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Date: Fri, 6 Aug 2010 13:10:53 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Greg Thelen <gthelen@...gle.com>
Cc: linux-mm@...ck.org,
"balbir\@linux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
"nishimura\@mxp.nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
vgoyal@...hat.com, m-ikeda@...jp.nec.com,
"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4 -mm][memcg] quick ID lookup in memcg
On Thu, 05 Aug 2010 21:12:50 -0700
Greg Thelen <gthelen@...gle.com> wrote:
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
>
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> >
> > Now, memory cgroup has an ID per cgroup and make use of it at
> > - hierarchy walk,
> > - swap recording.
> >
> > This patch is for making more use of it. The final purpose is
> > to replace page_cgroup->mem_cgroup's pointer to an unsigned short.
> >
> > This patch caches a pointer of memcg in an array. By this, we
> > don't have to call css_lookup() which requires radix-hash walk.
> > This saves some amount of memory footprint at lookup memcg via id.
> >
> > Changelog: 20100804
> > - fixed description in init/Kconfig
> >
> > Changelog: 20100730
> > - fixed rcu_read_unlock() placement.
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> > init/Kconfig | 10 ++++++++++
> > mm/memcontrol.c | 48 ++++++++++++++++++++++++++++++++++--------------
> > 2 files changed, 44 insertions(+), 14 deletions(-)
> >
> > Index: mmotm-0727/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0727.orig/mm/memcontrol.c
> > +++ mmotm-0727/mm/memcontrol.c
> > @@ -292,6 +292,30 @@ static bool move_file(void)
> > &mc.to->move_charge_at_immigrate);
> > }
> >
> > +/* 0 is unused */
> > +static atomic_t mem_cgroup_num;
> > +#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
> > +static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
> > +
> > +static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +{
> > + /*
> > + * This array is set to NULL when mem_cgroup is freed.
> > + * IOW, there are no more references && rcu_synchronized().
> > + * This lookup-caching is safe.
> > + */
> > + if (unlikely(!mem_cgroups[id])) {
> > + struct cgroup_subsys_state *css;
> > +
> > + rcu_read_lock();
> > + css = css_lookup(&mem_cgroup_subsys, id);
> > + rcu_read_unlock();
> > + if (!css)
> > + return NULL;
> > + mem_cgroups[id] = container_of(css, struct mem_cgroup, css);
> > + }
> > + return mem_cgroups[id];
> > +}
>
> I am worried that id may be larger than CONFIG_MEM_CGROUP_MAX_GROUPS and
> cause an illegal array index. I see that
> mem_cgroup_uncharge_swapcache() uses css_id() to compute 'id'.
> mem_cgroup_num ensures that there are never more than
> CONFIG_MEM_CGROUP_MAX_GROUPS memcg active. But do we have guarantee
> that the that all of the css_id of each active memcg are less than
> NR_MEMCG_GROUPS?
>
Yes. kernel/cgroup.c's ID assign routine use the smallest number, always.
> > /*
> > * Maximum loops in mem_cgroup_hierarchical_reclaim(), used for soft
> > * limit reclaim to prevent infinite loops, if they ever occur.
> > @@ -1824,18 +1848,7 @@ static void mem_cgroup_cancel_charge(str
> > * it's concern. (dropping refcnt from swap can be called against removed
> > * memcg.)
> > */
> > -static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
> > -{
> > - struct cgroup_subsys_state *css;
> >
> > - /* ID 0 is unused ID */
> > - if (!id)
> > - return NULL;
> > - css = css_lookup(&mem_cgroup_subsys, id);
> > - if (!css)
> > - return NULL;
> > - return container_of(css, struct mem_cgroup, css);
> > -}
> >
> > struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
> > {
> > @@ -1856,7 +1869,7 @@ struct mem_cgroup *try_get_mem_cgroup_fr
> > ent.val = page_private(page);
> > id = lookup_swap_cgroup(ent);
> > rcu_read_lock();
> > - mem = mem_cgroup_lookup(id);
> > + mem = id_to_memcg(id);
> > if (mem && !css_tryget(&mem->css))
> > mem = NULL;
> > rcu_read_unlock();
> > @@ -2208,7 +2221,7 @@ __mem_cgroup_commit_charge_swapin(struct
> >
> > id = swap_cgroup_record(ent, 0);
> > rcu_read_lock();
> > - memcg = mem_cgroup_lookup(id);
> > + memcg = id_to_memcg(id);
> > if (memcg) {
> > /*
> > * This recorded memcg can be obsolete one. So, avoid
> > @@ -2472,7 +2485,7 @@ void mem_cgroup_uncharge_swap(swp_entry_
> >
> > id = swap_cgroup_record(ent, 0);
> > rcu_read_lock();
> > - memcg = mem_cgroup_lookup(id);
> > + memcg = id_to_memcg(id);
> > if (memcg) {
> > /*
> > * We uncharge this because swap is freed.
> > @@ -3988,6 +4001,9 @@ static struct mem_cgroup *mem_cgroup_all
> > struct mem_cgroup *mem;
> > int size = sizeof(struct mem_cgroup);
> >
> > + if (atomic_read(&mem_cgroup_num) == NR_MEMCG_GROUPS)
> > + return NULL;
> > +
>
> I think that multiple tasks to be simultaneously running
> mem_cgroup_create(). Therefore more than NR_MEMCG_GROUPS memcg may be
> created.
>
No. cgroup_mutex() is held.
Thanks,
-Kame
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