Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the current sched domain

[Valentin Schneider]

On 20/02/23 15:22, hupu wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Specifically, I'm thinking something in the degenerate area where it
>> looks if a given domain has equal depth children or so.
>
> By the way, in the last email you mentioned "the degenerate area". I can't understand what it means because of cultural differences. Does it refer to this scenario: sched_domain at the SMT layer is destroyed after an SMT thread goes offline, so mc_domain->child is NULL?
>

This references sched domain degeneration, cf. sd_parent_degenerate() and
sd_degenerate() in sched/topology.c

Sched domains are built following a layout described in
sched_domain_topology[], and depending on the actual CPU layout some
domains end up not being worth keeping (e.g. they only contain one CPU - we
need at least 2 to balance things), so we destroy (degenerate) them.

> hupu