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    SubjectRe: [RFC PATCH v5 4/7] sched: bias task wakeups to preferred semi-idle packages
    * Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2008-12-11 23:13:04]:

    > 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 98345e4..939f2a1 100644
    > --- a/kernel/sched_fair.c
    > +++ b/kernel/sched_fair.c
    > @@ -1027,6 +1027,23 @@ static int wake_idle(int cpu, struct task_struct *p)
    > cpumask_t tmp;
    > struct sched_domain *sd;
    > int i;
    > + unsigned int chosen_wakeup_cpu;
    > + int this_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 =
    > + cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu;
    > +
    > + if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP &&
    > + idle_cpu(cpu) && idle_cpu(this_cpu) && p->mm &&

    The p->mm check is racy, it needs to be done under task_lock(). The
    best way to check for a kernel thread is get_task_mm(), followed by
    put_task_mm() is the mm is not NULL. We also need to check to see if
    the task is _hot_ on cpu. We should negate this optimization in case
    chosen_wakeup_cpu is idle, so check for that as well.

    > + 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.
    >
    >

    --
    Balbir


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