Re: [PATCH 2/2] powerpc/topology: Override cpu_smt_mask

On Tue, Aug 04, 2020 at 09:03:07AM +0530, Srikar Dronamraju wrote:

On Power9 a pair of cores can be presented by the firmware as a big-core
for backward compatibility reasons, with 4 threads per (small) core and 8
threads per big-core. cpu_smt_mask() should generally point to the cpu mask
of the (small)core.

In order to maintain userspace backward compatibility (with Power8 chips in
case of Power9) in enterprise Linux systems, the topology_sibling_cpumask
has to be set to big-core. Hence override the default cpu_smt_mask() to be
powerpc specific allowing for better scheduling behaviour on Power.

Why does Linux userspace care about this?