have a result in
the range [0..4kW[ which seems to be still enough

>
> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
> ---
> kernel/sched/fair.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7b8990fd4896..b517c9e79768 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6582,7 +6582,7 @@ static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
> * to compute what would be the energy if we decided to actually migrate that
> * task.
> */
> -static long
> +static u64
> compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> {
> struct cpumask *pd_mask = perf_domain_span(pd);
> @@ -6689,12 +6689,13 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> */
> static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> {
> - unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX;
> struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
> + u64 prev_delta = ULLONG_MAX, best_delta = ULLONG_MAX;
> int cpu, best_energy_cpu = prev_cpu, target = -1;
> - unsigned long cpu_cap, util, base_energy = 0;
> + unsigned long cpu_cap, util;
> struct sched_domain *sd;
> struct perf_domain *pd;
> + u64 base_energy = 0;
>
> rcu_read_lock();
> pd = rcu_dereference(rd->pd);
> @@ -6718,9 +6719,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> goto unlock;
>
> for (; pd; pd = pd->next) {
> - unsigned long cur_delta, spare_cap, max_spare_cap = 0;
> + unsigned long spare_cap, max_spare_cap = 0;
> bool compute_prev_delta = false;
> - unsigned long base_energy_pd;
> + u64 base_energy_pd, cur_delta;
> int max_spare_cap_cpu = -1;
>
> for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
> @@ -6790,7 +6791,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> * Pick the best CPU if prev_cpu cannot be used, or if it saves at
> * least 6% of the energy used by prev_cpu.
> */
> - if ((prev_delta == ULONG_MAX) ||
> + if ((prev_delta == ULLONG_MAX) ||
> (prev_delta - best_delta) > ((prev_delta + base_energy) >> 4))
> target = best_energy_cpu;
>
> --
> 2.17.1
>

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