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

[Peter Zijlstra]

On Mon, Feb 06, 2023 at 08:58:34PM -0800, Ricardo Neri wrote:
> SD_PREFER_SIBLING is set from the SMT scheduling domain up to the first
> non-NUMA domain (the exception is systems with SD_ASYM_CPUCAPACITY).
> 
> Above the SMT sched domain, all domains have a child. The SD_PREFER_
> SIBLING is honored always regardless of the scheduling domain at which the
> load balance takes place.
> 
> There are cases, however, in which the busiest CPU's sched domain has
> child but the destination CPU's does not. Consider, for instance a non-SMT
> core (or an SMT core with only one online sibling) doing load balance with
> an SMT core at the MC level. SD_PREFER_SIBLING will not be honored. We are
> left with a fully busy SMT core and an idle non-SMT core.
> 
> Avoid inconsistent behavior. Use the prefer_sibling behavior at the current
> scheduling domain, not its child.
> 
> The NUMA sched domain does not have the SD_PREFER_SIBLING flag. Thus, we
> will not spread load among NUMA sched groups, as desired.
> 

Like many of the others; I don't much like this.

Why not simply detect this asymmetric having of SMT and kill the
PREFER_SIBLING flag on the SMT leafs in that case?

Specifically, I'm thinking something in the degenerate area where it
looks if a given domain has equal depth children or so.

Note that this should not be tied to having special hardware, you can
create the very same weirdness by just offlining a few SMT siblings and
leaving a few on.