[PATCH 2/7] cgroup: introduce hook css_has_tasks_changed

[Song Liu]

This patch introduces a new hook css_has_tasks_changed:

   void (*css_has_tasks_changed)(struct cgroup_subsys_state *css,
                                 bool has_tasks);

The hook is called when the cgroup gets its first task and when the cgroup
loses its last task. It is called under css_set_lock.

Note: has_task is different to populated. It only considers directly
attached tasks.

Signed-off-by: Song Liu <songliubrav...@fb.com>
---
 include/linux/cgroup-defs.h |  2 ++
 kernel/cgroup/cgroup.c      | 14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 1c70803e9f77..ba499ed5309c 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -594,6 +594,8 @@ struct cgroup_subsys {
        void (*css_released)(struct cgroup_subsys_state *css);
        void (*css_free)(struct cgroup_subsys_state *css);
        void (*css_reset)(struct cgroup_subsys_state *css);
+       void (*css_has_tasks_changed)(struct cgroup_subsys_state *css,
+                                     bool has_tasks);
        void (*css_rstat_flush)(struct cgroup_subsys_state *css, int cpu);
        int (*css_extra_stat_show)(struct seq_file *seq,
                                   struct cgroup_subsys_state *css);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 3f2b4bde0f9c..b0df96132476 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -200,6 +200,7 @@ static u16 have_fork_callback __read_mostly;
 static u16 have_exit_callback __read_mostly;
 static u16 have_release_callback __read_mostly;
 static u16 have_canfork_callback __read_mostly;
+static u16 have_has_tasks_changed_callback __read_mostly;
 
 /* cgroup namespace for init task */
 struct cgroup_namespace init_cgroup_ns = {
@@ -762,8 +763,11 @@ static bool css_set_populated(struct css_set *cset)
  */
 static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 {
+       struct cgroup *orig_cgrp = cgrp;
        struct cgroup *child = NULL;
        int adj = populated ? 1 : -1;
+       struct cgroup_subsys *ss;
+       int ssid;
 
        lockdep_assert_held(&css_set_lock);
 
@@ -788,6 +792,14 @@ static void cgroup_update_populated(struct cgroup *cgrp, 
bool populated)
                child = cgrp;
                cgrp = cgroup_parent(cgrp);
        } while (cgrp);
+
+       do_each_subsys_mask(ss, ssid, have_has_tasks_changed_callback) {
+               struct cgroup_subsys_state *css;
+
+               css = cgroup_css(orig_cgrp, ss);
+               if (css)
+                       ss->css_has_tasks_changed(css, populated);
+       } while_each_subsys_mask();
 }
 
 /**
@@ -5370,6 +5382,8 @@ static void __init cgroup_init_subsys(struct 
cgroup_subsys *ss, bool early)
        have_exit_callback |= (bool)ss->exit << ss->id;
        have_release_callback |= (bool)ss->release << ss->id;
        have_canfork_callback |= (bool)ss->can_fork << ss->id;
+       have_has_tasks_changed_callback |=
+               (bool)ss->css_has_tasks_changed << ss->id;
 
        /* At system boot, before all subsystems have been
         * registered, no tasks have been forked, so we don't
-- 
2.17.1