cgroup: update debug controller to print out thread mode information

Update debug controller so that it prints out debug info about thread
mode.

 1) The relationship between proc_cset and threaded_csets are displayed.
 2) The status of being a thread root or threaded cgroup is displayed.

This patch is extracted from Waiman's larger patch.

v2: - Removed [thread root] / [threaded] from debug.cgroup_css_links
      file as the same information is available from cgroup.type.
      Suggested by Waiman.
    - Threaded marking is moved to the previous patch.

Patch-originally-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

3 files changed, 42 insertions, 16 deletions

diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index f10eb19ddf04..c167a40278e6 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -153,6 +153,8 @@ static inline void get_css_set(struct css_set *cset)
 
 bool cgroup_ssid_enabled(int ssid);
 bool cgroup_on_dfl(const struct cgroup *cgrp);
+bool cgroup_is_thread_root(struct cgroup *cgrp);
+bool cgroup_is_threaded(struct cgroup *cgrp);
 
 struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root);
 struct cgroup *task_cgroup_from_root(struct task_struct *task,
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index c396e701c206..e9a377dc5bdb 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -333,7 +333,7 @@ static bool cgroup_has_tasks(struct cgroup *cgrp)
         return cgrp->nr_populated_csets;
 }
 
-static bool cgroup_is_threaded(struct cgroup *cgrp)
+bool cgroup_is_threaded(struct cgroup *cgrp)
 {
         return cgrp->dom_cgrp != cgrp;
 }
@@ -372,7 +372,7 @@ static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
 }
 
 /* is @cgrp root of a threaded subtree? */
-static bool cgroup_is_thread_root(struct cgroup *cgrp)
+bool cgroup_is_thread_root(struct cgroup *cgrp)
 {
         /* thread root should be a domain */
         if (cgroup_is_threaded(cgrp))
diff --git a/kernel/cgroup/debug.c b/kernel/cgroup/debug.c
index 787a242fa69d..f661b4cc5efd 100644
--- a/kernel/cgroup/debug.c
+++ b/kernel/cgroup/debug.c
@@ -114,27 +114,49 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)
 {
         struct cgroup_subsys_state *css = seq_css(seq);
         struct cgrp_cset_link *link;
-        int dead_cnt = 0, extra_refs = 0;
+        int dead_cnt = 0, extra_refs = 0, threaded_csets = 0;
 
         spin_lock_irq(&css_set_lock);
+
         list_for_each_entry(link, &css->cgroup->cset_links, cset_link) {
                 struct css_set *cset = link->cset;
                 struct task_struct *task;
                 int count = 0;
                 int refcnt = refcount_read(&cset->refcount);
 
-                seq_printf(seq, " %d", refcnt);
-                if (refcnt - cset->nr_tasks > 0) {
-                        int extra = refcnt - cset->nr_tasks;
-
-                        seq_printf(seq, " +%d", extra);
-                        /*
-                         * Take out the one additional reference in
-                         * init_css_set.
-                         */
-                        if (cset == &init_css_set)
-                                extra--;
-                        extra_refs += extra;
+                /*
+                 * Print out the proc_cset and threaded_cset relationship
+                 * and highlight difference between refcount and task_count.
+                 */
+                seq_printf(seq, "css_set %pK", cset);
+                if (rcu_dereference_protected(cset->dom_cset, 1) != cset) {
+                        threaded_csets++;
+                        seq_printf(seq, "=>%pK", cset->dom_cset);
+                }
+                if (!list_empty(&cset->threaded_csets)) {
+                        struct css_set *tcset;
+                        int idx = 0;
+
+                        list_for_each_entry(tcset, &cset->threaded_csets,
+                                            threaded_csets_node) {
+                                seq_puts(seq, idx ? "," : "<=");
+                                seq_printf(seq, "%pK", tcset);
+                                idx++;
+                        }
+                } else {
+                        seq_printf(seq, " %d", refcnt);
+                        if (refcnt - cset->nr_tasks > 0) {
+                                int extra = refcnt - cset->nr_tasks;
+
+                                seq_printf(seq, " +%d", extra);
+                                /*
+                                 * Take out the one additional reference in
+                                 * init_css_set.
+                                 */
+                                if (cset == &init_css_set)
+                                        extra--;
+                                extra_refs += extra;
+                        }
                 }
                 seq_puts(seq, "\n");
 
@@ -163,10 +185,12 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)
         }
         spin_unlock_irq(&css_set_lock);
 
-        if (!dead_cnt && !extra_refs)
+        if (!dead_cnt && !extra_refs && !threaded_csets)
                 return 0;
 
         seq_puts(seq, "\n");
+        if (threaded_csets)
+                seq_printf(seq, "threaded css_sets = %d\n", threaded_csets);
         if (extra_refs)
                 seq_printf(seq, "extra references = %d\n", extra_refs);
         if (dead_cnt)