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    SubjectRe: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected
    On Mon, Sep 24, 2012 at 8:30 AM, Peter Zijlstra <> wrote:
    > No idea if its sufficient, but its a start.

    Can we please do this too?

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 96e2b18b6283..2010c1ece7b3 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -2634,25 +2634,12 @@ find_idlest_cpu(struct sched_group *group,
    struct task_struct *p, int this_cpu)
    static int select_idle_sibling(struct task_struct *p, int target)
    - int cpu = smp_processor_id();
    - int prev_cpu = task_cpu(p);
    struct sched_domain *sd;
    struct sched_group *sg;
    int i;

    - /*
    - * If the task is going to be woken-up on this cpu and if it is
    - * already idle, then it is the right target.
    - */
    - if (target == cpu && idle_cpu(cpu))
    - return cpu;
    - /*
    - * If the task is going to be woken-up on the cpu where it previously
    - * ran and if it is currently idle, then it the right target.
    - */
    - if (target == prev_cpu && idle_cpu(prev_cpu))
    - return prev_cpu;
    + if (idle_cpu(target))
    + return target;

    * Otherwise, iterate the domains and find an elegible idle cpu.

    (obviously whitespace-damaged). The whole "let's test prev_cpu or cpu"
    seems stupid and counter-productive. The only possible values for
    'target' are the two we test for.

    Your patch looks odd, though. Why do you use some complex initial
    value for 'candidate' (nr_cpu_ids) instead of a simple and readable
    one (-1)?

    And the whole "if we find any non-idle cpu, skip the whole domain"
    logic really seems a bit odd (that's not new to your patch, though).
    Can somebody explain what the whole point of that idiotically written
    function is?


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      Last update: 2012-09-24 18:21    [W:0.022 / U:13.536 seconds]
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