Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753148Ab1ELCXm (ORCPT ); Wed, 11 May 2011 22:23:42 -0400 Received: from mail-qy0-f174.google.com ([209.85.216.174]:40856 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750775Ab1ELCXl convert rfc822-to-8bit (ORCPT ); Wed, 11 May 2011 22:23:41 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=l9GtwLl6fFIhdW0CF3bXKlnjd8TyqdprDfW1rwDa7dPdWjvxWcfVTrjqT8IrNJWfzx ZlCsvBaeEVQ69x4sQ+pWwQNNPM6jC0ommAmDUUw6koA2/AY2VZeCilyYI5ZT1DS50cSh OcbcAEt75dt+5U/5CU5v0d2trdt98aMi1M1WE= MIME-Version: 1.0 In-Reply-To: <20110512105351.a57970d7.kamezawa.hiroyu@jp.fujitsu.com> References: <20110509182110.167F.A69D9226@jp.fujitsu.com> <20110510171335.16A7.A69D9226@jp.fujitsu.com> <20110510171641.16AF.A69D9226@jp.fujitsu.com> <20110512095243.c57e3e83.kamezawa.hiroyu@jp.fujitsu.com> <20110512105351.a57970d7.kamezawa.hiroyu@jp.fujitsu.com> Date: Thu, 12 May 2011 11:23:38 +0900 Message-ID: Subject: Re: [PATCH 2/4] oom: kill younger process first From: Minchan Kim To: KAMEZAWA Hiroyuki Cc: KOSAKI Motohiro , CAI Qian , avagin@gmail.com, Andrey Vagin , Andrew Morton , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Rientjes , Hugh Dickins , Oleg Nesterov Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3923 Lines: 118 On Thu, May 12, 2011 at 10:53 AM, KAMEZAWA Hiroyuki wrote: > On Thu, 12 May 2011 10:30:45 +0900 > Minchan Kim wrote: > >> Hi Kame, >> >> On Thu, May 12, 2011 at 9:52 AM, KAMEZAWA Hiroyuki >> wrote: >> > On Tue, 10 May 2011 17:15:01 +0900 (JST) >> > KOSAKI Motohiro wrote: >> > >> >> This patch introduces do_each_thread_reverse() and >> >> select_bad_process() uses it. The benefits are two, >> >> 1) oom-killer can kill younger process than older if >> >> they have a same oom score. Usually younger process >> >> is less important. 2) younger task often have PF_EXITING >> >> because shell script makes a lot of short lived processes. >> >> Reverse order search can detect it faster. >> >> >> >> Reported-by: CAI Qian >> >> Signed-off-by: KOSAKI Motohiro >> > >> > IIUC, for_each_thread() can be called under rcu_read_lock() but >> > for_each_thread_reverse() must be under tasklist_lock. >> >> Just out of curiosity. >> You mentioned it when I sent forkbomb killer patch. :) >> From at that time, I can't understand why we need holding >> tasklist_lock not rcu_read_lock. Sorry for the dumb question. >> >> At present, it seems that someone uses tasklist_lock and others uses >> rcu_read_lock. But I can't find any rule for that. >> > > for_each_list_rcu() makes use of RCU list's characteristics and allows > walk a list under rcu_read_lock() without taking any atomic locks. > > list_del() of RCU list works as folllowing. > > == >  1) assume  A, B, C, are linked in the list. >        (head)<->(A) <-> (B)  <-> (C) > >  2) remove B. >        (head)<->(A) <-> (C) >                        / >                     (B) > >  Because (B)'s next points to (C) even after (B) is removed, (B)->next >  points to the alive object. Even if (C) is removed at the same time, >  (C) is not freed until rcu glace period and (C)'s next points to (head) > > Then, for_each_list_rcu() can work well under rcu_read_lock(), it will visit > only alive objects (but may not be valid.) > > == > > please see include/linux/rculist.h and check list_add_rcu() ;) > > As above implies, (B)->prev pointer is invalid pointer after list_del(). > So, there will be race with list modification and for_each_list_reverse under > rcu_read__lock() > > So, when you need to take atomic lock (as tasklist lock is) is... > >  1) You can't check 'entry' is valid or not... >    In above for_each_list_rcu(), you may visit an object which is under removing. >    You need some flag or check to see the object is valid or not. > >  2) you want to use list_for_each_safe(). >    You can't do list_del() an object which is under removing... > >  3) You want to walk the list in reverse. > >  3) Some other reasons. For example, you'll access an object pointed by the >    'entry' and the object is not rcu safe. > > make sense ? Yes. Thanks, Kame. It seems It is caused by prev poisoning of list_del_rcu. If we remove it, isn't it possible to traverse reverse without atomic lock? > > Thanks, > -Kame > > >> Could you elaborate it, please? >> Doesn't it need document about it? >> >> -- >> Kind regards, >> Minchan Kim >> >> -- >> 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/ . >> Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ >> Don't email: email@kvack.org >> > > -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/