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SubjectRe: [PATCH 1/3] sched/rt: cpupri_find: implement fallback mechanism for !fit case
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On 14/02/2020 17:39, Qais Yousef wrote:

[...]

> /**
> * cpupri_find - find the best (lowest-pri) CPU in the system
> * @cp: The cpupri context
> @@ -62,80 +115,72 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
> struct cpumask *lowest_mask,
> bool (*fitness_fn)(struct task_struct *p, int cpu))
> {
> - int idx = 0;
> int task_pri = convert_prio(p->prio);
> + int best_unfit_idx = -1;
> + int idx = 0, cpu;
>
> BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
>
> for (idx = 0; idx < task_pri; idx++) {
> - struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
> - int skip = 0;
>
> - if (!atomic_read(&(vec)->count))
> - skip = 1;
> - /*
> - * When looking at the vector, we need to read the counter,
> - * do a memory barrier, then read the mask.
> - *
> - * Note: This is still all racey, but we can deal with it.
> - * Ideally, we only want to look at masks that are set.
> - *
> - * If a mask is not set, then the only thing wrong is that we
> - * did a little more work than necessary.
> - *
> - * If we read a zero count but the mask is set, because of the
> - * memory barriers, that can only happen when the highest prio
> - * task for a run queue has left the run queue, in which case,
> - * it will be followed by a pull. If the task we are processing
> - * fails to find a proper place to go, that pull request will
> - * pull this task if the run queue is running at a lower
> - * priority.
> - */
> - smp_rmb();
> -
> - /* Need to do the rmb for every iteration */
> - if (skip)
> - continue;
> -
> - if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids)
> + if (!__cpupri_find(cp, p, lowest_mask, idx))
> continue;
>
> - if (lowest_mask) {
> - int cpu;

Shouldn't we add an extra condition here?

+ if (!static_branch_unlikely(&sched_asym_cpucapacity))
+ return 1;
+

Otherwise non-heterogeneous systems have to got through this
for_each_cpu(cpu, lowest_mask) further below for no good reason.

> + if (!lowest_mask || !fitness_fn)
> + return 1;
>
> - cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
> + /* Ensure the capacity of the CPUs fit the task */
> + for_each_cpu(cpu, lowest_mask) {
> + if (!fitness_fn(p, cpu))
> + cpumask_clear_cpu(cpu, lowest_mask);
> + }

[...]

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