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    SubjectRe: [PATCH] Fix smpnice high priority task hopping problem
    Siddha, Suresh B wrote:
    > Andrew, Please don't apply this patch. This breaks the existing HT
    > (and multi-core) scheduler optimizations.
    > Peter, on a DP system with HT, if we have only two runnable processes
    > and they end up running on the two threads of the same package,
    > with your patch, migration thread will never move one of those processes
    > to the idle package..

    On a normal system, would either of them be moved anyway?

    > To fix my reported problem, we need to make sure that find_busiest_group()
    > doesn't find an imbalance..

    I disagree. If this causes a problem with your "optimizations" then I
    think that you need to fix the "optimizations".

    There's a rational argument (IMHO) that this patch should be applied
    even in the absence of the smpnice patches as it prevents
    active_load_balance() doing unnecessary work. If this isn't good for
    hypo threading then hypo threading is a special case and needs to handle
    it as such.

    > thanks,
    > suresh
    > On Thu, Feb 16, 2006 at 11:39:34AM +1100, Peter Williams wrote:
    >>Suresh B. Siddha has reported:
    >>"on a lightly loaded system, this will result in higher priority job
    >>hopping around from one processor to another processor.. This is because
    >>of the code in find_busiest_group() which assumes that SCHED_LOAD_SCALE
    >>represents a unit process load and with nice_to_bias calculations this
    >>is no longer true (in the presence of non nice-0 tasks)"
    >>Analysis of this problem as revealed that the smpnice code results in
    >>the weighted load being larger than 1 and this triggers the active load
    >>balancing code. However, in active_load_balance(), the migration thread
    >>fails to take into account itself when deciding if there are any tasks
    >>to be migrated from its run queue. I.e. even if there is only one other
    >>task on the run queue other than itself it will still migrate that other
    >>The attached patch fixes that anomaly.
    >>Signed-off-by: Peter Williams <>
    >>Peter Williams
    >>"Learning, n. The kind of ignorance distinguishing the studious."
    >> -- Ambrose Bierce
    >>Index: MM-2.6.X/kernel/sched.c
    >>--- MM-2.6.X.orig/kernel/sched.c 2006-02-16 10:51:52.000000000 +1100
    >>+++ MM-2.6.X/kernel/sched.c 2006-02-16 11:02:45.000000000 +1100
    >>@@ -2406,7 +2406,7 @@ static void active_load_balance(runqueue
    >> runqueue_t *target_rq;
    >> int target_cpu = busiest_rq->push_cpu;
    >>- if (busiest_rq->nr_running <= 1)
    >>+ if (busiest_rq->nr_running <= 2)
    >> /* no task to move */
    >> return;

    Peter Williams

    "Learning, n. The kind of ignorance distinguishing the studious."
    -- Ambrose Bierce
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