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    SubjectRe: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor
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    On Fri, 2012-12-21 at 18:56 -0500, Rik van Riel wrote:
    > Argh, the first one had a typo in it that did not influence
    > performance with fewer threads running, but that made things
    > worse with more than a dozen threads...

    > +
    > + /*
    > + * The lock is still busy, the delay was not long enough.
    > + * Going through here 2.7 times will, on average, cancel
    > + * out the decrement above. Using a non-integer number
    > + * gets rid of performance artifacts and reduces oversleeping.
    > + */
    > + if (delay < MAX_SPINLOCK_DELAY &&
    > + ((inc.head & 3) == 0 || (inc.head & 7) == 1))
    > + delay++;

    ((inc.head & 3) == 0 || (inc.head & 7) == 1)) seems a strange condition
    to me...




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