linux-kernel - Re: [RFC PATCH 2/3] sched: change scheduler to give preference to soft affinity CPUs

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Date:   Tue, 2 Jul 2019 19:28:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        prakash.sangappa@...cle.com, dhaval.giani@...cle.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, tim.c.chen@...ux.intel.com,
        mgorman@...hsingularity.net, Paul Turner <pjt@...gle.com>
Subject: Re: [RFC PATCH 2/3] sched: change scheduler to give preference to
 soft affinity CPUs

On Wed, Jun 26, 2019 at 03:47:17PM -0700, subhra mazumdar wrote:
> The soft affinity CPUs present in the cpumask cpus_preferred is used by the
> scheduler in two levels of search. First is in determining wake affine
> which choses the LLC domain and secondly while searching for idle CPUs in
> LLC domain. In the first level it uses cpus_preferred to prune out the
> search space. In the second level it first searches the cpus_preferred and
> then cpus_allowed. Using affinity_unequal flag it breaks early to avoid
> any overhead in the scheduler fast path when soft affinity is not used.
> This only changes the wake up path of the scheduler, the idle balancing
> is unchanged; together they achieve the "softness" of scheduling.

I really dislike this implementation.

I thought the idea was to remain work conserving (in so far as that
we're that anyway), so changing select_idle_sibling() doesn't make sense
to me. If there is idle, we use it.

Same for newidle; which you already retained.

This then leaves regular balancing, and for that we can fudge with
can_migrate_task() and nr_balance_failed or something.

And I also really don't want a second utilization tipping point; we
already have the overloaded thing.

I also still dislike how you never looked into the numa balancer, which
already has peferred_nid stuff.