From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751095Ab1FHECq (ORCPT ); Wed, 8 Jun 2011 00:02:46 -0400 Received: from smtp-out.google.com ([74.125.121.67]:19540 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750846Ab1FHECp convert rfc822-to-8bit (ORCPT ); Wed, 8 Jun 2011 00:02:45 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-type:content-transfer-encoding; b=PGL4tJd57/+kK+K9hh3Fqjke5MWmnWk1aHW7JP9YR/B1HkMNHy0bv5vxCzmskAHamA Z+T/yD+Zznc1+JVKqEkA== MIME-Version: 1.0 In-Reply-To: <20110608091815.fdef924d.kamezawa.hiroyu@jp.fujitsu.com> References: <1307117538-14317-1-git-send-email-gthelen@google.com> <1307117538-14317-12-git-send-email-gthelen@google.com> <20110607193835.GD26965@redhat.com> <20110607210540.GB30919@redhat.com> <20110608091815.fdef924d.kamezawa.hiroyu@jp.fujitsu.com> From: Greg Thelen Date: Tue, 7 Jun 2011 21:02:21 -0700 Message-ID: Subject: Re: [PATCH v8 11/12] writeback: make background writeback cgroup aware To: KAMEZAWA Hiroyuki Cc: Vivek Goyal , Andrew Morton , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , containers@lists.osdl.org, linux-fsdevel@vger.kernel.org, Andrea Righi , Balbir Singh , Daisuke Nishimura , Minchan Kim , Johannes Weiner , Ciju Rajan K , David Rientjes , Wu Fengguang , Dave Chinner Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 7, 2011 at 5:18 PM, KAMEZAWA Hiroyuki wrote: > On Tue, 7 Jun 2011 17:05:40 -0400 > Vivek Goyal wrote: > >> On Tue, Jun 07, 2011 at 01:43:08PM -0700, Greg Thelen wrote: >> > Vivek Goyal writes: >> > >> > > On Fri, Jun 03, 2011 at 09:12:17AM -0700, Greg Thelen wrote: >> > >> When the system is under background dirty memory threshold but a cgroup >> > >> is over its background dirty memory threshold, then only writeback >> > >> inodes associated with the over-limit cgroup(s). >> > >> >> > > >> > > [..] >> > >> -static inline bool over_bground_thresh(void) >> > >> +static inline bool over_bground_thresh(struct bdi_writeback *wb, >> > >> +                                       struct writeback_control *wbc) >> > >>  { >> > >>          unsigned long background_thresh, dirty_thresh; >> > >> >> > >>          global_dirty_limits(&background_thresh, &dirty_thresh); >> > >> >> > >> -        return (global_page_state(NR_FILE_DIRTY) + >> > >> -                global_page_state(NR_UNSTABLE_NFS) > background_thresh); >> > >> +        if (global_page_state(NR_FILE_DIRTY) + >> > >> +            global_page_state(NR_UNSTABLE_NFS) > background_thresh) { >> > >> +                wbc->for_cgroup = 0; >> > >> +                return true; >> > >> +        } >> > >> + >> > >> +        wbc->for_cgroup = 1; >> > >> +        wbc->shared_inodes = 1; >> > >> +        return mem_cgroups_over_bground_dirty_thresh(); >> > >>  } >> > > >> > > Hi Greg, >> > > >> > > So all the logic of writeout from mem cgroup works only if system is >> > > below background limit. The moment we cross background limit, looks >> > > like we will fall back to existing way of writting inodes? >> > >> > Correct.  If the system is over its background limit then the previous >> > cgroup-unaware background writeback occurs.  I think of the system >> > limits as those of the root cgroup.  If the system is over the global >> > limit than all cgroups are eligible for writeback.  In this situation >> > the current code does not distinguish between cgroups over or under >> > their dirty background limit. >> > >> > Vivek Goyal writes: >> > > If yes, then from design point of view it is little odd that as long >> > > as we are below background limit, we share the bdi between different >> > > cgroups. The moment we are above background limit, we fall back to >> > > algorithm of sharing the disk among individual inodes and forget >> > > about memory cgroups. Kind of awkward. >> > > >> > > This kind of cgroup writeback I think will atleast not solve the problem >> > > for CFQ IO controller, as we fall back to old ways of writting back inodes >> > > the moment we cross dirty ratio. >> > >> > It might make more sense to reverse the order of the checks in the >> > proposed over_bground_thresh(): the new version would first check if any >> > memcg are over limit; assuming none are over limit, then check global >> > limits.  Assuming that the system is over its background limit and some >> > cgroups are also over their limits, then the over limit cgroups would >> > first be written possibly getting the system below its limit.  Does this >> > address your concern? >> >> Do you treat root group also as any other cgroup? If no, then above logic >> can lead to issue of starvation of root group inode. Or unfair writeback. >> So I guess it will be important to treat root group same as other groups. >> > > As far as I can say, you should not place programs onto ROOT cgroups if you need > performance isolation. Agreed. > From the code, I think if the system hits dirty_ratio, "1" bit of bitmap should be > set and background writeback can work for ROOT cgroup seamlessly. > > Thanks, > -Kame Not quite. The proposed patches do not set the "1" bit (css_id of root is 1). mem_cgroup_balance_dirty_pages() (from patch 10/12) introduces the following balancing loop: + /* balance entire ancestry of current's mem. */ + for (; mem_cgroup_has_dirty_limit(mem); mem = parent_mem_cgroup(mem)) { The loop terminates when mem_cgroup_has_dirty_limit() is called for the root cgroup. The bitmap is set in the body of the loop. So the root cgroup's bit (bit 1) will never be set in the bitmap. However, I think the effect is the same. The proposed changes in this patch (11/12) have background writeback first checking if the system is over limit and if yes, then b_dirty inodes from any cgroup written. This means that a small system background limit with an over-{fg or bg}-limit cgroup could cause other cgroups that are not over their limit to have their inodes written back. In an system-over-limit situation normal system-wide bdi writeback is used (writing inodes in b_dirty order). For those who want isolation, a simple rule to avoid this is to ensure that that sum of all cgroup background_limits is less than the system background limit.