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SubjectRe: [PATCH v3 1/6] sched/rt: cpupri_find: Implement fallback mechanism for !fit case
On Mon,  2 Mar 2020 13:27:16 +0000
Qais Yousef <qais.yousef@arm.com> 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;

Nit, but if you moved idx, might as well remove the unnecessary
initialization of it as well ;-)

>
> BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
>
> for (idx = 0; idx < task_pri; idx++) {

It's initialized here.

> - 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;
> + 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);
> + }
>
> + /*
> + * If no CPU at the current priority can fit the task
> + * continue looking
> + */
> + if (cpumask_empty(lowest_mask)) {
> /*
> - * We have to ensure that we have at least one bit
> - * still set in the array, since the map could have
> - * been concurrently emptied between the first and
> - * second reads of vec->mask. If we hit this
> - * condition, simply act as though we never hit this
> - * priority level and continue on.
> + * Store our fallback priority in case we
> + * didn't find a fitting CPU
> */
> - if (cpumask_empty(lowest_mask))
> - continue;
> + if (best_unfit_idx == -1)
> + best_unfit_idx = idx;
>
> - if (!fitness_fn)
> - return 1;
> -
> - /* 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);
> - }
> -
> - /*
> - * If no CPU at the current priority can fit the task
> - * continue looking
> - */
> - if (cpumask_empty(lowest_mask))
> - continue;
> + continue;
> }
>
> return 1;
> }
>
> + /*
> + * If we failed to find a fitting lowest_mask, make sure we fall back
> + * to the last known unfitting lowest_mask.
> + *
> + * Note that the map of the recorded idx might have changed since then,
> + * so we must ensure to do the full dance to make sure that level still
> + * holds a valid lowest_mask.
> + *
> + * As per above, the map could have been concurrently emptied while we
> + * were busy searching for a fitting lowest_mask at the other priority
> + * levels.
> + *
> + * This rule favours honouring priority over fitting the task in the
> + * correct CPU (Capacity Awareness being the only user now).
> + * The idea is that if a higher priority task can run, then it should
> + * run even if this ends up being on unfitting CPU.
> + *
> + * The cost of this trade-off is not entirely clear and will probably
> + * be good for some workloads and bad for others.
> + *
> + * The main idea here is that if some CPUs were overcommitted, we try
> + * to spread which is what the scheduler traditionally did. Sys admins
> + * must do proper RT planning to avoid overloading the system if they
> + * really care.
> + */
> + if (best_unfit_idx != -1)
> + return __cpupri_find(cp, p, lowest_mask, best_unfit_idx);

Hmm, this only checks the one index, which can change and then we miss
everything. I think we can do better. What about this:


for (idx = 0; idx < task_pri; idx++) {
int found = -1;

if (!__cpupri_find(cp, p, lowest_mask, idx))
continue;

if (!lowest_mask || !fitness_fn)
return 1;

/* Make sure we have one fit CPU before clearing */
for_each_cpu(cpu, lowest_mask) {
if (fitness_fn(p, cpu)) {
found = cpu;
break;
}
}

if (found == -1)
continue;

/* Ensure the capacity of the CPUs fit the task */
for_each_cpu(cpu, lowest_mask) {
if (cpu < found || !fitness_fn(p, cpu))
cpumask_clear_cpu(cpu, lowest_mask);
}

return 1;
}

This way, if nothing fits we return the untouched lowest_mask, and only
clear the lowest_mask bits if we found a fitness cpu.

-- Steve

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