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    Subject[RFC][PATCH RT 1/2] futex_requeue-optimize


    In futex_requeue(), when the 2 futexes keys hash to the same bucket, there
    is no need to move the futex_q to the end of the bucket list.

    futex.c | 13 ++++++++-----
    1 files changed, 8 insertions(+), 5 deletions(-)

    Signed-off-by: Sébastien Dugué <sebastien.dugue@bull.net>

    Index: linux-2.6.16-rt20/kernel/futex.c
    ===================================================================
    --- linux-2.6.16-rt20.orig/kernel/futex.c 2006-05-04 10:58:38.000000000 +0200
    +++ linux-2.6.16-rt20/kernel/futex.c 2006-05-04 10:58:55.000000000 +0200
    @@ -835,17 +835,20 @@ static int futex_requeue(u32 __user *uad
    if (++ret <= nr_wake) {
    wake_futex(this);
    } else {
    - list_move_tail(&this->list, &hb2->chain);
    - this->lock_ptr = &hb2->lock;
    + /*
    + * If key1 and key2 hash to the same bucket, no
    + * need to requeue.
    + */
    + if (likely(head1 != &hb2->chain)) {
    + list_move_tail(&this->list, &hb2->chain);
    + this->lock_ptr = &hb2->lock;
    + }
    this->key = key2;
    get_key_refs(&key2);
    drop_count++;

    if (ret - nr_wake >= nr_requeue)
    break;
    - /* Make sure to stop if key1 == key2: */
    - if (head1 == &hb2->chain && head1 != &next->list)
    - head1 = &this->list;
    }
    }

    -
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