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    Subject[tip:sched/balancing] sched: Add comments to find_busiest_group() function
    Commit-ID:  7b6340ef884aff69a54f8a530c73ad9da0a7c388
    Gitweb: http://git.kernel.org/tip/7b6340ef884aff69a54f8a530c73ad9da0a7c388
    Author: Gautham R Shenoy <ego@in.ibm.com>
    AuthorDate: Wed, 25 Mar 2009 14:44:27 +0530
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Wed, 25 Mar 2009 10:30:49 +0100

    sched: Add comments to find_busiest_group() function

    Impact: cleanup

    Add /** style comments around find_busiest_group(). Also add a few
    explanatory comments.

    This concludes the find_busiest_group() cleanup. The function is
    now down to 72 lines from the original 313 lines.

    Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Suresh Siddha <suresh.b.siddha@intel.com>
    Cc: "Balbir Singh" <balbir@in.ibm.com>
    Cc: Nick Piggin <nickpiggin@yahoo.com.au>
    Cc: "Dhaval Giani" <dhaval@linux.vnet.ibm.com>
    Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
    Cc: "Vaidyanathan Srinivasan" <svaidy@linux.vnet.ibm.com>
    LKML-Reference: <20090325091427.13992.18933.stgit@sofia.in.ibm.com>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    kernel/sched.c | 50 ++++++++++++++++++++++++++++++++++++++++++--------
    1 files changed, 42 insertions(+), 8 deletions(-)

    diff --git a/kernel/sched.c b/kernel/sched.c
    index 5f21658..b7723bd 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -3676,10 +3676,30 @@ static inline void calculate_imbalance(struct sd_lb_stats *sds, int this_cpu,
    }
    /******* find_busiest_group() helpers end here *********************/

    -/*
    - * find_busiest_group finds and returns the busiest CPU group within the
    - * domain. It calculates and returns the amount of weighted load which
    - * should be moved to restore balance via the imbalance parameter.
    +/**
    + * find_busiest_group - Returns the busiest group within the sched_domain
    + * if there is an imbalance. If there isn't an imbalance, and
    + * the user has opted for power-savings, it returns a group whose
    + * CPUs can be put to idle by rebalancing those tasks elsewhere, if
    + * such a group exists.
    + *
    + * Also calculates the amount of weighted load which should be moved
    + * to restore balance.
    + *
    + * @sd: The sched_domain whose busiest group is to be returned.
    + * @this_cpu: The cpu for which load balancing is currently being performed.
    + * @imbalance: Variable which stores amount of weighted load which should
    + * be moved to restore balance/put a group to idle.
    + * @idle: The idle status of this_cpu.
    + * @sd_idle: The idleness of sd
    + * @cpus: The set of CPUs under consideration for load-balancing.
    + * @balance: Pointer to a variable indicating if this_cpu
    + * is the appropriate cpu to perform load balancing at this_level.
    + *
    + * Returns: - the busiest group if imbalance exists.
    + * - If no imbalance and user has opted for power-savings balance,
    + * return the least loaded group whose CPUs can be
    + * put to idle by rebalancing its tasks onto our group.
    */
    static struct sched_group *
    find_busiest_group(struct sched_domain *sd, int this_cpu,
    @@ -3697,17 +3717,31 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
    update_sd_lb_stats(sd, this_cpu, idle, sd_idle, cpus,
    balance, &sds);

    + /* Cases where imbalance does not exist from POV of this_cpu */
    + /* 1) this_cpu is not the appropriate cpu to perform load balancing
    + * at this level.
    + * 2) There is no busy sibling group to pull from.
    + * 3) This group is the busiest group.
    + * 4) This group is more busy than the avg busieness at this
    + * sched_domain.
    + * 5) The imbalance is within the specified limit.
    + * 6) Any rebalance would lead to ping-pong
    + */
    if (balance && !(*balance))
    goto ret;

    - if (!sds.busiest || sds.this_load >= sds.max_load
    - || sds.busiest_nr_running == 0)
    + if (!sds.busiest || sd.busiest_nr_running == 0)
    + goto out_balanced;
    +
    + if (sds.this_load >= sds.max_load)
    goto out_balanced;

    sds.avg_load = (SCHED_LOAD_SCALE * sds.total_load) / sds.total_pwr;

    - if (sds.this_load >= sds.avg_load ||
    - 100*sds.max_load <= sd->imbalance_pct * sds.this_load)
    + if (sds.this_load >= sds.avg_load)
    + goto out_balanced;
    +
    + if (100 * sds.max_load <= sd->imbalance_pct * sds.this_load)
    goto out_balanced;

    sds.busiest_load_per_task /= sds.busiest_nr_running;

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