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

[Peter Zijlstra]

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.