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    SubjectRe: [PATCH] hotplug: Optimize {get,put}_online_cpus()
    On Tue, Oct 01, 2013 at 08:07:50PM +0200, Oleg Nesterov wrote:
    > On 10/01, Peter Zijlstra wrote:
    > >
    > > On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote:
    > > >
    > > > I tend to agree with Srivatsa... Without a strong reason it would be better
    > > > to preserve the current logic: "some time after" should not be after the
    > > > next CPU_DOWN/UP*. But I won't argue too much.
    > >
    > > Nah, I think breaking it is the right thing :-)
    >
    > I don't really agree but I won't argue ;)

    The authors of arch/x86/kernel/cpu/mcheck/mce.c would seem to be the
    guys who would need to complain, given that they seem to have the only
    use in 3.11.

    Thanx, Paul

    > > > But note that you do not strictly need this change. Just kill cpuhp_waitcount,
    > > > then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in
    > > > another thread, this should likely "join" all synchronize_sched's.
    > >
    > > That would still be 4k * sync_sched() == terribly long.
    >
    > No? the next xxx_enter() avoids sync_sched() if rcu callback is still
    > pending. Unless __cpufreq_remove_dev_finish() is "too slow" of course.
    >
    > > > Or split cpu_hotplug_begin() into 2 helpers which handle FAST -> SLOW and
    > > > SLOW -> BLOCK transitions, then move the first "FAST -> SLOW" handler outside
    > > > of for_each_online_cpu().
    > >
    > > Right, that's more messy but would work if we cannot teach cpufreq (and
    > > possibly others) to not rely on state you shouldn't rely on anyway.
    >
    > Yes,
    >
    > > I tihnk the only guarnatee POST_DEAD should have is that it should be
    > > called before UP_PREPARE of the same cpu ;-) Nothing more, nothing less.
    >
    > See above... This makes POST_DEAD really "special" compared to other
    > CPU_* events.
    >
    > And again. Something like a global lock taken by CPU_DOWN_PREPARE and
    > released by POST_DEAD or DOWN_FAILED does not look "too wrong" to me.
    >
    > But I leave this to you and Srivatsa.
    >
    > Oleg.
    >



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