Re: [PATCH] sched/fair: Optimize select_idle_cpu

[Valentin Schneider]

On 13/12/2019 12:09, Peter Zijlstra wrote:
>> Like you said the gains here would probably be small - the highest SMT
>> count I'm aware of is SMT8 (POWER9). Still, if we end up with both
>> select_idle_core() and select_idle_cpu() using that pattern, it would make
>> sense IMO to align select_idle_smt() with those.
> 
> The cpumask_and() operation added would also have cost. I really don't
> see that paying off.
> 
> The other sites have the problem that we combine an iteration limit with
> affinity constraints. This loop doesn't do that and therefore doesn't
> suffer the problem.
> 

select_idle_core() doesn't really have an iteration limit, right? That
being said, yeah, the cpumask_and() for e.g. SMT2 systems would be
mostly wasteful.