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

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Date:	Mon, 11 Jul 2016 13:13:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/13] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

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.