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    Subject[PATCH v3 2/5] cpusets, hotplug: Restructure functions that are invoked during hotplug
    Date
    Separate out the cpuset related handling for CPU/Memory online/offline.
    This also helps us exploit the most obvious and basic level of optimization
    that any notification mechanism (CPU/Mem online/offline) has to offer us:
    "We *know* why we have been invoked. So stop pretending that we are lost,
    and do only the necessary amount of processing!".

    And while at it, rename scan_for_empty_cpusets() to
    scan_cpusets_upon_hotplug(), which will be more appropriate, considering
    the upcoming changes.

    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Cc: stable@vger.kernel.org
    ---

    include/linux/cpuset.h | 4 +-
    kernel/cpuset.c | 91 +++++++++++++++++++++++++++++++++---------------
    kernel/sched/core.c | 4 +-
    3 files changed, 67 insertions(+), 32 deletions(-)

    diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
    index 668f66b..838320f 100644
    --- a/include/linux/cpuset.h
    +++ b/include/linux/cpuset.h
    @@ -20,7 +20,7 @@ extern int number_of_cpusets; /* How many cpusets are defined in system? */

    extern int cpuset_init(void);
    extern void cpuset_init_smp(void);
    -extern void cpuset_update_active_cpus(void);
    +extern void cpuset_update_active_cpus(bool cpu_online);
    extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
    extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
    extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
    @@ -124,7 +124,7 @@ static inline void set_mems_allowed(nodemask_t nodemask)
    static inline int cpuset_init(void) { return 0; }
    static inline void cpuset_init_smp(void) {}

    -static inline void cpuset_update_active_cpus(void)
    +static inline void cpuset_update_active_cpus(bool cpu_online)
    {
    partition_sched_domains(1, NULL, NULL);
    }
    diff --git a/kernel/cpuset.c b/kernel/cpuset.c
    index 23e5da6..87b0048 100644
    --- a/kernel/cpuset.c
    +++ b/kernel/cpuset.c
    @@ -147,6 +147,12 @@ typedef enum {
    CS_SPREAD_SLAB,
    } cpuset_flagbits_t;

    +/* the type of hotplug event */
    +enum hotplug_event {
    + CPUSET_CPU_OFFLINE,
    + CPUSET_MEM_OFFLINE,
    +};
    +
    /* convenient tests for these bits */
    static inline int is_cpu_exclusive(const struct cpuset *cs)
    {
    @@ -2018,8 +2024,10 @@ static struct cpuset *traverse_cpusets(struct list_head *queue)


    /*
    - * Walk the specified cpuset subtree and look for empty cpusets.
    - * The tasks of such cpuset must be moved to a parent cpuset.
    + * Walk the specified cpuset subtree upon a hotplug operation (CPU/Memory
    + * online/offline) and update the cpusets accordingly.
    + * For regular CPU/Mem hotplug, look for empty cpusets; the tasks of such
    + * cpuset must be moved to a parent cpuset.
    *
    * Called with cgroup_mutex held. We take callback_mutex to modify
    * cpus_allowed and mems_allowed.
    @@ -2032,39 +2040,60 @@ static struct cpuset *traverse_cpusets(struct list_head *queue)
    * that has tasks along with an empty 'mems'. But if we did see such
    * a cpuset, we'd handle it just like we do if its 'cpus' was empty.
    */
    -static void scan_for_empty_cpusets(struct cpuset *root)
    +static void
    +scan_cpusets_upon_hotplug(struct cpuset *root, enum hotplug_event event)
    {
    LIST_HEAD(queue);
    - struct cpuset *cp; /* scans cpusets being updated */
    + struct cpuset *cp; /* scans cpusets being updated */
    static nodemask_t oldmems; /* protected by cgroup_mutex */

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

    - while (!list_empty(&queue)) {
    - cp = traverse_cpusets(&queue);
    + switch (event) {
    + case CPUSET_CPU_OFFLINE:
    + while (!list_empty(&queue)) {
    + cp = traverse_cpusets(&queue);

    - /* Continue past cpusets with all cpus, mems online */
    - if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&
    - nodes_subset(cp->mems_allowed, node_states[N_HIGH_MEMORY]))
    - continue;
    + /* Continue past cpusets with all cpus online */
    + if (cpumask_subset(cp->cpus_allowed, cpu_active_mask))
    + continue;

    - oldmems = cp->mems_allowed;
    + /* Remove offline cpus from this cpuset. */
    + mutex_lock(&callback_mutex);
    + cpumask_and(cp->cpus_allowed, cp->cpus_allowed,
    + cpu_active_mask);
    + mutex_unlock(&callback_mutex);

    - /* Remove offline cpus and mems from this cpuset. */
    - mutex_lock(&callback_mutex);
    - cpumask_and(cp->cpus_allowed, cp->cpus_allowed,
    - cpu_active_mask);
    - nodes_and(cp->mems_allowed, cp->mems_allowed,
    + /* Move tasks from the empty cpuset to a parent */
    + if (cpumask_empty(cp->cpus_allowed))
    + remove_tasks_in_empty_cpuset(cp);
    + else
    + update_tasks_cpumask(cp, NULL);
    + }
    + break;
    +
    + case CPUSET_MEM_OFFLINE:
    + while (!list_empty(&queue)) {
    + cp = traverse_cpusets(&queue);
    +
    + /* Continue past cpusets with all mems online */
    + if (nodes_subset(cp->mems_allowed,
    + node_states[N_HIGH_MEMORY]))
    + continue;
    +
    + oldmems = cp->mems_allowed;
    +
    + /* Remove offline mems from this cpuset. */
    + mutex_lock(&callback_mutex);
    + nodes_and(cp->mems_allowed, cp->mems_allowed,
    node_states[N_HIGH_MEMORY]);
    - mutex_unlock(&callback_mutex);
    + mutex_unlock(&callback_mutex);

    - /* Move tasks from the empty cpuset to a parent */
    - if (cpumask_empty(cp->cpus_allowed) ||
    - nodes_empty(cp->mems_allowed))
    - remove_tasks_in_empty_cpuset(cp);
    - else {
    - update_tasks_cpumask(cp, NULL);
    - update_tasks_nodemask(cp, &oldmems, NULL);
    + /* Move tasks from the empty cpuset to a parent */
    + if (nodes_empty(cp->mems_allowed))
    + remove_tasks_in_empty_cpuset(cp);
    + else
    + update_tasks_nodemask(cp, &oldmems, NULL);
    }
    }
    }
    @@ -2080,8 +2109,11 @@ static void scan_for_empty_cpusets(struct cpuset *root)
    *
    * Called within get_online_cpus(). Needs to call cgroup_lock()
    * before calling generate_sched_domains().
    + *
    + * @cpu_online: Indicates whether this is a CPU online event (true) or
    + * a CPU offline event (false).
    */
    -void cpuset_update_active_cpus(void)
    +void cpuset_update_active_cpus(bool cpu_online)
    {
    struct sched_domain_attr *attr;
    cpumask_var_t *doms;
    @@ -2091,7 +2123,10 @@ void cpuset_update_active_cpus(void)
    mutex_lock(&callback_mutex);
    cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
    mutex_unlock(&callback_mutex);
    - scan_for_empty_cpusets(&top_cpuset);
    +
    + if (!cpu_online)
    + scan_cpusets_upon_hotplug(&top_cpuset, CPUSET_CPU_OFFLINE);
    +
    ndoms = generate_sched_domains(&doms, &attr);
    cgroup_unlock();

    @@ -2122,9 +2157,9 @@ static int cpuset_track_online_nodes(struct notifier_block *self,
    case MEM_OFFLINE:
    /*
    * needn't update top_cpuset.mems_allowed explicitly because
    - * scan_for_empty_cpusets() will update it.
    + * scan_cpusets_upon_hotplug() will update it.
    */
    - scan_for_empty_cpusets(&top_cpuset);
    + scan_cpusets_upon_hotplug(&top_cpuset, CPUSET_MEM_OFFLINE);
    break;
    default:
    break;
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 0533a68..55cfe8c 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -6812,7 +6812,7 @@ static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
    switch (action & ~CPU_TASKS_FROZEN) {
    case CPU_ONLINE:
    case CPU_DOWN_FAILED:
    - cpuset_update_active_cpus();
    + cpuset_update_active_cpus(true);
    return NOTIFY_OK;
    default:
    return NOTIFY_DONE;
    @@ -6824,7 +6824,7 @@ static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action,
    {
    switch (action & ~CPU_TASKS_FROZEN) {
    case CPU_DOWN_PREPARE:
    - cpuset_update_active_cpus();
    + cpuset_update_active_cpus(false);
    return NOTIFY_OK;
    default:
    return NOTIFY_DONE;


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