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SubjectRe: [RFC v5 4/6] sched/fair: Tune task wake-up logic to pack small background tasks on fewer cores
On Wed, 9 Oct 2019 at 11:23, Parth Shah <parth@linux.ibm.com> wrote:
>
>
>
> On 10/8/19 6:58 PM, Hillf Danton wrote:
> >
> > On Mon, 7 Oct 2019 14:00:49 +0530 Parth Shah wrote:
> >> +/*
> >> + * Try to find a non idle core in the system based on few heuristics:
> >> + * - Keep track of overutilized (>80% util) and busy (>12.5% util) CPUs
> >> + * - If none CPUs are busy then do not select the core for task packing
> >> + * - If atleast one CPU is busy then do task packing unless overutilized CPUs
> >> + * count is < busy/2 CPU count
> >> + * - Always select idle CPU for task packing
> >> + */
> >> +static int select_non_idle_core(struct task_struct *p, int prev_cpu, int target)
> >> +{
> >> + struct cpumask *cpus = this_cpu_cpumask_var_ptr(turbo_sched_mask);
> >> + int iter_cpu, sibling;
> >> +
> >> + cpumask_and(cpus, cpu_online_mask, p->cpus_ptr);
> >> +
> >> + for_each_cpu_wrap(iter_cpu, cpus, prev_cpu) {
> >> + int idle_cpu_count = 0, non_idle_cpu_count = 0;
> >> + int overutil_cpu_count = 0;
> >> + int busy_cpu_count = 0;
> >> + int best_cpu = iter_cpu;
> >> +
> >> + for_each_cpu(sibling, cpu_smt_mask(iter_cpu)) {
> >> + __cpumask_clear_cpu(sibling, cpus);
> >> + if (idle_cpu(iter_cpu)) {
> >
> > Would you please elaborate the reasons that the iter cpu is checked idle
> > more than once for finding a busy core?
> >
>
> Thanks for looking at the patches.
> Could you please point me out where iter_cpu is checked more than once?

I think that point is that you have a sibling that there is
for_each_cpu(sibling, cpu_smt_mask(iter_cpu) but you never use sibling
in the loop except for clearing it on the cpumask cpus
All the tests are done with iter_cpu so you will test several time
iter_cpus but never the other sibling
Should you use sibling instead ?


>
> >> + idle_cpu_count++;
> >> + best_cpu = iter_cpu;
> >> + } else {
> >> + non_idle_cpu_count++;
> >> + if (cpu_overutilized(iter_cpu))
> >> + overutil_cpu_count++;
> >> + if (is_cpu_busy(cpu_util(iter_cpu)))
> >> + busy_cpu_count++;
> >> + }
> >> + }
> >> +
> >
>

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