linux-kernel - Re: [PATCH 2/7] cgroups: New resource counter inheritance API

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Date:	Wed, 13 Jul 2011 14:34:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Paul Menage <menage@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Aditya Kali <adityakali@...gle.com>
Subject: Re: [PATCH 2/7] cgroups: New resource counter inheritance API

On Mon, Jul 11, 2011 at 01:41:31PM -0700, Paul Menage wrote:
> On Mon, Jul 11, 2011 at 7:15 AM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > Provide an API to inherit a counter value from a parent.
> > This can be useful to implement cgroup.clone_children on
> > a resource counter.
> >
> > Still the resources of the children are limited by those
> > of the parent, so this is only to provide a default setting
> > behaviour when clone_children is set.
> >
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> 
> Can't this be just:
> 
> void res_counter_inherit(struct res_counter *counter, int member) {
>   struct res_counter *parent;
>   parent = counter->parent;
>   if (parent)
>     res_counter_write_u64(counter, member,
> res_counter_read_u64(parent, member));
> }
> 
> This is just used at cgroup creation time, right? So the performance
> impact of an extra cli/sti shouldn't matter.

Yeah indeed.
 
> Also, looking at the code res_counter_read_u64() appears to not do any
> locking. I don't recall why I added it like that, but it probably
> ought to do at least an atomic64_read().

It does in 32 bits, using the res counter spinlock.
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