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    SubjectRe: bisected: futex regression >= 3.14 - was - Slowdown due to threads bouncing between HT cores
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    On Wed, Oct 8, 2014 at 11:37 AM, Mike Galbraith
    <umgwanakikbuti@gmail.com> wrote:
    >
    > 11d4616bd07f38d496bd489ed8fad1dc4d928823 is the first bad commit
    > commit 11d4616bd07f38d496bd489ed8fad1dc4d928823
    > Author: Linus Torvalds <torvalds@linux-foundation.org>
    > Date: Thu Mar 20 22:11:17 2014 -0700
    >
    > futex: revert back to the explicit waiter counting code

    While that revert might make things a tiny bit slower (I hated doing
    it, but the clever approach sadly didn't work on powerpc and depended
    on x86 locking semantics), I seriously doubt it's really relevant.
    It's more likely that the *real* problem itself is very
    timing-dependent, and the subtle synchronization changes here then
    expose it or hide it, rather than really fixing anything.

    So like Thomas, I would suspect a race condition in the futex use, and
    then the exact futex implementation details are just exposing it
    incidentally.

    Linus


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