cgroups: add the trigger callback to struct cftype

Trigger callback can be used to receive a kick-up from the user space. The
string written is ignored.

The cftype->private is used for multiplexing events.

Signed-off-by: Pavel Emelyanov <[e-mail]>
Acked-by: Paul Menage <[e-mail]>
Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/cgroup.h

···
 	 */
 	int (*write_s64) (struct cgroup *cgrp, struct cftype *cft, s64 val);
 
+	/*
+	 * trigger() callback can be used to get some kick from the
+	 * userspace, when the actual string written is not important
+	 * at all. The private field can be used to determine the
+	 * kick type for multiplexing.
+	 */
+	int (*trigger)(struct cgroup *cgrp, unsigned int event);
+
 	int (*release) (struct inode *inode, struct file *file);
 };
 

kernel/cgroup.c

···
 		return cft->write(cgrp, cft, file, buf, nbytes, ppos);
 	if (cft->write_u64 || cft->write_s64)
 		return cgroup_write_X64(cgrp, cft, file, buf, nbytes, ppos);
+	if (cft->trigger) {
+		int ret = cft->trigger(cgrp, (unsigned int)cft->private);
+		return ret ? ret : nbytes;
+	}
 	return -EINVAL;
 }