cgroup: only need to check oldcgrp==newgrp once

In cgroup_attach_proc it is now sufficient to only check that
oldcgrp==newcgrp once. Now that we are using threadgroup_lock()
during the migrations, oldcgrp will not change.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>

1 files changed, 6 insertions, 16 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a85a7002ca33..1042b3c41314 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2067,7 +2067,7 @@ static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
  */
 int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 {
-        int retval, i, group_size, nr_migrating_tasks;
+        int retval, i, group_size;
         struct cgroup_subsys *ss, *failed_ss = NULL;
         /* guaranteed to be initialized later, but the compiler needs this */
         struct css_set *oldcg;
@@ -2118,7 +2118,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
         }
 
         tsk = leader;
-        i = nr_migrating_tasks = 0;
+        i = 0;
         do {
                 struct task_and_cgroup ent;
 
@@ -2134,11 +2134,12 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                  */
                 ent.task = tsk;
                 ent.cgrp = task_cgroup_from_root(tsk, root);
+                /* nothing to do if this task is already in the cgroup */
+                if (ent.cgrp == cgrp)
+                        continue;
                 retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
                 BUG_ON(retval != 0);
                 i++;
-                if (ent.cgrp != cgrp)
-                        nr_migrating_tasks++;
         } while_each_thread(leader, tsk);
         /* remember the number of threads in the array for later. */
         group_size = i;
@@ -2148,7 +2149,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 
         /* methods shouldn't be called if no task is actually migrating */
         retval = 0;
-        if (!nr_migrating_tasks)
+        if (!group_size)
                 goto out_free_group_list;
 
         /*
@@ -2171,14 +2172,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
         INIT_LIST_HEAD(&newcg_list);
         for (i = 0; i < group_size; i++) {
                 tc = flex_array_get(group, i);
-                /* nothing to do if this task is already in the cgroup */
-                if (tc->cgrp == cgrp)
-                        continue;
-                /*
-                 * get old css_set pointer. threadgroup is locked so this is
-                 * safe against concurrent cgroup_exit() changing this to
-                 * init_css_set.
-                 */
                 oldcg = tc->task->cgroups;
 
                 /* if we don't already have it in the list get a new one */
@@ -2194,9 +2187,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          */
         for (i = 0; i < group_size; i++) {
                 tc = flex_array_get(group, i);
-                /* leave current thread as it is if it's already there */
-                if (tc->cgrp == cgrp)
-                        continue;
                 retval = cgroup_task_migrate(cgrp, tc->cgrp, tc->task, true);
                 BUG_ON(retval);
         }