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From: Matt Helsley <matthltc@us.ibm.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matt Helsley <matthltc@us.ibm.com>,
	Paul Menage <menage@google.com>, Li Zefan <lizf@cn.fujitsu.com>,
	containers@lists.linux-foundation.org,
	jacob.jun.pan@linux.intel.com,
	Arjan van de Ven <arjan@linux.intel.com>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-api@vger.kernel.org
Subject: Re: [PATCH, v6 3/3] cgroups: introduce timer slack controller
Date: Mon, 14 Feb 2011 16:00:55 -0800	[thread overview]
Message-ID: <20110215000055.GR16432@count0.beaverton.ibm.com> (raw)
In-Reply-To: <20110214225940.GB6230@shutemov.name>

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?

Cheers,
	-Matt Helsley

  reply	other threads:[~2011-02-15  0:01 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-02-14 13:06 [PATCH, v6 0/3] Introduce " Kirill A. Shutsemov
2011-02-14 13:06 ` [PATCH, v6 1/3] cgroups: export cgroup_iter_{start,next,end} Kirill A. Shutsemov
2011-02-14 13:27   ` Thomas Gleixner
2011-02-14 14:39     ` Kirill A. Shutemov
2011-02-14 15:09       ` Thomas Gleixner
2011-02-14 13:06 ` [PATCH, v6 2/3] Implement timer slack notifier chain Kirill A. Shutsemov
2011-02-14 13:32   ` Thomas Gleixner
2011-02-14 14:52     ` Kirill A. Shutemov
2011-02-14 15:16       ` Thomas Gleixner
2011-02-14 13:06 ` [PATCH, v6 3/3] cgroups: introduce timer slack controller Kirill A. Shutsemov
2011-02-14 13:59   ` Matt Helsley
2011-02-14 22:59     ` Kirill A. Shutemov
2011-02-15  0:00       ` Matt Helsley [this message]
2011-02-15  0:10         ` Kirill A. Shutemov
2011-02-14 14:00   ` Thomas Gleixner
2011-02-14 15:19     ` Kirill A. Shutemov
2011-02-14 17:01       ` Thomas Gleixner
2011-02-14 22:39         ` Kirill A. Shutemov
2011-02-14 23:39           ` Matt Helsley
2011-02-15  6:04           ` Thomas Gleixner
2011-02-14 13:26 ` [PATCH, v6 0/3] Introduce " Thomas Gleixner

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