From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f71.google.com (mail-lf0-f71.google.com [209.85.215.71]) by kanga.kvack.org (Postfix) with ESMTP id 948066B0003 for ; Sat, 28 Apr 2018 11:08:34 -0400 (EDT) Received: by mail-lf0-f71.google.com with SMTP id l14-v6so1529991lfc.16 for ; Sat, 28 Apr 2018 08:08:34 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id n7-v6sor833983ljh.47.2018.04.28.08.08.31 for (Google Transport Security); Sat, 28 Apr 2018 08:08:32 -0700 (PDT) Date: Sat, 28 Apr 2018 18:08:27 +0300 From: Vladimir Davydov Subject: Re: [PATCH v2 04/12] mm: Assign memcg-aware shrinkers bitmap to memcg Message-ID: <20180428150827.b2bh7hhma7pp4av5@esperanza> References: <152397794111.3456.1281420602140818725.stgit@localhost.localdomain> <152399121146.3456.5459546288565589098.stgit@localhost.localdomain> <20180422175900.dsjmm7gt2nsqj3er@esperanza> <14ebcccf-3ea8-59f4-d7ea-793aaba632c0@virtuozzo.com> <20180424112844.626madzs4cwoz5gh@esperanza> <7bf5372d-7d9d-abee-27dd-5044da5ec489@virtuozzo.com> <20180424121516.ihn6lewpidc34ayl@esperanza> <402281e6-6fea-3541-1435-a2f81e705e2b@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <402281e6-6fea-3541-1435-a2f81e705e2b@virtuozzo.com> Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai Cc: akpm@linux-foundation.org, shakeelb@google.com, viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org, tglx@linutronix.de, pombredanne@nexb.com, stummala@codeaurora.org, gregkh@linuxfoundation.org, sfr@canb.auug.org.au, guro@fb.com, mka@chromium.org, penguin-kernel@I-love.SAKURA.ne.jp, chris@chris-wilson.co.uk, longman@redhat.com, minchan@kernel.org, hillf.zj@alibaba-inc.com, ying.huang@intel.com, mgorman@techsingularity.net, jbacik@fb.com, linux@roeck-us.net, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@infradead.org, lirongqing@baidu.com, aryabinin@virtuozzo.com On Tue, Apr 24, 2018 at 03:24:53PM +0300, Kirill Tkhai wrote: > >>>>>> +int expand_shrinker_maps(int old_nr, int nr) > >>>>>> +{ > >>>>>> + int id, size, old_size, node, ret; > >>>>>> + struct mem_cgroup *memcg; > >>>>>> + > >>>>>> + old_size = old_nr / BITS_PER_BYTE; > >>>>>> + size = nr / BITS_PER_BYTE; > >>>>>> + > >>>>>> + down_write(&shrinkers_max_nr_rwsem); > >>>>>> + for_each_node(node) { > >>>>> > >>>>> Iterating over cgroups first, numa nodes second seems like a better idea > >>>>> to me. I think you should fold for_each_node in memcg_expand_maps. > >>>>> > >>>>>> + idr_for_each_entry(&mem_cgroup_idr, memcg, id) { > >>>>> > >>>>> Iterating over mem_cgroup_idr looks strange. Why don't you use > >>>>> for_each_mem_cgroup? > >>>> > >>>> We want to allocate shrinkers maps in mem_cgroup_css_alloc(), since > >>>> mem_cgroup_css_online() mustn't fail (it's a requirement of currently > >>>> existing design of memcg_cgroup::id). > >>>> > >>>> A new memcg is added to parent's list between two of these calls: > >>>> > >>>> css_create() > >>>> ss->css_alloc() > >>>> list_add_tail_rcu(&css->sibling, &parent_css->children) > >>>> ss->css_online() > >>>> > >>>> for_each_mem_cgroup() does not see allocated, but not linked children. > >>> > >>> Why don't we move shrinker map allocation to css_online then? > >> > >> Because the design of memcg_cgroup::id prohibits mem_cgroup_css_online() to fail. > >> This function can't fail. > > > > I fail to understand why it is so. Could you please elaborate? > > mem_cgroup::id is freed not in mem_cgroup_css_free(), but earlier. It's freed > between mem_cgroup_css_offline() and mem_cgroup_free(), after the last reference > is put. > > In case of sometimes we want to free it in mem_cgroup_css_free(), this will > introduce assymmetric in the logic, which makes it more difficult. There is > already a bug, which I fixed in > > "memcg: remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure" > > new change will make this code completely not-modular and unreadable. How is that? AFAIU all we need to do to handle css_online failure properly is call mem_cgroup_id_remove() from mem_cgroup_css_free(). That's it, as mem_cgroup_id_remove() is already safe to call more than once for the same cgroup - the first call will free the id while all subsequent calls will do nothing. > > >> > >> I don't think it will be good to dive into reworking of this stuff for this patchset, > >> which is really already big. Also, it will be assymmetric to allocate one part of > >> data in css_alloc(), while another data in css_free(). This breaks cgroup design, > >> which specially introduces this two function to differ allocation and onlining. > >> Also, I've just move the allocation to alloc_mem_cgroup_per_node_info() like it was > >> suggested in comments to v1... > > > > Yeah, but (ab)using mem_cgroup_idr for iterating over all allocated > > memory cgroups looks rather dubious to me... > > But we have to iterate over all allocated memory cgroups in any way, > as all of them must have expanded maps. What is the problem? > It's rather simple method, and it faster then for_each_mem_cgroup() > cycle, since it does not have to play with get and put of refcounters. I don't like this, because mem_cgroup_idr was initially introduced to avoid depletion of css ids by offline cgroups. We could fix that problem by extending swap_cgroup to UINT_MAX, in which case mem_cgroup_idr wouldn't be needed at all. Reusing mem_cgroup_idr for iterating over allocated cgroups deprives us of the ability to reconsider that design decision in future, neither does it look natural IMO. Besides, in order to use mem_cgroup_idr for your purpose, you have to reshuffle the code of mem_cgroup_css_alloc in a rather contrived way IMO. I agree that allocating parts of struct mem_cgroup in css_online may look dubious, but IMHO it's better than inventing a new way to iterate over cgroups instead of using the iterator provided by cgroup core. May be, you should ask Tejun which way he thinks is better. Thanks, Vladimir