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SubjectRe: [PATCH 08/10] sched/fair: Steal work from an overloaded CPU when CPU goes idle
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Hi Steve,

On 22/10/2018 15:59, Steve Sistare wrote:
[...]
> @@ -9683,6 +9698,141 @@ void trigger_load_balance(struct rq *rq)
> nohz_balancer_kick(rq);
> }
>
> +/*
> + * Search the runnable tasks in @cfs_rq in order of next to run, and find
> + * the first one that can be migrated to @dst_rq. @cfs_rq is locked on entry.
> + * On success, dequeue the task from @cfs_rq and return it, else return NULL.
> + */
> +static struct task_struct *
> +detach_next_task(struct cfs_rq *cfs_rq, struct rq *dst_rq)
> +{
> + int dst_cpu = dst_rq->cpu;
> + struct task_struct *p;
> + struct rq *rq = rq_of(cfs_rq);
> +
> + lockdep_assert_held(&rq_of(cfs_rq)->lock);
> +
> + list_for_each_entry_reverse(p, &rq->cfs_tasks, se.group_node) {
> + if (can_migrate_task_llc(p, rq, dst_rq)) {
> + detach_task(p, rq, dst_cpu);
> + return p;
> + }
> + }
> + return NULL;
> +}
> +
> +/*
> + * Attempt to migrate a CFS task from @src_cpu to @dst_rq. @locked indicates
> + * whether @dst_rq is already locked on entry. This function may lock or
> + * unlock @dst_rq, and updates @locked to indicate the locked state on return.
> + * The locking protocol is based on idle_balance().
> + * Returns 1 on success and 0 on failure.
> + */
> +static int steal_from(struct rq *dst_rq, struct rq_flags *dst_rf, bool *locked,
> + int src_cpu)
> +{
> + struct task_struct *p;
> + struct rq_flags rf;
> + int stolen = 0;
> + int dst_cpu = dst_rq->cpu;
> + struct rq *src_rq = cpu_rq(src_cpu);
> +
> + if (dst_cpu == src_cpu || src_rq->cfs.h_nr_running < 2)
> + return 0;
> +
> + if (*locked) {
> + rq_unpin_lock(dst_rq, dst_rf);
> + raw_spin_unlock(&dst_rq->lock);
> + *locked = false;
> + }
> + rq_lock_irqsave(src_rq, &rf);
> + update_rq_clock(src_rq);
> +
> + if (src_rq->cfs.h_nr_running < 2 || !cpu_active(src_cpu))
> + p = NULL;
> + else
> + p = detach_next_task(&src_rq->cfs, dst_rq);
> +
> + rq_unlock(src_rq, &rf);
> +
> + if (p) {
> + raw_spin_lock(&dst_rq->lock);
> + rq_repin_lock(dst_rq, dst_rf);
> + *locked = true;
> + update_rq_clock(dst_rq);
> + attach_task(dst_rq, p);
> + stolen = 1;
> + }
> + local_irq_restore(rf.flags);
> +
> + return stolen;
> +}
> +
> +/*
> + * 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 */
> +
> + /* Accept any suitable task in the LLC */
> +
> + 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")

[...]

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