Re: [PATCH, v6 3/3] cgroups: introduce timer slack controller

On Mon, 14 Feb 2011, Kirill A. Shutsemov wrote:

From: Kirill A. Shutemov <redacted>

Every task_struct has timer_slack_ns value. This value uses to round up
poll() and select() timeout values. This feature can be useful in
mobile environment where combined wakeups are desired.

cgroup subsys "timer_slack" implement timer slack controller. It
provides a way to group tasks by timer slack value and manage the
value of group's tasks.

I have no objections against the whole thing in general, but why do we
need a module for this? Why can't we add this to the cgroups muck and
compile it in?

+struct cgroup_subsys timer_slack_subsys;
+struct timer_slack_cgroup {
+	struct cgroup_subsys_state css;
+	unsigned long min_slack_ns;
+	unsigned long max_slack_ns;
+};
+
+enum {
+	TIMER_SLACK_MIN,
+	TIMER_SLACK_MAX,
+};
+
+static struct timer_slack_cgroup *cgroup_to_tslack_cgroup(struct cgroup *cgroup)
+{
+	struct cgroup_subsys_state *css;
+
+	css = cgroup_subsys_state(cgroup, timer_slack_subsys.subsys_id);
+	return container_of(css, struct timer_slack_cgroup, css);
+}
+
+static int is_timer_slack_allowed(struct timer_slack_cgroup *tslack_cgroup,

  bool perhaps ?

+		unsigned long slack_ns)
+{
+	if (slack_ns < tslack_cgroup->min_slack_ns ||
+			slack_ns > tslack_cgroup->max_slack_ns)
+		return false;
+	return true;
+}
+
+static int cgroup_timer_slack_check(struct notifier_block *nb,
+		unsigned long slack_ns, void *data)
+{
+	struct cgroup_subsys_state *css;
+	struct timer_slack_cgroup *tslack_cgroup;
+
+	/* XXX: lockdep false positive? */

  What? Either this has a reason or not. If it's a false positive then
  it needs to be fixed in lockdep. If not, ....

+	rcu_read_lock();
+	css = task_subsys_state(current, timer_slack_subsys.subsys_id);
+	tslack_cgroup = container_of(css, struct timer_slack_cgroup, css);
+	rcu_read_unlock();
+
+	if (!is_timer_slack_allowed(tslack_cgroup, slack_ns))
+		return notifier_from_errno(-EPERM);

  If the above needs rcu read lock, why is the acess safe ?

+	return NOTIFY_OK;

+/*
+ * Adjust ->timer_slack_ns and ->default_max_slack_ns of the task to fit
+ * limits of the cgroup.
+ */
+static void tslack_adjust_task(struct timer_slack_cgroup *tslack_cgroup,
+		struct task_struct *tsk)
+{
+	if (tslack_cgroup->min_slack_ns > tsk->timer_slack_ns)
+		tsk->timer_slack_ns = tslack_cgroup->min_slack_ns;
+	else if (tslack_cgroup->max_slack_ns < tsk->timer_slack_ns)
+		tsk->timer_slack_ns = tslack_cgroup->max_slack_ns;
+
+	if (tslack_cgroup->min_slack_ns > tsk->default_timer_slack_ns)
+		tsk->default_timer_slack_ns = tslack_cgroup->min_slack_ns;
+	else if (tslack_cgroup->max_slack_ns < tsk->default_timer_slack_ns)
+		tsk->default_timer_slack_ns = tslack_cgroup->max_slack_ns;

  Why is there not a default slack value for the whole group ?

+static u64 tslack_read_range(struct cgroup *cgroup, struct cftype *cft)
+{
+	struct timer_slack_cgroup *tslack_cgroup;
+
+	tslack_cgroup = cgroup_to_tslack_cgroup(cgroup);
+	switch (cft->private) {
+	case TIMER_SLACK_MIN:
+		return tslack_cgroup->min_slack_ns;
+	case TIMER_SLACK_MAX:
+		return tslack_cgroup->max_slack_ns;
+	default:
+		BUG();

  BUG() for soemthing which can be dealt with sensible ?

+	}
+}
+
+static int validate_change(struct cgroup *cgroup, u64 val, int type)
+{
+	struct timer_slack_cgroup *tslack_cgroup, *child;
+	struct cgroup *cur;
+
+	BUG_ON(type != TIMER_SLACK_MIN && type != TIMER_SLACK_MAX);

  Ditto. That should be -EINVAL or such.

+	if (val > ULONG_MAX)
+		return -EINVAL;
+
+	if (cgroup->parent) {
+		struct timer_slack_cgroup *parent;
+		parent = cgroup_to_tslack_cgroup(cgroup->parent);
+		if (!is_timer_slack_allowed(parent, val))
+			return -EPERM;
+	}
+
+	tslack_cgroup = cgroup_to_tslack_cgroup(cgroup);
+	if (type == TIMER_SLACK_MIN && val > tslack_cgroup->max_slack_ns)
+		return -EINVAL;
+	if (type == TIMER_SLACK_MAX && val < tslack_cgroup->min_slack_ns)
+		return -EINVAL;
+
+	list_for_each_entry(cur, &cgroup->children, sibling) {
+		child = cgroup_to_tslack_cgroup(cur);
+		if (type == TIMER_SLACK_MIN && val > child->min_slack_ns)
+			return -EBUSY;

  I thought the whole point is to propagate values through the group.

+		if (type == TIMER_SLACK_MAX && val < child->max_slack_ns)
+			return -EBUSY;

  This is completely confusing w/o any line of comment.

Thanks

	tglx