Messages in this thread Patch in this message | | | Date | Wed, 21 Mar 2018 15:35:18 +0000 | From | Patrick Bellasi <> | Subject | Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up |
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On 20-Mar 09:43, Dietmar Eggemann wrote: > From: Quentin Perret <quentin.perret@arm.com>
[...]
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 76bd46502486..65a1bead0773 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6513,6 +6513,60 @@ static unsigned long compute_energy(struct task_struct *p, int dst_cpu) > return energy; > } > > +static bool task_fits(struct task_struct *p, int cpu) > +{ > + unsigned long next_util = cpu_util_next(cpu, p, cpu); > + > + return util_fits_capacity(next_util, capacity_orig_of(cpu)); ^^^^^^^^^^^^^^^^^^^^^
Since here we are at scheduling CFS tasks, should we not better use capacity_of() to account for RT/IRQ pressure ?
> +} > + > +static int find_energy_efficient_cpu(struct sched_domain *sd, > + struct task_struct *p, int prev_cpu) > +{ > + unsigned long cur_energy, prev_energy, best_energy; > + int cpu, best_cpu = prev_cpu; > + > + if (!task_util(p))
We are still waking up a task... what if the task was previously running on a big CPU which is now idle?
I understand that from a _relative_ energy_diff standpoint there is not much to do for a 0 utilization task. However, for those tasks we can still try to return the most energy efficient CPU among the ones in their cpus_allowed mask.
It should be a relatively low overhead (maybe contained in a fallback most_energy_efficient_cpu() kind of function) which allows, for example on ARM big.LITTLE systems, to consolidate those tasks on LITTLE CPUs instead for example keep running them on a big CPU.
> + return prev_cpu; > + > + /* Compute the energy impact of leaving the task on prev_cpu. */ > + prev_energy = best_energy = compute_energy(p, prev_cpu); > + > + /* Look for the CPU that minimizes the energy. */ ^^^^^^^^^^ nit-pick: would say explicitly "best_energy" here, just to avoid confusion about (non) possible overflows in the following if check ;)
> + for_each_cpu_and(cpu, &p->cpus_allowed, sched_domain_span(sd)) { > + if (!task_fits(p, cpu) || cpu == prev_cpu)
nit-pick: to me it would read better as:
if (cpu == prev_cpu) continue; if (!task_fits(p, cpu)) continue;
but it's more matter of (personal) taste then efficiency.
> + continue; > + cur_energy = compute_energy(p, cpu); > + if (cur_energy < best_energy) { > + best_energy = cur_energy; > + best_cpu = cpu; > + } > + } > + > + /* > + * We pick the best CPU only if it saves at least 1.5% of the > + * energy used by prev_cpu. > + */ > + if ((prev_energy - best_energy) > (prev_energy >> 6)) > + return best_cpu; > + > + return prev_cpu; > +}
[...]
> @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > break; > } > > + /* > + * Energy-aware task placement is performed on the highest > + * non-overutilized domain spanning over cpu and prev_cpu. > + */ > + if (want_energy && !sd_overutilized(tmp) && > + cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) > + energy_sd = tmp; > +
Not entirely sure, but I was trying to understand if we can avoid to modify the definition of want_affine (in the previous chunk) and move this block before the previous "if (want_affine..." (in mainline but not in this chunk), which will became an else, e.g.
if (want_energy && !sd_overutilized(tmp) && // ... else if (want_energy && !sd_overutilized(tmp) && // ...
Isn't that the same?
Maybe there is a code path I'm missing... but otherwise it seems a more self contained modification of select_task_rq_fair...
> if (tmp->flags & sd_flag) > sd = tmp; > else if (!want_affine) > @@ -6586,6 +6652,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > if (want_affine) > current->recent_used_cpu = cpu; > } > + } else if (energy_sd) { > + new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu); > } else { > new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag); > }
-- #include <best/regards.h>
Patrick Bellasi
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