On 1/23/21 1:32 PM, Vincent Guittot wrote: >> PowerPC PowerNV Host: (160 cpus) >> num_online_cpus 1 num_present_cpus 160 num_possible_cpus 160 nr_cpu_ids 160 >> >> PowerPC pseries KVM guest: (-smp 16,maxcpus=160) >> num_online_cpus 1 num_present_cpus 16 num_possible_cpus 160 nr_cpu_ids 160 >> >> That's what I see on powerpc, hence I thought num_present_cpus() could >> be the correct one to use in slub page order calculation. > > num_present_cpus() is set to 1 on arm64 until secondaries cpus boot > > arm64 224cpus acpi host: > num_online_cpus 1 num_present_cpus 1 num_possible_cpus 224 nr_cpu_ids 224 > arm64 8cpus DT host: > num_online_cpus 1 num_present_cpus 1 num_possible_cpus 8 nr_cpu_ids 8 > arm64 8cpus qemu-system-aarch64 (-smp 8,maxcpus=256) > num_online_cpus 1 num_present_cpus 1 num_possible_cpus 8 nr_cpu_ids 8 I would have expected num_present_cpus to be 224, 8, 8, respectively. > Then present and online increase to num_possible_cpus once all cpus are booted > >> >> > >> > What about heuristic: >> > - num_online_cpus() > 1 - we trust that and use it >> > - otherwise nr_cpu_ids >> > Would that work? Too arbitrary? >> >> Looking at the following snippet from include/linux/cpumask.h, it >> appears that num_present_cpus() should be reasonable compromise >> between online and possible/nr_cpus_ids to use here. >> >> /* >> * The following particular system cpumasks and operations manage >> * possible, present, active and online cpus. >> * >> * cpu_possible_mask- has bit 'cpu' set iff cpu is populatable >> * cpu_present_mask - has bit 'cpu' set iff cpu is populated >> * cpu_online_mask - has bit 'cpu' set iff cpu available to scheduler >> * cpu_active_mask - has bit 'cpu' set iff cpu available to migration >> * >> * If !CONFIG_HOTPLUG_CPU, present == possible, and active == online. >> * >> * The cpu_possible_mask is fixed at boot time, as the set of CPU id's >> * that it is possible might ever be plugged in at anytime during the >> * life of that system boot. The cpu_present_mask is dynamic(*), >> * representing which CPUs are currently plugged in. And >> * cpu_online_mask is the dynamic subset of cpu_present_mask, >> * indicating those CPUs available for scheduling. >> * >> * If HOTPLUG is enabled, then cpu_possible_mask is forced to have >> * all NR_CPUS bits set, otherwise it is just the set of CPUs that >> * ACPI reports present at boot. >> * >> * If HOTPLUG is enabled, then cpu_present_mask varies dynamically, >> * depending on what ACPI reports as currently plugged in, otherwise >> * cpu_present_mask is just a copy of cpu_possible_mask. >> * >> * (*) Well, cpu_present_mask is dynamic in the hotplug case. If not >> * hotplug, it's a copy of cpu_possible_mask, hence fixed at boot. >> */ >> >> So for host systems, present is (usually) equal to possible and for > > But "cpu_present_mask varies dynamically, depending on what ACPI > reports as currently plugged in" > > So it should varies when secondaries cpus are booted Hm, but booting the secondaries is just a software (kernel) action? They are already physically there, so it seems to me as if the cpu_present_mask is not populated correctly on arm64, and it's just a mirror of cpu_online_mask?
