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    Subject[RFC PATCH v3 3/5] sched: nominate preferred wakeup cpu
    Date
    When the system utilisation is low and more cpus are idle,
    then the process waking up from sleep should prefer to
    wakeup an idle cpu from semi-idle cpu package (multi core
    package) rather than a completely idle cpu package which
    would waste power.

    Use the sched_mc balance logic in find_busiest_group() to
    nominate a preferred wakeup cpu.

    This info can be sored in appropriate sched_domain, but
    updating this info in all copies of sched_domain is not
    practical. For now lets try with a per-cpu variable
    pointing to a common storage in partition sched domain
    attribute. Global variable may not work in partitioned
    sched domain case.

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

    include/linux/sched.h | 1 +
    kernel/sched.c | 34 +++++++++++++++++++++++++++++++++-
    2 files changed, 34 insertions(+), 1 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 715028a..8363d02 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -810,6 +810,7 @@ enum sched_domain_level {

    struct sched_domain_attr {
    int relax_domain_level;
    + unsigned int preferred_wakeup_cpu;
    };

    #define SD_ATTR_INIT (struct sched_domain_attr) { \
    diff --git a/kernel/sched.c b/kernel/sched.c
    index d910496..16c5e1f 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -1612,6 +1612,21 @@ static void cfs_rq_set_shares(struct cfs_rq *cfs_rq, unsigned long shares)
    }
    #endif

    +#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
    +
    +/*
    + * Preferred wake up cpu nominated by sched_mc balance that will be used when
    + * most cpus are idle in the system indicating overall very low system
    + * utilisation. Triggered at POWERSAVINGS_BALANCE_WAKEUP (2).
    + */
    +
    +DEFINE_PER_CPU(unsigned int *, sched_mc_preferred_wakeup_cpu);
    +
    +/* Default storage allocation for non-partitioned sched domains */
    +unsigned int fallback_preferred_wakeup_cpu;
    +
    +#endif
    +
    #include "sched_stats.h"
    #include "sched_idletask.c"
    #include "sched_fair.c"
    @@ -3078,6 +3093,7 @@ static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
    return 0;
    }

    +
    /*
    * find_busiest_group finds and returns the busiest CPU group within the
    * domain. It calculates and returns the amount of weighted load which
    @@ -3394,6 +3410,10 @@ out_balanced:

    if (this == group_leader && group_leader != group_min) {
    *imbalance = min_load_per_task;
    + if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP)
    + *per_cpu(sched_mc_preferred_wakeup_cpu,
    + smp_processor_id()) =
    + first_cpu(group_leader->cpumask);
    return group_min;
    }
    #endif
    @@ -7372,7 +7392,7 @@ static void set_domain_attribute(struct sched_domain *sd,
    static int __build_sched_domains(const cpumask_t *cpu_map,
    struct sched_domain_attr *attr)
    {
    - int i;
    + int i, cpu;
    struct root_domain *rd;
    SCHED_CPUMASK_DECLARE(allmasks);
    cpumask_t *tmpmask;
    @@ -7472,6 +7492,18 @@ static int __build_sched_domains(const cpumask_t *cpu_map,
    sd->parent = p;
    p->child = sd;
    cpu_to_core_group(i, cpu_map, &sd->groups, tmpmask);
    + /* Set the preferred wake up CPU */
    + if (attr) {
    + for_each_cpu_mask_nr(cpu, sd->span) {
    + per_cpu(sched_mc_preferred_wakeup_cpu, cpu) =
    + &attr->preferred_wakeup_cpu;
    + }
    + } else {
    + for_each_cpu_mask_nr(cpu, sd->span) {
    + per_cpu(sched_mc_preferred_wakeup_cpu, cpu) =
    + &fallback_preferred_wakeup_cpu;
    + }
    + }
    #endif

    #ifdef CONFIG_SCHED_SMT


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