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    Subject[PATCH v5 2/5] cpusets, hotplug: Implement cpuset tree traversal in a helper function
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    At present, the functions that deal with cpusets during CPU/Mem hotplug
    are quite messy, since a lot of the functionality is mixed up without clear
    separation. And this takes a toll on optimization as well. For example,
    the function cpuset_update_active_cpus() is called on both CPU offline and CPU
    online events; and it invokes scan_for_empty_cpusets(), which makes sense
    only for CPU offline events. And hence, the current code ends up unnecessarily
    traversing the cpuset tree during CPU online also.

    As a first step towards cleaning up those functions, encapsulate the cpuset
    tree traversal in a helper function, so as to facilitate upcoming changes.

    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    kernel/cpuset.c | 36 +++++++++++++++++++++++++++---------
    1 files changed, 27 insertions(+), 9 deletions(-)

    diff --git a/kernel/cpuset.c b/kernel/cpuset.c
    index 5fc1570..15926f8 100644
    --- a/kernel/cpuset.c
    +++ b/kernel/cpuset.c
    @@ -2001,6 +2001,32 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
    }

    /*
    + * Helper function to traverse cpusets.
    + * It can be used to walk the cpuset tree from top to bottom, completing
    + * one layer before dropping down to the next (thus always processing a
    + * node before any of its children).
    + */
    +static struct cpuset *cpuset_next(struct list_head *queue)
    +{
    + struct cpuset *cp;
    + struct cpuset *child; /* scans child cpusets of cp */
    + struct cgroup *cont;
    +
    + if (list_empty(queue))
    + return NULL;
    +
    + cp = list_first_entry(queue, struct cpuset, stack_list);
    + list_del(queue->next);
    + list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
    + child = cgroup_cs(cont);
    + list_add_tail(&child->stack_list, queue);
    + }
    +
    + return cp;
    +}
    +
    +
    +/*
    * Walk the specified cpuset subtree and look for empty cpusets.
    * The tasks of such cpuset must be moved to a parent cpuset.
    *
    @@ -2019,19 +2045,11 @@ static void scan_for_empty_cpusets(struct cpuset *root)
    {
    LIST_HEAD(queue);
    struct cpuset *cp; /* scans cpusets being updated */
    - struct cpuset *child; /* scans child cpusets of cp */
    - struct cgroup *cont;
    static nodemask_t oldmems; /* protected by cgroup_mutex */

    list_add_tail((struct list_head *)&root->stack_list, &queue);

    - while (!list_empty(&queue)) {
    - cp = list_first_entry(&queue, struct cpuset, stack_list);
    - list_del(queue.next);
    - list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
    - child = cgroup_cs(cont);
    - list_add_tail(&child->stack_list, &queue);
    - }
    + while ((cp = cpuset_next(&queue)) != NULL) {

    /* Continue past cpusets with all cpus, mems online */
    if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&


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