On Thu, Aug 03, 2017 at 02:03:34PM +0200, Peter Zijlstra wrote:
> This one I'm not sure on.. at the very least we should exclude all of
> the prefer sibling domain when we do the next domain, and if there are
> multiple prefer sibling levels, we should only pick the first
> fallback_cpu -- there is no point is overriding it with a possible CPU
> further away.
I agree.
> I implemented that below -- although the find_cpu() function is really
> rather horrible.
>
> But still this isn't quite right, because when we consider this for SMT
> (as was the intent here) we'll happily occupy a full sibling core over
> finding an empty one.
>
> Now, the problem is that actually doing the right thing quickly ends up
> very expensive, we'd have to scan the entire cache domain at least once,
> so maybe this is good enough.. no idea :/
>
>
> ---
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1793,12 +1793,35 @@ static struct task_struct *pick_earliest
>
> static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
>
> +/*
> + * Find the first cpu in: mask & sd & ~prefer
^
Yes, I missed it.
> + */
> +static int find_cpu(const struct cpumask *mask,
> + const struct sched_domain *sd,
> + const struct sched_domain *prefer)
> +{
> + const struct cpumask *sds = sched_domain_span(sd);
> + const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
> + int cpu = -1;
> +
> + while ((cpu = cpumask_next(cpu, mask)) < nr_cpu_ids) {
> + if (!cpumask_test_cpu(cpu, sds))
> + continue;
> + if (ps && cpumask_test_cpu(cpu, ps))
> + continue;
> + break;
> + }
> +
> + return cpu;
> +}
> +
> static int find_later_rq(struct task_struct *task)
> {
> - struct sched_domain *sd;
> + struct sched_domain *sd, *prefer = NULL;
> struct cpumask *later_mask =
> this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> int this_cpu = smp_processor_id();
> int cpu = task_cpu(task);
> + int fallback_cpu = -1;
>
> /* Make sure the mask is initialized first */
> if (unlikely(!later_mask))
> @@ -1850,8 +1873,7 @@ static int find_later_rq(struct task_str
> return this_cpu;
> }
>
> - best_cpu = cpumask_first_and(later_mask,
> - sched_domain_span(sd));
> + best_cpu = find_cpu(later_mask, sd, prefer);
> /*
> * Last chance: if a cpu being in both later_mask
> * and current sd span is valid, that becomes our
> @@ -1859,6 +1881,17 @@ static int find_later_rq(struct task_str
> * already under consideration through later_mask.
> */
> if (best_cpu < nr_cpu_ids) {
> + /*
> + * If current domain is SD_PREFER_SIBLING
> + * flaged, we have to get more chances to
> + * check other siblings.
> + */
> + if (sd->flags & SD_PREFER_SIBLING) {
> + prefer = sd;
> + if (fallback_cpu == -1)
^
I like the 'if' statement.
I should have done this.
> + fallback_cpu = best_cpu;
> + continue;
> + }
> rcu_read_unlock();
> return best_cpu;
> }
Thank you.
