Messages in this thread | | | Date | Fri, 6 Dec 2019 13:32:54 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/fair: Optimize select_idle_core |
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On Fri, Dec 06, 2019 at 12:00:00PM +0000, Valentin Schneider wrote:
> Say you have a 4-core SMT2 system with the usual numbering scheme: > > {0, 4} {1, 5} {2, 6} {3, 7} > CORE0 CORE1 CORE2 CORE3 > > > Say 'target' is the prev_cpu, in that case let's pick 5. Because we do a > for_each_cpu_wrap(), our iteration for 'core' would start with > > 5, 6, 7, ... > > So say CORE2 is entirely idle and CORE1 isn't, we would go through the > inner loop on CORE1 (with 'core' == 5), then go through CORE2 (with > 'core' == 6) and return 'core'. I find it a bit unusual that we wouldn't > return the first CPU in the SMT mask, usually we try to fill sched_groups > in cpumask order. > > > If we could have 'cpus' start with only primary CPUs, that would simplify > things methinks: > > for_each_cpu_wrap(core, cpus, target) { > bool idle = true; > > for_each_cpu(cpu, cpu_smt_mask(core)) { > if (!available_idle_cpu(cpu)) { > idle = false; > break; > } > > __cpumask_clear_cpu(core, cpus); > > if (idle) > return core; > > > Food for thought;
See here:
https://lkml.kernel.org/r/20180530142236.667774973@infradead.org
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