On Mon, Mar 10, 2025 at 06:07:21AM -1000, Tejun Heo wrote: > Hello, > > On Sun, Mar 09, 2025 at 04:39:40PM +0100, Andrea Righi wrote: > > > 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 pre-allocate two additional (per-cpu) cpumasks to do: > > - cpumask_and(numa_cpus, numa_span(cpu), cpus_allowed) > > - cpumask_and(llc_cpus, llc_span(cpu), cpus_allowed) > > > > And update/use them only when it's needed. In this way the API would be > > generic without making any implicit assumption about @cpus_allowed. > > I'm not quite following why two masks would be necessary. The user is > providing two masks and and'ing those two masks result in a single > cpus_allowed mask which can then be passed down to the existing pick > functions, no? When you say the user is providing two masks, you mean p->cpus_ptr and @cpus_allowed, right? Or am I missing something? So, internally we have three levels of cpumasks, used in this order: 1) p->cpus_ptr & cpus_allowed & llc_span(prev_cpu) 2) p->cpus_ptr & cpus_allowed & numa_span(prev_cpu) 3) p->cpus_ptr & cpus_allowed The current logic (without @cpus_allowed) is applying LLC and NUMA optimization only for tasks that can run on all CPUs (p->cpus_ptr == all), to avoid doing extra "and" operations internally and simply use llc_span(prev_cpu) and numa_span(prev_cpu). With @cpus_allowed this optimization doesn't work anymore and we can't just re-apply the current logic to "p->cpus_ptr & cpus_allowed", since it would result in ignoring the LLC and NUMA cpumasks. Maybe we could use a single pre-allocated temporary cpumask and do the "and" at each step when it's needed, instead of using two separate cpumasks to evaluate "cpus_allowed & llc_span(prev_cpu)" and "cpus_allowed & numa_span(prev_cpu). Is this what you mean? Thanks, -Andrea
