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To:     Vincent Guittot <vincent.guittot@linaro.org>,
        Bharata B Rao <bharata@linux.ibm.com>
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From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the
 slub page order
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Date:   Mon, 25 Jan 2021 12:20:14 +0100
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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?