cgroup: reimplement reading "cgroup.procs" on cgroup v2

On v1, "tasks" and "cgroup.procs" are expected to be sorted which
makes the implementation expensive and unnecessarily complicated
involving result cache management.

v2 doesn't have the sorting requirement, so it can just iterate and
print processes one by one.  seq_files are either read sequentially or
reset to position zero, so the implementation doesn't even need to
worry about seeking.

This keeps the css_task_iter across multiple read(2) calls and
migrations of new processes always append won't miss processes which
are newly migrated in before each read(2).

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Acked-by: Zefan Li <lizefan@huawei.com>

kernel/cgroup.c

@@ -4426,6 +4426,60 @@ out_err:
 	return ret;
 }
 
+static void cgroup_procs_release(struct kernfs_open_file *of)
+{
+	if (of->priv) {
+		css_task_iter_end(of->priv);
+		kfree(of->priv);
+	}
+}
+
+static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
+{
+	struct kernfs_open_file *of = s->private;
+	struct css_task_iter *it = of->priv;
+	struct task_struct *task;
+
+	do {
+		task = css_task_iter_next(it);
+	} while (task && !thread_group_leader(task));
+
+	return task;
+}
+
+static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
+{
+	struct kernfs_open_file *of = s->private;
+	struct cgroup *cgrp = seq_css(s)->cgroup;
+	struct css_task_iter *it = of->priv;
+
+	/*
+	 * When a seq_file is seeked, it's always traversed sequentially
+	 * from position 0, so we can simply keep iterating on !0 *pos.
+	 */
+	if (!it) {
+		if (WARN_ON_ONCE((*pos)++))
+			return ERR_PTR(-EINVAL);
+
+		it = kzalloc(sizeof(*it), GFP_KERNEL);
+		if (!it)
+			return ERR_PTR(-ENOMEM);
+		of->priv = it;
+		css_task_iter_start(&cgrp->self, it);
+	} else if (!(*pos)++) {
+		css_task_iter_end(it);
+		css_task_iter_start(&cgrp->self, it);
+	}
+
+	return cgroup_procs_next(s, NULL, NULL);
+}
+
+static int cgroup_procs_show(struct seq_file *s, void *v)
+{
+	seq_printf(s, "%d\n", task_tgid_vnr(v));
+	return 0;
+}
+
 /*
  * Stuff for reading the 'tasks'/'procs' files.
  *
@@ -4914,11 +4968,10 @@ static struct cftype cgroup_dfl_base_files[] = {
 	{
 		.name = "cgroup.procs",
 		.file_offset = offsetof(struct cgroup, procs_file),
-		.seq_start = cgroup_pidlist_start,
-		.seq_next = cgroup_pidlist_next,
-		.seq_stop = cgroup_pidlist_stop,
-		.seq_show = cgroup_pidlist_show,
-		.private = CGROUP_FILE_PROCS,
+		.release = cgroup_procs_release,
+		.seq_start = cgroup_procs_start,
+		.seq_next = cgroup_procs_next,
+		.seq_show = cgroup_procs_show,
 		.write = cgroup_procs_write,
 	},
 	{