Messages in this thread | | | Date | Tue, 10 Apr 2018 19:29:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2 5/6] sched/fair: Select an energy-efficient CPU on task wake-up |
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On Fri, Apr 06, 2018 at 04:36:06PM +0100, Dietmar Eggemann wrote: > + for_each_freq_domain(fd) { > + 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(fd), sched_domain_span(sd)) { > + if (!cpumask_test_cpu(cpu, &p->cpus_allowed)) > + continue; > + > + if (cpu == prev_cpu) > + continue; > + > + util = cpu_util_wake(cpu, p); > + cpu_cap = capacity_of(cpu); > + if (!util_fits_capacity(util + task_util, cpu_cap)) > + 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; > + } > + } > + }
If each CPU has its own frequency domain, then the above loop ends up being O(n^2), no? Is there really nothing we can do about that? Also, I feel that warrants a comment warning about this.
Someone, somewhere will try and build a 64+64 cpu system and get surprised it doesn't work :-)
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