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    SubjectRe: [patch] sched: fix broken smt/mc optimizations with CFS
    On Thu, Aug 23, 2007 at 02:13:41PM +0200, Ingo Molnar wrote:
    >
    > * Ingo Molnar <mingo@elte.hu> wrote:
    >
    > > [...] So how about the patch below instead?
    >
    > the right patch attached.
    >
    > -------------------------------->
    > Subject: sched: fix broken SMT/MC optimizations
    > From: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
    >
    > On a four package system with HT - HT load balancing optimizations were
    > broken. For example, if two tasks end up running on two logical threads
    > of one of the packages, scheduler is not able to pull one of the tasks
    > to a completely idle package.
    >
    > In this scenario, for nice-0 tasks, imbalance calculated by scheduler
    > will be 512 and find_busiest_queue() will return 0 (as each cpu's load
    > is 1024 > imbalance and has only one task running).
    >
    > Similarly MC scheduler optimizations also get fixed with this patch.
    >
    > [ mingo@elte.hu: restored fair balancing by increasing the fuzz and
    > adding it back to the power decision, without the /2
    > factor. ]
    >
    > Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
    > Signed-off-by: Ingo Molnar <mingo@elte.hu>
    > ---
    >
    > include/linux/sched.h | 2 +-
    > kernel/sched.c | 2 +-
    > 2 files changed, 2 insertions(+), 2 deletions(-)
    >
    > Index: linux/include/linux/sched.h
    > ===================================================================
    > --- linux.orig/include/linux/sched.h
    > +++ linux/include/linux/sched.h
    > @@ -681,7 +681,7 @@ enum cpu_idle_type {
    > #define SCHED_LOAD_SHIFT 10
    > #define SCHED_LOAD_SCALE (1L << SCHED_LOAD_SHIFT)
    >
    > -#define SCHED_LOAD_SCALE_FUZZ (SCHED_LOAD_SCALE >> 1)
    > +#define SCHED_LOAD_SCALE_FUZZ SCHED_LOAD_SCALE
    >
    > #ifdef CONFIG_SMP
    > #define SD_LOAD_BALANCE 1 /* Do load balancing on this domain. */
    > Index: linux/kernel/sched.c
    > ===================================================================
    > --- linux.orig/kernel/sched.c
    > +++ linux/kernel/sched.c
    > @@ -2517,7 +2517,7 @@ 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/2) {
    > + if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {

    Ingo, this is still broken. This condition is always false for nice-0 tasks..

    thanks,
    suresh
    -
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