linux-kernel - Re: [PATCH, v3 2/2] cgroups: introduce timer slack subsystem

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Date:	Sat, 5 Feb 2011 18:49:51 -0800
From:	Matt Helsley <matthltc@...ibm.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	jacob.jun.pan@...ux.intel.com,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH, v3 2/2] cgroups: introduce timer slack subsystem

On Thu, Feb 03, 2011 at 11:41:38AM +0200, Kirill A. Shutemov wrote:
> On Wed, Feb 02, 2011 at 09:46:16PM -0800, Matt Helsley wrote:
> > On Wed, Feb 02, 2011 at 10:47:36PM +0200, Kirill A. Shutsemov wrote:
> > > From: Kirill A. Shutemov <kirill@...temov.name>

<snip>

> > > diff --git a/kernel/cgroup_timer_slack.c b/kernel/cgroup_timer_slack.c
> > > new file mode 100644
> > > index 0000000..a343a50
> > > --- /dev/null
> > > +++ b/kernel/cgroup_timer_slack.c

<snip>

> > > +static int tslack_write_set_slack_ns(struct cgroup *cgroup, struct cftype *cft,
> > > +		u64 val)
> > > +{
> > > +	struct timer_slack_cgroup *tslack_cgroup;
> > > +	struct cgroup_iter it;
> > > +	struct task_struct *task;
> > > +
> > > +	tslack_cgroup = cgroup_to_tslack_cgroup(cgroup);
> > > +	if (!val || val < tslack_cgroup->min_slack_ns ||
> > 
> > Why is a val of 0 disallowed? I know having slack is good, but for
> > an administrator or tool that doesn't care about number of wakeups
> > and cares more about wringing out performance a slack of
> > 0 seems acceptable. Is this just here to be consistent with the
> > values passed in via prctl?
> 
> Yes, it's to consistent with the prctl(). I don't think that it's good
> idea to allow to set timer_slack outside of range prctl() allows. It may
> lead to interface abuse.

Hmm, I was just thinking that 0 timer slack might be useful. But I
suppose you could just as easily set it to 1 and nobody would notice.

> > > +			val > tslack_cgroup->max_slack_ns )
> > > +		return -EINVAL;
> > 
> > Shouldn't it be EPERM and not EINVAL?
> > 
> > The write(2) man page says: "Other errors may occur, depending on the
> > object connected to fd." So I think EPERM is fine and more descriptive.
> 
> What do you think about -EINVAL for (val == 0) and -EPERM for rest?

OK, that makes sense to me given both of our points above.

Cheers,
	-Matt Helsley
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