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    SubjectRe: 'global' rq->clock

    * David Miller <davem@davemloft.net> wrote:

    > > there's already such a mechanism in sched-devel.git (and has been
    > > there for a week or two): an architecture can set time_sync_thresh
    > > to -1LL during early bootup and essentially disable all the
    > > synchronization logic.
    >
    > Does it remove all of the code too? :-)
    >
    > Please give us a config boolean. The only platform for which a
    > run-time knob is even necessary is x86.

    yeah, will try something like that too.

    the thing is, core kernel folks have to resist such pressures
    _somewhat_.

    Architecture maintainers will put pressure on us from two directions: if
    a particular generic feature only helps _another_ architecture, they
    find it a nuisance and want it to be gone as much as possible.

    If a particular feature helps them, they want it supported and
    default-enabled as much as possible. In fact, complaints come if a
    generic-looking feature shows up in one architecture only. (recent
    example: inlining optimizations ;-)

    And you are totally right about sched_clock() being dead on accurate an
    globally synchronous on sparc64 - and you are right to find _any_ issue
    about it a nuisance. I totally envy you that sparc64's sched_clock() is
    so simple - it should have been like that on x86 years ago, if hw
    designers werent so impotent about it.

    ( although please note that the growing generalization that goes on
    _did_ find a subtle nohz problem on sparc64 early in the merge window,
    so it's not like these changes are totally useless to you. )

    but it all goes in the other direction as well: many folks find
    endianness problems a nuisance on x86, many folks find TLB and explicit
    cache coherence complications a nuisance on x86 as well. The bus-to-phys
    complication which is an identity on x86. Etc. etc.

    But the core kernel is a conscious and intelligent union of all
    architecture's needs, and often we maintain complications even if they
    make no sense on the most popular platform. I think it makes strategic
    sense because it keep the kernel truly generic and truly clean. It also
    keeps architectures honest: even today the x86 architecture is still not
    as clean as sparc64 for example.

    so please be patient, we are working on it :)

    Ingo


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