From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id DD09C6B0005 for ; Tue, 13 Mar 2018 13:55:23 -0400 (EDT) Received: by mail-wr0-f197.google.com with SMTP id o23so381147wrc.9 for ; Tue, 13 Mar 2018 10:55:23 -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 j10sor231962wrg.72.2018.03.13.10.55.21 for (Google Transport Security); Tue, 13 Mar 2018 10:55:22 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20180313134902.GW12772@dhcp22.suse.cz> References: <20180221223757.127213-1-shakeelb@google.com> <20180221223757.127213-2-shakeelb@google.com> <20180313134902.GW12772@dhcp22.suse.cz> From: Shakeel Butt Date: Tue, 13 Mar 2018 10:55:18 -0700 Message-ID: Subject: Re: [PATCH v3 1/2] mm: memcg: remote memcg charging for kmem allocations Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Jan Kara , Amir Goldstein , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Greg Thelen , Johannes Weiner , Vladimir Davydov , Mel Gorman , Vlastimil Babka , linux-fsdevel , Linux MM , Cgroups , LKML On Tue, Mar 13, 2018 at 6:49 AM, Michal Hocko wrote: > On Wed 21-02-18 14:37:56, Shakeel Butt wrote: > [...] >> +#ifdef CONFIG_MEMCG >> +static inline struct mem_cgroup *memalloc_memcg_save(struct mem_cgroup *memcg) >> +{ >> + struct mem_cgroup *old_memcg = current->target_memcg; >> + current->target_memcg = memcg; >> + return old_memcg; >> +} > > So you are relying that the caller will handle the reference counting > properly? I do not think this is a good idea. For the fsnotify use-case, this assumption makes sense as fsnotify has an abstraction of fsnotify_group which is created by the person/process interested in the events and thus can be used to hold the reference to the person/process's memcg. Another use-case I have in mind is the filesystem mount. Basically attaching a mount with a memcg and thus all user pages and kmem allocations (inodes, dentries) for that mount will be charged to the attached memcg. In this use-case the super_block is the perfect structure to hold the reference to the memcg. If in future we find a use-case where this assumption does not make sense we can evolve the API and since this is kernel internal API, it should not be hard to evolve. > Also do we need some kind > of debugging facility to detect unbalanced save/restore scopes? > I am not sure, I didn't find other similar patterns (like PF_MEMALLOC) having debugging facility. Maybe we can add such debugging facility when we find more users other than kmalloc & kmem_cache_alloc. Vmalloc may be one but I could not think of a use-case for vmalloc for remote charging, so, no need to add more code at this time. > [...] >> @@ -2260,7 +2269,10 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep) >> if (current->memcg_kmem_skip_account) >> return cachep; >> >> - memcg = get_mem_cgroup_from_mm(current->mm); >> + if (current->target_memcg) >> + memcg = get_mem_cgroup(current->target_memcg); >> + if (!memcg) >> + memcg = get_mem_cgroup_from_mm(current->mm); >> kmemcg_id = READ_ONCE(memcg->kmemcg_id); >> if (kmemcg_id < 0) >> goto out; > > You are also adding one branch for _each_ charge path even though the > usecase is rather limited. > I understand the concern but the charging path, IMO, is much complex than just one or couple of additional branches. I can run a simple microbenchmark to see if there is anything noticeable here. > I will have to think about this approach more. It is clearly less code > than your previous attempt but I cannot say I would be really impressed. > Thanks for your time.