linux-kernel - Re: [RFC 4/5] memcg: make sure that memcg is not offline when charging

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Message-ID: <20140203164455.GL2495@dhcp22.suse.cz>
Date:	Mon, 3 Feb 2014 17:44:55 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [RFC 4/5] memcg: make sure that memcg is not offline when
 charging

On Mon 03-02-14 11:18:23, Johannes Weiner wrote:
> On Mon, Feb 03, 2014 at 02:33:13PM +0100, Michal Hocko wrote:
> > On Thu 30-01-14 12:29:06, Johannes Weiner wrote:
> > > On Tue, Dec 17, 2013 at 04:45:29PM +0100, Michal Hocko wrote:
[...]
> > > > In order to make this raceless we would need to hold rcu_read_lock since
> > > > css_tryget until res_counter_charge. This is not so easy unfortunately
> > > > because mem_cgroup_do_charge might sleep so we would need to do drop rcu
> > > > lock and do css_tryget tricks after each reclaim.
> > > 
> > > Yes, why not?
> > 
> > Although css_tryget is cheap these days I thought that a simple flag
> > check would be even heaper in this hot path. Changing the patch to use
> > css_tryget rather than offline check is trivial if you really think it
> > is better?
> 
> You already changed it to do css_tryget() on every single charge.

Fair point

Thanks!
-- 
Michal Hocko
SUSE Labs
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