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    Subject[PATCH v2 09/10] cpuset: allow to move tasks to empty cpusets
    Currently some cpuset behaviors are not friendly when cpuset is co-mounted
    with other cgroup controllers.

    Now with this patchset if cpuset is mounted with sane_behavior option,
    it behaves differently:

    - Tasks will be kept in empty cpusets when hotplug happens and take
    masks of ancestors with non-empty cpus/mems, instead of being moved to
    an ancestor.

    - A task can be moved into an empty cpuset, and again it takes masks of
    ancestors, so the user can drop a task into a newly created cgroup without
    having to do anything for it.

    As tasks can reside in empy cpusets, here're some rules:

    - They can be moved to another cpuset, regardless it's empty or not.

    - Though it takes masks from ancestors, it takes other configs from the
    empty cpuset.

    - If the ancestors' masks are changed, those tasks will also be updated
    to take new masks.

    v2: add documentation in include/linux/cgroup.h

    Signed-off-by: Li Zefan <lizefan@huawei.com>
    ---
    include/linux/cgroup.h | 3 +++
    kernel/cpuset.c | 12 +++++++++---
    2 files changed, 12 insertions(+), 3 deletions(-)

    diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
    index 53e81a6..74e8b8e 100644
    --- a/include/linux/cgroup.h
    +++ b/include/linux/cgroup.h
    @@ -281,6 +281,9 @@ enum {
    * and take masks of ancestors with non-empty cpus/mems, instead of
    * being moved to an ancestor.
    *
    + * - cpuset: a task can be moved into an empty cpuset, and again it
    + * takes masks of ancestors.
    + *
    * - memcg: use_hierarchy is on by default and the cgroup file for
    * the flag is not created.
    *
    diff --git a/kernel/cpuset.c b/kernel/cpuset.c
    index 3b93098..9bb6a47 100644
    --- a/kernel/cpuset.c
    +++ b/kernel/cpuset.c
    @@ -483,7 +483,7 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
    */
    ret = -ENOSPC;
    if ((cgroup_task_count(cur->css.cgroup) || cur->attach_in_progress) &&
    - (cpumask_empty(trial->cpus_allowed) ||
    + (cpumask_empty(trial->cpus_allowed) &&
    nodes_empty(trial->mems_allowed)))
    goto out;

    @@ -1478,8 +1478,13 @@ static int cpuset_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)

    mutex_lock(&cpuset_mutex);

    + /*
    + * We allow to move tasks into an empty cpuset if sane_behavior
    + * flag is set.
    + */
    ret = -ENOSPC;
    - if (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed))
    + if (!cgroup_sane_behavior(cgrp) &&
    + (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed)))
    goto out_unlock;

    cgroup_taskset_for_each(task, cgrp, tset) {
    @@ -1584,7 +1589,8 @@ static void cpuset_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
    * propagation if @cs doesn't have any CPU or memory. It will move
    * the newly added tasks to the nearest parent which can execute.
    */
    - if (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed))
    + if (!cgroup_sane_behavior(cgrp) &&
    + (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed)))
    schedule_cpuset_propagate_hotplug(cs);

    mutex_unlock(&cpuset_mutex);
    --
    1.8.0.2

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