From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx122.postini.com [74.125.245.122]) by kanga.kvack.org (Postfix) with SMTP id 4BAE16B0070 for ; Wed, 27 Jun 2012 04:55:09 -0400 (EDT) Message-ID: <4FEAC9CB.2010800@parallels.com> Date: Wed, 27 Jun 2012 12:52:27 +0400 From: Glauber Costa MIME-Version: 1.0 Subject: Re: [PATCH 2/2] memcg: first step towards hierarchical controller References: <1340725634-9017-1-git-send-email-glommer@parallels.com> <1340725634-9017-3-git-send-email-glommer@parallels.com> <20120626180451.GP3869@google.com> <20120626220809.GA4653@tiehlicka.suse.cz> <20120626221452.GA15811@google.com> In-Reply-To: <20120626221452.GA15811@google.com> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo Cc: Michal Hocko , cgroups@vger.kernel.org, linux-mm@kvack.org, kamezawa.hiroyu@jp.fujitsu.com, Johannes Weiner , Andrew Morton On 06/27/2012 02:14 AM, Tejun Heo wrote: > Hello, Michal. > > On Wed, Jun 27, 2012 at 12:08:09AM +0200, Michal Hocko wrote: >> According to my experience, people usually create deeper subtrees >> just because they want to have memcg hierarchy together with other >> controller(s) and the other controller requires a different topology >> but then they do not care about memory.* attributes in parents. >> Those cases are not affected by this change because parents are >> unlimited by default. >> Deeper subtrees without hierarchy and independent limits are usually >> mis-configurations, and we would like to hear about those to help to fix >> them, or they are unfixable usecases which we want to know about as well >> (because then we have a blocker for the unified cgroup hierarchy, don't >> we). > > Yeah, this is something I'm seriously considering doing from cgroup > core. ie. generating a warning message if the user nests cgroups w/ > controllers which don't support full hierarchy. > >>> Note that the default should still be flat hierarchy. >>> >>> 2. Mark flat hierarchy deprecated and produce a warning message if >>> memcg is mounted w/o hierarchy option for a year or two. >> >> I would agree with you on this with many kernel configurables but >> this one doesn't fall in. There is a trivial fallback (set root to >> use_hierarchy=0) so the mount option seems like an overkill - yet >> another API to keep for some time... > > Just disallow clearing .use_hierarchy if it was mounted with the > option? We can later either make the file RO 1 for compatibility's > sake or remove it. How will it buy us anything, if it is clear by default?? >> So in short, I do think we should go the sanity path and end up >> with hierarchical trees and sooner we start the better. > > I do agree with you in principle, but I still don't think we can > switch the default behavior underneath the users. > I think we all agree with that. I can't speak for Johannes here, but I risk saying that he agrees with that as well. The problem is that we may differ in what means "default behavior". It is very clear in a system call, API, or any documented feature. We never made the guarantee, *ever*, that non-hierarchical might be the default. I understand that users may have grown accustomed to it. But users grow accustomed to bugs as well! Bugs change behaviors. In fact, in hardware emulation - where it matters, because it is harder to change it - we have emulator people actually emulating bugs - because that is what software expects. Is this reason for us to keep bugs around, because people grew accustomed to it? Hell no. Well, it might be: If we have a proven user base that is big and solid on top of that, it may be fair to say: "Well, this is unfortunate, but this is how it plays". Here, we're discussing - or handwaving as Hannes stated, about whether or not we have *some* users relying on this behavior. We must certainly agree that this is not by far a solid and big usersbase, or anything at the like. Another analogy I believe it is pertinent: I consider this change much closer to icon or button placement in the Desktop market: No one *ever* said a particular button stays at a particular place. Yet it was there for many releases. If you change it, some people will feel it. So what? People change it anyway. Because that is *not* anything set in stone. -- 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