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    SubjectRe: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order
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    On 1/22/21 9:03 AM, Vincent Guittot wrote:
    > On Thu, 21 Jan 2021 at 19:19, Vlastimil Babka <vbabka@suse.cz> wrote:
    >>
    >> On 1/21/21 11:01 AM, Christoph Lameter wrote:
    >> > On Thu, 21 Jan 2021, Bharata B Rao wrote:
    >> >
    >> >> > The problem is that calculate_order() is called a number of times
    >> >> > before secondaries CPUs are booted and it returns 1 instead of 224.
    >> >> > This makes the use of num_online_cpus() irrelevant for those cases
    >> >> >
    >> >> > After adding in my command line "slub_min_objects=36" which equals to
    >> >> > 4 * (fls(num_online_cpus()) + 1) with a correct num_online_cpus == 224
    >> >> > , the regression diseapears:
    >> >> >
    >> >> > 9 iterations of hackbench -l 16000 -g 16: 3.201sec (+/- 0.90%)
    >>
    >> I'm surprised that hackbench is that sensitive to slab performance, anyway. It's
    >> supposed to be a scheduler benchmark? What exactly is going on?
    >>
    >
    > From hackbench description:
    > Hackbench is both a benchmark and a stress test for the Linux kernel
    > scheduler. It's main
    > job is to create a specified number of pairs of schedulable
    > entities (either threads or
    > traditional processes) which communicate via either sockets or
    > pipes and time how long it
    > takes for each pair to send data back and forth.

    Yep, so I wonder which slab entities this is stressing that much.

    >> Things would be easier if we could trust *on all arches* either
    >>
    >> - num_present_cpus() to count what the hardware really physically has during
    >> boot, even if not yet onlined, at the time we init slab. This would still not
    >> handle later hotplug (probably mostly in a VM scenario, not that somebody would
    >> bring bunch of actual new cpu boards to a running bare metal system?).
    >>
    >> - num_possible_cpus()/nr_cpu_ids not to be excessive (broken BIOS?) on systems
    >> where it's not really possible to plug more CPU's. In a VM scenario we could
    >> still have an opposite problem, where theoretically "anything is possible" but
    >> the virtual cpus are never added later.
    >
    > On all the system that I have tested num_possible_cpus()/nr_cpu_ids
    > were correctly initialized
    >
    > large arm64 acpi system
    > small arm64 DT based system
    > VM on x86 system

    So it's just powerpc that has this issue with too large nr_cpu_ids? Is it caused
    by bios or the hypervisor? How does num_present_cpus() look there?

    What about heuristic:
    - num_online_cpus() > 1 - we trust that and use it
    - otherwise nr_cpu_ids
    Would that work? Too arbitrary?


    >> We could also start questioning the very assumption that number of cpus should
    >> affect slab page size in the first place. Should it? After all, each CPU will
    >> have one or more slab pages privately cached, as we discuss in the other
    >> thread... So why make the slab pages also larger?
    >>
    >> > Or the num_online_cpus needs to be up to date earlier. Why does this issue
    >> > not occur on x86? Does x86 have an up to date num_online_cpus earlier?
    >> >
    >> >
    >>
    >

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