On Mon, Jan 25, 2021 at 12:20:14PM +0100, Vlastimil Babka wrote:

On 1/23/21 1:32 PM, Vincent Guittot wrote:

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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.

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Then present and online increase to num_possible_cpus once all cpus are booted

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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