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    SubjectRe: [patch] sched: fix broken smt/mc optimizations with CFS
    On Mon, Aug 27, 2007 at 12:31:03PM -0700, Siddha, Suresh B wrote:
    > Essentially I observed that nice 0 tasks still endup on two cores of same
    > package, with out getting spread out to two different packages. This behavior
    > is same with out this fix and this fix doesn't help in any way.

    Ingo, Appended patch seems to fix the issue and as far as I can test, seems ok
    to me.

    This is a quick fix for .23. Peter Williams and myself plan to look at
    code cleanups in this area (HT/MC optimizations) post .23

    BTW, with this fix, do you want to retain the current FUZZ value?


    Try to fix MC/HT scheduler optimization breakage again, with out breaking
    the FUZZ logic.

    First fix the check
    if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task)
    with this
    if (*imbalance < busiest_load_per_task)

    As the current check is always false for nice 0 tasks (as SCHED_LOAD_SCALE_FUZZ
    is same as busiest_load_per_task for nice 0 tasks).

    With the above change, imbalance was getting reset to 0 in the corner case
    condition, making the FUZZ logic fail. Fix it by not corrupting the
    imbalance and change the imbalance, only when it finds that the
    HT/MC optimization is needed.

    Signed-off-by: Suresh Siddha <>

    diff --git a/kernel/sched.c b/kernel/sched.c
    index 9fe473a..03e5e8d 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -2511,7 +2511,7 @@ group_next:
    * a think about bumping its value to force at least one task to be
    * moved
    - if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
    + if (*imbalance < busiest_load_per_task) {
    unsigned long tmp, pwr_now, pwr_move;
    unsigned int imbn;

    @@ -2563,10 +2563,8 @@ small_imbalance:
    pwr_move /= SCHED_LOAD_SCALE;

    /* Move if we gain throughput */
    - if (pwr_move <= pwr_now)
    - goto out_balanced;
    - *imbalance = busiest_load_per_task;
    + if (pwr_move > pwr_now)
    + *imbalance = busiest_load_per_task;

    return busiest;
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      Last update: 2007-08-29 00:29    [W:0.029 / U:0.740 seconds]
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