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

["Kirill A. Shutemov" <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>]

On Mon, Feb 14, 2011 at 03:00:03PM +0100, Thomas Gleixner wrote:
> On Mon, 14 Feb 2011, Kirill A. Shutsemov wrote:
> > From: Kirill A. Shutemov <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>
> > 
> > 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?

It was easier to test and debug with module.
What is wrong with module? Do you worry about number of exports?

> > +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 ?

Right.

> > +		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, ....

I was not sure about it. There is similar workaround in freezer_fork().

> > +	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 ?

I think it breaks prctl() semantic. default slack value is a value on
fork().

> > +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 ?

tslack_read_range() and tslack_write_range() have written to handle
defined cftypes. If it used for other cftype it's a bug().

> > +	}
> > +}
> > +
> > +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.

I think silent change here is wrong. cpuset returns -EBUSY in similar
case.

> > +		if (type == TIMER_SLACK_MAX && val < child->max_slack_ns)
> > +			return -EBUSY;
> 
>   This is completely confusing w/o any line of comment.

Ok, I'll add a comment here.
> 
> Thanks
> 
> 	tglx

-- 
 Kirill A. Shutemov