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SubjectRe: [PATCH 08/10] sched/fair: Steal work from an overloaded CPU when CPU goes idle
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On 10/25/2018 9:48 AM, Valentin Schneider wrote:
> Hi Steve,
>
> On 22/10/2018 15:59, Steve Sistare wrote:
> [...]
>> +/*
>> + * Try to steal a runnable CFS task from a CPU in the same LLC as @dst_rq,
>> + * and migrate it to @dst_rq. rq_lock is held on entry and return, but
>> + * may be dropped in between. Return 1 on success, 0 on failure, and -1
>> + * if a task in a different scheduling class has become runnable on @dst_rq.
>> + */
>> +static int try_steal(struct rq *dst_rq, struct rq_flags *dst_rf)
>> +{
>> + int src_cpu;
>> + int dst_cpu = dst_rq->cpu;
>> + bool locked = true;
>> + int stolen = 0;
>> + struct sparsemask *overload_cpus;
>> +
>> + if (!sched_feat(STEAL))
>> + return 0;
>> +
>> + if (!cpu_active(dst_cpu))
>> + return 0;
>> +
>> + /* Get bitmap of overloaded CPUs in the same LLC as @dst_rq */
>> +
>> + rcu_read_lock();
>> + overload_cpus = rcu_dereference(dst_rq->cfs_overload_cpus);
>> + if (!overload_cpus) {
>> + rcu_read_unlock();
>> + return 0;
>> + }
>> +
>> +#ifdef CONFIG_SCHED_SMT
>> + /*
>> + * First try overloaded CPUs on the same core to preserve cache warmth.
>> + */
>> + if (static_branch_likely(&sched_smt_present)) {
>> + for_each_cpu(src_cpu, cpu_smt_mask(dst_cpu)) {
>> + if (sparsemask_test_elem(src_cpu, overload_cpus) &&
>> + steal_from(dst_rq, dst_rf, &locked, src_cpu)) {
>> + stolen = 1;
>> + goto out;
>> + }
>> + }
>> + }
>> +#endif /* CONFIG_SCHED_SMT */
>> +
>> [...]
>> +
>> + for_each_sparse_wrap(src_cpu, overload_cpus, dst_cpu) {
>> + if (steal_from(dst_rq, dst_rf, &locked, src_cpu)) {
>> + stolen = 1;
>> + break;
> ^^^^^^
> You might want to have a 'goto out' there for consistency and to make GCC
> happy for !CONFIG_SCHED_SMT (I get a "warning: label ‘out’ defined but not
> used")

Absolutely, will fix in next version. Thanks for the careful review.

- Steve

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