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

[Thomas Gleixner]

On Mon, 14 Feb 2011, Kirill A. Shutsemov wrote:
> From: Kirill A. Shutemov <kirill at shutemov.name>
> 
> 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
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