Re: [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask

From: peterz
Date: Tue Aug 04 2020 - 06:45:55 EST

On Tue, Aug 04, 2020 at 09:03:06AM +0530, Srikar Dronamraju wrote:
> cpu_smt_mask tracks topology_sibling_cpumask. This would be good for
> most architectures. One of the users of cpu_smt_mask(), would be to
> identify idle-cores. On Power9, a pair of cores can be presented by the
> firmware as a big-core for backward compatibility reasons.
> In order to maintain userspace backward compatibility with previous
> versions of processor, (since Power8 had SMT8 cores), Power9 onwards there
> is option to the firmware to advertise a pair of SMT4 cores as a fused
> cores (referred to as the big_core mode in the Linux Kernel). On Power9
> this pair shares the L2 cache as well. However, from the scheduler's point
> of view, a core should be determined by SMT4. The load-balancer already
> does this. Hence allow PowerPc architecture to override the default
> cpu_smt_mask() to point to the SMT4 cores in a big_core mode.

I'm utterly confused.

Why can't you set your regular siblings mask to the smt4 thing? Who
cares about the compat stuff, I thought that was an LPAR/AIX thing.