Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order

Vlastimil Babka
2021-01-25T13:49:15Z

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?