mm: memcontrol: use CSS_TASK_ITER_PROCS at mem_cgroup_scan_tasks()

Since commit c03cd773 ("cgroup: Include dying leaders with live
threads in PROCS iterations") corrected how CSS_TASK_ITER_PROCS works,
mem_cgroup_scan_tasks() can use CSS_TASK_ITER_PROCS in order to check
only one thread from each thread group.

[penguin-kernel@I-love.SAKURA.ne.jp: remove thread group leader check in oom_evaluate_task()]
  Link: http://lkml.kernel.org/r/1560853257-14934-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Link: http://lkml.kernel.org/r/c763afc8-f0ae-756a-56a7-395f625b95fc@i-love.sakura.ne.jpSigned-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1167,7 +1167,7 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 		struct css_task_iter it;
 		struct task_struct *task;
 
-		css_task_iter_start(&iter->css, 0, &it);
+		css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it);
 		while (!ret && (task = css_task_iter_next(&it)))
 			ret = fn(task, arg);
 		css_task_iter_end(&it);

mm/oom_kill.c

@@ -346,9 +346,6 @@ static int oom_evaluate_task(struct task_struct *task, void *arg)
 	if (!points || points < oc->chosen_points)
 		goto next;
 
-	/* Prefer thread group leaders for display purposes */
-	if (points == oc->chosen_points && thread_group_leader(oc->chosen))
-		goto next;
 select:
 	if (oc->chosen)
 		put_task_struct(oc->chosen);