Hello, On Sat, Mar 08, 2025 at 07:48:42AM +0100, Andrea Righi wrote: > > > With this concept the idle CPU selection policy becomes the following: > > > - always prioritize CPUs from fully idle SMT cores (if SMT is enabled), > > > - select the same CPU if it's idle and in the allowed domain, > > > - select an idle CPU within the same LLC domain, if the LLC domain is a > > > subset of the allowed domain, > > > > Why not select from the intersection of the same LLC domain and the cpumask? > > We could do that, but to guarantee the intersection we need to introduce > other temporary cpumasks (one for the LLC intersection and another for the > NUMA), which is not a big problem, but it can introduce overhead. And most > of the time the LLC group is either a subset of the allowed CPUs or > vice-versa, so in this case the current logic already works. > > The extra cpumask work is needed only when the allowed cpumask spans > multiple partial LLCs, which should be rare. So maybe in such cases, we > could tolerate the additional overhead of updating an additional temporary > cpumask to ensure proper hierarchical semantics (maintaining consistency > with the topology hierarchy). WDYT? Would just using a pre-allocated cpumask to do pre-and on @cpus_allowed work? This won't only be used for topology support (e.g. soft partitioning in scx_layered and scx_mitosis may want to use multi-topology-unit spanning subsets) and I'm not sure assuming and optimizing for that is a good idea for generic API. We can do something simple now. Note that if we want to optimize it, we can introduce cpumask_any_and_and_distribute(). There already is cpumask_first_and_and(), so the pattern isn't new and the only extra bitops we need to add is find_next_and_and_bit_wrap(). There's already find_first_and_and_bit(), so I don't think it will be all that difficult to add. Thanks. -- tejun
