Re: [PATCH v2 07/13] sched/fair: Let asymmetric cpu configurations balance at wake-up

From: Peter Zijlstra
Date: Mon Jul 11 2016 - 07:13:53 EST

On Wed, Jun 22, 2016 at 06:03:18PM +0100, Morten Rasmussen wrote:
> Currently, SD_WAKE_AFFINE always takes priority over wakeup balancing if
> SD_BALANCE_WAKE is set on the sched_domains. For asymmetric
> configurations SD_WAKE_AFFINE is only desirable if the waking task's
> compute demand (utilization) is suitable for all the cpu capacities
> available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
> balancing take over (find_idlest_{group, cpu}()).

I think I tripped over this one the last time around, and I'm not sure
this Changelog is any clearer.

This is about the case where the waking cpu and prev_cpu are both in the
'wrong' cluster, right?

> This patch makes affine wake-ups conditional on whether both the waker
> cpu and prev_cpu has sufficient capacity for the waking task, or not.
> It is assumed that the sched_group(s) containing the waker cpu and
> prev_cpu only contain cpu with the same capacity (homogeneous).

> Ideally, we shouldn't set 'want_affine' in the first place, but we don't
> know if SD_BALANCE_WAKE is enabled on the sched_domain(s) until we start
> traversing them.

Is this again more fallout from that weird ASYM_CAP thing?

> +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> +{
> + long min_cap, max_cap;
> +
> + min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
> + max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;
> +
> + /* Minimum capacity is close to max, no need to abort wake_affine */
> + if (max_cap - min_cap < max_cap >> 3)
> + return 0;
> +
> + return min_cap * 1024 < task_util(p) * capacity_margin;
> +}

I'm most puzzled by these inequalities, how, why ?

I would figure you'd compare task_util to the current remaining util of
the small group, and if it fits, place it there. This seems to do
something entirely different.