From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ea0-f171.google.com (mail-ea0-f171.google.com [209.85.215.171]) by kanga.kvack.org (Postfix) with ESMTP id 198D86B0035 for ; Tue, 4 Feb 2014 10:55:19 -0500 (EST) Received: by mail-ea0-f171.google.com with SMTP id f15so4479928eak.16 for ; Tue, 04 Feb 2014 07:55:18 -0800 (PST) Received: from zene.cmpxchg.org (zene.cmpxchg.org. [2a01:238:4224:fa00:ca1f:9ef3:caee:a2bd]) by mx.google.com with ESMTPS id x3si43100788eea.55.2014.02.04.07.55.17 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 04 Feb 2014 07:55:17 -0800 (PST) Date: Tue, 4 Feb 2014 10:55:08 -0500 From: Johannes Weiner Subject: Re: [PATCH -v2 1/6] memcg: do not replicate try_get_mem_cgroup_from_mm in __mem_cgroup_try_charge Message-ID: <20140204155508.GM6963@cmpxchg.org> References: <1391520540-17436-1-git-send-email-mhocko@suse.cz> <1391520540-17436-2-git-send-email-mhocko@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1391520540-17436-2-git-send-email-mhocko@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Andrew Morton , KAMEZAWA Hiroyuki , KOSAKI Motohiro , LKML , linux-mm@kvack.org On Tue, Feb 04, 2014 at 02:28:55PM +0100, Michal Hocko wrote: > Johannes Weiner has pointed out that __mem_cgroup_try_charge duplicates > try_get_mem_cgroup_from_mm for charges which came without a memcg. The > only reason seems to be a tiny optimization when css_tryget is not > called if the charge can be consumed from the stock. Nevertheless > css_tryget is very cheap since it has been reworked to use per-cpu > counting so this optimization doesn't give us anything these days. > > So let's drop the code duplication so that the code is more readable. > While we are at it also remove a very confusing comment in > try_get_mem_cgroup_from_mm. > > Signed-off-by: Michal Hocko > --- > mm/memcontrol.c | 49 ++++++++----------------------------------------- > 1 file changed, 8 insertions(+), 41 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 53385cd4e6f0..042e4ff36c05 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1081,11 +1081,7 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm) > > if (!mm) > return NULL; While you're at it, this check also seems unnecessary. > - /* > - * Because we have no locks, mm->owner's may be being moved to other > - * cgroup. We use css_tryget() here even if this looks > - * pessimistic (rather than adding locks here). > - */ > + > rcu_read_lock(); > do { > memcg = mem_cgroup_from_task(rcu_dereference(mm->owner)); > @@ -2759,45 +2755,15 @@ again: > goto done; > css_get(&memcg->css); > } else { > - struct task_struct *p; > - > - rcu_read_lock(); > - p = rcu_dereference(mm->owner); > - /* > - * Because we don't have task_lock(), "p" can exit. > - * In that case, "memcg" can point to root or p can be NULL with > - * race with swapoff. Then, we have small risk of mis-accouning. > - * But such kind of mis-account by race always happens because > - * we don't have cgroup_mutex(). It's overkill and we allo that > - * small race, here. > - * (*) swapoff at el will charge against mm-struct not against > - * task-struct. So, mm->owner can be NULL. > - */ > - memcg = mem_cgroup_from_task(p); > - if (!memcg) > + memcg = try_get_mem_cgroup_from_mm(mm); > + if (!memcg) { > memcg = root_mem_cgroup; > - if (mem_cgroup_is_root(memcg)) { > - rcu_read_unlock(); > - goto done; > - } > - if (consume_stock(memcg, nr_pages)) { > - /* > - * It seems dagerous to access memcg without css_get(). > - * But considering how consume_stok works, it's not > - * necessary. If consume_stock success, some charges > - * from this memcg are cached on this cpu. So, we > - * don't need to call css_get()/css_tryget() before > - * calling consume_stock(). > - */ > - rcu_read_unlock(); > goto done; > } > - /* after here, we may be blocked. we need to get refcnt */ > - if (!css_tryget(&memcg->css)) { > - rcu_read_unlock(); > - goto again; > - } > - rcu_read_unlock(); > + if (mem_cgroup_is_root(memcg)) > + goto done_put; > + if (consume_stock(memcg, nr_pages)) > + goto done_put; These two are actually the same in the if (*ptr) branch. -- 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