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    SubjectRe: load balancing regression since commit 367456c7
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    On Tue, 2012-04-10 at 18:06 -0700, Tim Chen wrote:
    > Peter,
    >
    > We noticed in a hackbench test (./hackbench 100 process 2000)
    > on a Sandy bridge 2 socket server, there has been a slow down
    > by a factor of 4 since commit 367456c7 was applied
    > (sched: Ditch per cgroup task lists for load-balancing).
    >
    > The commit 5d6523e (sched: Fix load-balance wreckage) did
    > not fix the regression.
    >
    > In the profile, there is heavy spin lock contention in the load_balance path of 3.4-rc2
    > where it was less than .003% of cpu before commit 367456c7.

    I can't actually reproduce but does the below help?

    If not, can you shoot your .config over?

    ---
    kernel/sched/fair.c | 8 +++++---
    1 files changed, 5 insertions(+), 3 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 0d97ebd..e1da5c6 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3215,6 +3215,8 @@ static int move_one_task(struct lb_env *env)

    static unsigned long task_h_load(struct task_struct *p);

    +static const unsigned int sched_nr_migrate_break = IS_ENABLED(CONFIG_PREEMPT) ? 8 : 32;
    +
    /*
    * move_tasks tries to move up to load_move weighted load from busiest to
    * this_rq, as part of a balancing operation within domain "sd".
    @@ -3242,7 +3244,7 @@ static int move_tasks(struct lb_env *env)

    /* take a breather every nr_migrate tasks */
    if (env->loop > env->loop_break) {
    - env->loop_break += sysctl_sched_nr_migrate;
    + env->loop_break += sched_nr_migrate_break;
    env->flags |= LBF_NEED_BREAK;
    break;
    }
    @@ -4407,7 +4409,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
    .dst_cpu = this_cpu,
    .dst_rq = this_rq,
    .idle = idle,
    - .loop_break = sysctl_sched_nr_migrate,
    + .loop_break = sched_nr_migrate_break,
    + .loop_max = sysctl_sched_nr_migrate,
    };

    cpumask_copy(cpus, cpu_active_mask);
    @@ -4448,7 +4451,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
    env.load_move = imbalance;
    env.src_cpu = busiest->cpu;
    env.src_rq = busiest;
    - env.loop_max = busiest->nr_running;

    more_balance:
    local_irq_save(flags);



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