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SubjectRe: [PATCH] sched/core: forced idle accounting
On Thu, Oct 07, 2021 at 05:08:25PM -0700, Josh Don wrote:
> Adds accounting for "forced idle" time, which is time where a cookie'd
> task forces its SMT sibling to idle, despite the presence of runnable
> tasks.
>
> Forced idle time is one means to measure the cost of enabling core
> scheduling (ie. the capacity lost due to the need to force idle).

It seems an excessive amount of code for what it says to do.

> + smt_count = cpumask_weight(smt_mask);

That's a fairly expensive operation to find a number that's going the be
to same over and over and over...

> + if (smt_count > 2) {
> + unsigned int nr_forced_idle = 0, nr_running = 0;
> +
> + for_each_cpu(i, smt_mask) {
> + rq_i = cpu_rq(i);
> + p = rq_i->core_pick ?: rq_i->curr;
> +
> + if (p != rq_i->idle)
> + nr_running++;
> + else if (rq_i->nr_running)
> + nr_forced_idle++;
> + }
> +
> + if (WARN_ON_ONCE(!nr_running)) {
> + /* can't be forced idle without a running task */
> + } else {
> + delta *= nr_forced_idle;
> + delta /= nr_running;
> + }

Now the comment sayeth:

> + /*
> + * For larger SMT configurations, we need to scale the charged
> + * forced idle amount since there can be more than one forced idle
> + * sibling and more than one running cookied task.
> + */

But why?

> + }
> +
> + for_each_cpu(i, smt_mask) {
> + rq_i = cpu_rq(i);
> + p = rq_i->core_pick ?: rq_i->curr;
> +
> + if (!p->core_cookie)
> + continue;
> +
> + p->core_forceidle_sum += delta;
> +
> + /* Optimize for common case. */
> + if (smt_count == 2)
> + break;
> + }
> +}

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