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    SubjectRe: [RFC PATCH 2/4] x86: use dmi check to treat disabled cpus as hotplug cpus.

    * Linus Torvalds <> wrote:

    > On Mon, 11 Jan 2010, Yinghai Lu wrote:
    > >
    > > some systems that have disable cpus entries because same
    > > BIOS will support 2 sockets and 4 sockets and more at
    > > same time, BIOS just leave some disable entries, but
    > > those system do not support cpu hotplug. we don't need
    > > treat disabled_cpus as hotplug cpus.
    > >
    > > so we can make nr_cpu_ids smaller and save more space
    > > (pcpu data allocations), and could make some systems run
    > > with logical flat instead of physical flat apic mode
    > .. but this one I detest.
    > We can't play games that depend on us always filling in some DMI table
    > correctly. Things need to "just work".
    > So while 1/4 looks fine, 2/4 looks fundamentally unacceptable.
    > Is the flat APIC mode really so important?

    it's not important at all. When it's proven safe it's fine but it's clearly
    not unconditionally safe ...

    > I would suggest a few alternatives:
    > Truly static:
    > - only use that flat apic mode when you _know_ that you absolutely will
    > never have more than 8 cpu's. Ie when CONFIG_NR_CPUS <= 8 (or, with
    > 1/3, when nr_cpu_ids <= 8) and/or when <= 8 CPU's were detected, and
    > CPU hotplug is disabled entirely.
    > Slightly more intelligent:
    > - Look at the ACPI socket count, and hey, if it says it might have more
    > sockets - whether they are really hotplug or not, don't use flat mode,
    > because we simply don't know. But do _not_ do some kind of DMI table to
    > say one way or the other.
    > And if it's _really_ important:
    > - if flat mode is so important that you want to enable it whenever
    > possible, what about enabling/disabling it dynamically at CPU hotplug
    > time? That does sound _very_ painful, but it's still better than having
    > to maintain some list of all systems that can ever hot-plug.
    > Hmm?

    I'd go for #1. If the (modest) micro-performance advantages of flat delivery
    matter to a hardware vendor the system/BIOS can be built in a way to trigger
    the optimization. But even single socket systems are quickly running out of
    the space of 8 APIC ids so the relevance is dwindling ...


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