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    SubjectRe: [PATCH, v6 3/3] cgroups: introduce timer slack controller
    On Mon, Feb 14, 2011 at 04:00:55PM -0800, Matt Helsley wrote:
    > On Tue, Feb 15, 2011 at 12:59:40AM +0200, Kirill A. Shutemov wrote:
    > > On Mon, Feb 14, 2011 at 05:59:26AM -0800, Matt Helsley wrote:
    > > > On Mon, Feb 14, 2011 at 03:06:27PM +0200, Kirill A. Shutsemov wrote:
    > > > > From: Kirill A. Shutemov <kirill@shutemov.name>
    >
    > <snip>
    >
    > > > > + 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;
    > > > > + if (type == TIMER_SLACK_MAX && val < child->max_slack_ns)
    > > > > + return -EBUSY;
    > > > > + }
    > > >
    > > > This doesn't look right. Child cgroups should not constrain their
    > > > parents. Instead you should allow the change and propagate the
    > > > constraint to the children.
    > >
    > > See discussion with Thomas.
    >
    > <OK, shifting this topic to that thread>
    > <snip>
    >
    > > > > +static struct cftype files[] = {
    > > > > + {
    > > > > + .name = "set_slack_ns",
    > > > > + .write_u64 = tslack_write_set_slack_ns,
    > > > > + },
    > > > > + {
    > > > > + .name = "min_slack_ns",
    > > > > + .private = TIMER_SLACK_MIN,
    > > > > + .read_u64 = tslack_read_range,
    > > > > + .write_u64 = tslack_write_range,
    > > > > + },
    > > > > + {
    > > > > + .name = "max_slack_ns",
    > > > > + .private = TIMER_SLACK_MAX,
    > > > > + .read_u64 = tslack_read_range,
    > > > > + .write_u64 = tslack_write_range,
    > > > > + },
    > > >
    > > > I didn't get a reply on how a max_slack_ns is useful. It seems
    > > > prudent to add as little interface as possible and only when
    > > > we clearly see the utility of it.
    > >
    > > For example, you can create two groups (excluding root cgroup):
    > >
    > > default - timer slack range 50000-50000
    > > relaxed - timer slack range 500000-unlimited.
    > >
    > > Now you can drag tasks between these group without need to reset value on
    > > relaxed -> default transition.
    >
    > Perhaps you misunderstood my point.
    >
    > Yes, I can see that a maximum allows you to do counter-productive/pointless
    > little tricks like "setting" the timer slack when you move the task. I
    > just don't get the point of it. Why is setting a maximum timer slack useful?
    > If anything it seems like it would be quite counterproductive or pointless
    > *at best* because limiting the amount of timer slack would not improve
    > the wakeup situation -- it could easily make it worse. Are there
    > *any* negative consequences to allowing timer slacks as large as
    > userspace requests -- perhaps even up to ULLONG_MAX? If there are none then
    > why should we bother providing userspace a knob to set and enforce such a
    > limit?

    Could you describe the interface how you see it?

    --
    Kirill A. Shutemov


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