Messages in this thread | | | Date | Tue, 19 Jun 2018 08:57:23 +0100 | From | Quentin Perret <> | Subject | Re: [RFC PATCH v3 09/10] sched/fair: Select an energy-efficient CPU on task wake-up |
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Hi Pavan,
On Tuesday 19 Jun 2018 at 10:36:01 (+0530), Pavan Kondeti wrote: > On Mon, May 21, 2018 at 03:25:04PM +0100, Quentin Perret wrote: > > <snip> > > > + if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed)) > > + prev_energy = best_energy = compute_energy(p, prev_cpu); > > + else > > + prev_energy = best_energy = ULONG_MAX; > > + > > + for_each_freq_domain(sfd) { > > + unsigned long spare_cap, max_spare_cap = 0; > > + int max_spare_cap_cpu = -1; > > + unsigned long util; > > + > > + /* Find the CPU with the max spare cap in the freq. dom. */ > > + for_each_cpu_and(cpu, freq_domain_span(sfd), sched_domain_span(sd)) { > > + if (!cpumask_test_cpu(cpu, &p->cpus_allowed)) > > + continue; > > + > > + if (cpu == prev_cpu) > > + continue; > > + > > + /* Skip CPUs that will be overutilized */ > > + util = cpu_util_wake(cpu, p) + task_util; > > + cpu_cap = capacity_of(cpu); > > + if (cpu_cap * 1024 < util * capacity_margin) > > + continue; > > + > > + spare_cap = cpu_cap - util; > > + if (spare_cap > max_spare_cap) { > > + max_spare_cap = spare_cap; > > + max_spare_cap_cpu = cpu; > > + } > > + } > > + > > + /* Evaluate the energy impact of using this CPU. */ > > + if (max_spare_cap_cpu >= 0) { > > + cur_energy = compute_energy(p, max_spare_cap_cpu); > > + if (cur_energy < best_energy) { > > + best_energy = cur_energy; > > + best_energy_cpu = max_spare_cap_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_energy_cpu; > > + > > + return prev_cpu; > > +} > > We are comparing the best_energy_cpu against prev_cpu even when prev_cpu > can't accommodate the waking task. Is this intentional? Should not we > discard the prev_cpu if it can't accommodate the task. > > This can potentially make a BIG task run on a lower capacity CPU until > load balancer kicks in and corrects the situation.
We shouldn't enter find_energy_efficient_cpu() in the first place if the system is overutilized, so that shouldn't too much of an issue in general.
But yeah, there is one small corner case where prev_cpu is overutilized and the system has not been flagged as such yet (when the tasks wakes-up shortly before the tick for ex). I think it's possible to cover this case by extending the "if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))" condition at the top of the function with a check on capacity_margin.
I'll change that in v4.
Thanks ! Quentin
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