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    Subject[RFC PATCH v3 4/5] sched: bias task wakeups to preferred semi-idle packages
    Date
    Preferred wakeup cpu (from a semi idle package) has been
    nominated in find_busiest_group() in the previous patch. Use
    this information in sched_mc_preferred_wakeup_cpu in function
    wake_idle() to bias task wakeups if the following conditions
    are satisfied:
    - The present cpu that is trying to wakeup the process is
    idle and waking the target process on this cpu will
    potentially wakeup a completely idle package
    - The previous cpu on which the target process ran is
    also idle and hence selecting the previous cpu may
    wakeup a semi idle cpu package
    - The task being woken up is allowed to run in the
    nominated cpu (cpu affinity and restrictions)

    Basically if both the current cpu and the previous cpu on
    which the task ran is idle, select the nominated cpu from semi
    idle cpu package for running the new task that is waking up.

    Cache hotness is considered since the actual biasing happens
    in wake_idle() only if the application is cache cold.

    This technique will effectively move short running bursty jobs in
    a mostly idle system.

    Wakeup biasing for power savings gets automatically disabled if
    system utilisation increases due to the fact that the probability
    of finding both this_cpu and prev_cpu idle decreases.

    Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
    ---

    kernel/sched_fair.c | 17 +++++++++++++++++
    1 files changed, 17 insertions(+), 0 deletions(-)

    diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
    index ce514af..ad5269a 100644
    --- a/kernel/sched_fair.c
    +++ b/kernel/sched_fair.c
    @@ -1026,6 +1026,23 @@ static int wake_idle(int cpu, struct task_struct *p)
    cpumask_t tmp;
    struct sched_domain *sd;
    int i;
    + int this_cpu;
    + unsigned int *chosen_wakeup_cpu;
    +
    + /*
    + * At POWERSAVINGS_BALANCE_WAKEUP level, if both this_cpu and prev_cpu
    + * are idle and this is not a kernel thread and this task's affinity
    + * allows it to be moved to preferred cpu, then just move!
    + */
    +
    + this_cpu = smp_processor_id();
    + chosen_wakeup_cpu = per_cpu(sched_mc_preferred_wakeup_cpu, this_cpu);
    +
    + if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP &&
    + chosen_wakeup_cpu &&
    + idle_cpu(cpu) && idle_cpu(this_cpu) && p->mm &&
    + cpu_isset(*chosen_wakeup_cpu, p->cpus_allowed))
    + return *chosen_wakeup_cpu;

    /*
    * If it is idle, then it is the best cpu to run this task.


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