From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx164.postini.com [74.125.245.164]) by kanga.kvack.org (Postfix) with SMTP id A5FAD6B009B for ; Fri, 5 Apr 2013 06:27:36 -0400 (EDT) Message-ID: <515EA73C.8050602@parallels.com> Date: Fri, 5 Apr 2013 14:28:12 +0400 From: Glauber Costa MIME-Version: 1.0 Subject: Re: [RFC][PATCH 2/7] memcg: don't use mem_cgroup_get() when creating a kmemcg cache References: <515BF233.6070308@huawei.com> <515BF275.5080408@huawei.com> <20130403153133.GM16471@dhcp22.suse.cz> In-Reply-To: <20130403153133.GM16471@dhcp22.suse.cz> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Li Zefan , linux-mm@kvack.org, LKML , Cgroups , Tejun Heo , KAMEZAWA Hiroyuki , Johannes Weiner On 04/03/2013 07:31 PM, Michal Hocko wrote: > On Wed 03-04-13 17:12:21, Li Zefan wrote: >> Use css_get()/css_put() instead of mem_cgroup_get()/mem_cgroup_put(). >> >> Signed-off-by: Li Zefan >> --- >> mm/memcontrol.c | 10 +++++----- >> 1 file changed, 5 insertions(+), 5 deletions(-) >> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 43ca91d..dafacb8 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -3191,7 +3191,7 @@ void memcg_release_cache(struct kmem_cache *s) >> list_del(&s->memcg_params->list); >> mutex_unlock(&memcg->slab_caches_mutex); >> >> - mem_cgroup_put(memcg); >> + css_put(&memcg->css); >> out: >> kfree(s->memcg_params); >> } >> @@ -3350,16 +3350,18 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg, >> >> mutex_lock(&memcg_cache_mutex); >> new_cachep = cachep->memcg_params->memcg_caches[idx]; >> - if (new_cachep) >> + if (new_cachep) { >> + css_put(&memcg->css); >> goto out; >> + } >> >> new_cachep = kmem_cache_dup(memcg, cachep); >> if (new_cachep == NULL) { >> new_cachep = cachep; >> + css_put(&memcg->css); >> goto out; >> } >> >> - mem_cgroup_get(memcg); >> atomic_set(&new_cachep->memcg_params->nr_pages , 0); >> >> cachep->memcg_params->memcg_caches[idx] = new_cachep; >> @@ -3449,8 +3451,6 @@ static void memcg_create_cache_work_func(struct work_struct *w) >> >> cw = container_of(w, struct create_work, work); >> memcg_create_kmem_cache(cw->memcg, cw->cachep); >> - /* Drop the reference gotten when we enqueued. */ >> - css_put(&cw->memcg->css); >> kfree(cw); >> } > > You are putting references but I do not see any single css_{try}get > here. /me puzzled. > There are two things being done in this code: First, we acquired a css_ref to make sure that the underlying cgroup would not go away. That is a short lived reference, and it is put as soon as the cache is created. At this point, we acquire a long-lived per-cache memcg reference count to guarantee that the memcg will still be alive. so it is: enqueue: css_get create : memcg_get, css_put destroy: css_put If I understand Li's patch correctly, he is not touching the first css_get, only turning that into the long lived reference (which was not possible before, since that would prevent rmdir). Then he only needs to get rid of the memcg_get, change the memcg_put to css_put, and get rid of the now extra css_put. He is issuing extra css_puts in memcg_create_kmem_cache, but only in failure paths. So the code reads as: * css_get on enqueue (already done, so not shown in patch) * if it fails, css_put * if it succeeds, don't do anything. This is already the long-lived reference count. put it at release time. The code looks correct, and of course, extremely simpler due to the use of a single reference. Li, am I right in my understanding that this is your intention? -- 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