From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with ESMTP id 6C4C66008E4 for ; Tue, 3 Aug 2010 00:51:12 -0400 (EDT) Date: Tue, 3 Aug 2010 13:51:29 +0900 From: Daisuke Nishimura Subject: Re: [PATCH -mm 1/5] quick lookup memcg by ID Message-Id: <20100803135129.4316dfff.nishimura@mxp.nes.nec.co.jp> In-Reply-To: <20100803133723.bb6487a0.kamezawa.hiroyu@jp.fujitsu.com> References: <20100802191113.05c982e4.kamezawa.hiroyu@jp.fujitsu.com> <20100802191304.8e520808.kamezawa.hiroyu@jp.fujitsu.com> <20100803133109.c0e6f150.nishimura@mxp.nes.nec.co.jp> <20100803133723.bb6487a0.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki Cc: linux-mm@kvack.org, "balbir@linux.vnet.ibm.com" , vgoyal@redhat.com, m-ikeda@ds.jp.nec.com, gthelen@google.com, "akpm@linux-foundation.org" , "linux-kernel@vger.kernel.org" , Daisuke Nishimura List-ID: On Tue, 3 Aug 2010 13:37:23 +0900 KAMEZAWA Hiroyuki wrote: > On Tue, 3 Aug 2010 13:31:09 +0900 > Daisuke Nishimura wrote: > (snip) > > > +/* 0 is unused */ > > > +static atomic_t mem_cgroup_num; > > > +#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1) > > > +static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly; > > > + > > > +static struct mem_cgroup *id_to_memcg(unsigned short id) > > > +{ > > > + /* > > > + * This array is set to NULL when mem_cgroup is freed. > > > + * IOW, there are no more references && rcu_synchronized(). > > > + * This lookup-caching is safe. > > > + */ > > > + if (unlikely(!mem_cgroups[id])) { > > > + struct cgroup_subsys_state *css; > > > + > > > + rcu_read_lock(); > > > + css = css_lookup(&mem_cgroup_subsys, id); > > > + rcu_read_unlock(); > > > + if (!css) > > > + return NULL; > > > + mem_cgroups[id] = container_of(css, struct mem_cgroup, css); > > > + } > > > + return mem_cgroups[id]; > > > +} > > id_to_memcg() seems to be called under rcu_read_lock() already, so I think > > rcu_read_lock()/unlock() would be unnecessary. > > > > Maybe. I thought about which is better to add > > VM_BUG_ON(!rcu_read_lock_held); > or > rcu_read_lock() > .. > rcu_read_unlock() > > Do you like former ? If so, it's ok to remove rcu-read-lock. > Yes, I personally like the former. -- 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