From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx156.postini.com [74.125.245.156]) by kanga.kvack.org (Postfix) with SMTP id E45496B0044 for ; Tue, 4 Dec 2012 03:23:19 -0500 (EST) Date: Tue, 4 Dec 2012 09:23:16 +0100 From: Michal Hocko Subject: Re: [PATCH 4/4] memcg: replace cgroup_lock with memcg specific memcg_lock Message-ID: <20121204082316.GB31319@dhcp22.suse.cz> References: <1354282286-32278-1-git-send-email-glommer@parallels.com> <1354282286-32278-5-git-send-email-glommer@parallels.com> <20121203171532.GG17093@dhcp22.suse.cz> <50BDAD38.6030200@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50BDAD38.6030200@parallels.com> Sender: owner-linux-mm@kvack.org List-ID: To: Glauber Costa Cc: cgroups@vger.kernel.org, linux-mm@kvack.org, Tejun Heo , kamezawa.hiroyu@jp.fujitsu.com, Johannes Weiner On Tue 04-12-12 11:58:48, Glauber Costa wrote: > On 12/03/2012 09:15 PM, Michal Hocko wrote: > > On Fri 30-11-12 17:31:26, Glauber Costa wrote: > > [...] > >> +/* > >> + * must be called with memcg_lock held, unless the cgroup is guaranteed to be > >> + * already dead (like in mem_cgroup_force_empty, for instance). > >> + */ > >> +static inline bool memcg_has_children(struct mem_cgroup *memcg) > >> +{ > >> + return mem_cgroup_count_children(memcg) != 1; > >> +} > > > > Why not just keep list_empty(&cgrp->children) which is much simpler much > > more effective and correct here as well because cgroup cannot vanish > > while we are at the call because all callers come from cgroup fs? > > > Because it depends on cgroup's internal representation, which I think > we're better off not depending upon, even if this is not as serious a > case as the locking stuff. But also, technically, cgrp->children is > protected by the cgroup_lock(), while since we'll hold the memcg_lock > during creation and also around the iterators, we cover everything with > the same lock. The list is RCU safe so we do not have to use cgroup_lock there for this kind of test. > That said, of course we don't need to do the full iteration here, and > mem_cgroup_count_children is indeed overkill. We could just as easily > verify if any child exist - it is just an emptiness test after all. But > it is not living in any fast path, though, and I just assumed code reuse > to win over efficiency in this particular case - > mem_cgroup_count_children already existed... Yes but the function name suggests a more generic usage and the test is really an overkill. Maybe we can get a cgroup generic helper cgroup_as_children which would do the thing without exhibiting cgroup internals. What do you think? -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org