On Mon, Jan 25, 2021 at 12:20:14PM +0100, Vlastimil Babka wrote: > 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? I think the present_mask retains CPUs if they are hotplugged off, whereas the online mask does not. We can't really do any better on arm64, as there's no way of telling that a CPU is present until we've seen it. Will
