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From: Greg Thelen on 6 Aug 2010 00:20
KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> writes:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.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(a)jp.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?

> /*
> * 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.

> /* Can be very big if MAX_NUMNODES is very big */
> if (size < PAGE_SIZE)
> mem = kmalloc(size, GFP_KERNEL);
> @@ -4025,7 +4041,10 @@ static void __mem_cgroup_free(struct mem
> int node;
>
> mem_cgroup_remove_from_trees(mem);
> + /* No more lookup against this ID */
> + mem_cgroups[css_id(&mem->css)] = NULL;
> free_css_id(&mem_cgroup_subsys, &mem->css);
> + atomic_dec(&mem_cgroup_num);
>
> for_each_node_state(node, N_POSSIBLE)
> free_mem_cgroup_per_zone_info(mem, node);
> @@ -4162,6 +4181,7 @@ mem_cgroup_create(struct cgroup_subsys *
> atomic_set(&mem->refcnt, 1);
> mem->move_charge_at_immigrate = 0;
> mutex_init(&mem->thresholds_lock);
> + atomic_inc(&mem_cgroup_num);
> return &mem->css;
> free_out:
> __mem_cgroup_free(mem);
> Index: mmotm-0727/init/Kconfig
> ===================================================================
> --- mmotm-0727.orig/init/Kconfig
> +++ mmotm-0727/init/Kconfig
> @@ -594,6 +594,16 @@ config CGROUP_MEM_RES_CTLR_SWAP
> Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page
> size is 4096bytes, 512k per 1Gbytes of swap.
>
> +config MEM_CGROUP_MAX_GROUPS
> + int "Maximum number of memory cgroups on a system"
> + range 1 65535
> + default 8192 if 64BIT
> + default 2048 if 32BIT
> + help
> + Memory cgroup has limitation of the number of groups created.
> + Please select your favorite value. The more you allow, the more
> + memory(a pointer per group) will be consumed.
> +
> menuconfig CGROUP_SCHED
> bool "Group CPU scheduler"
> depends on EXPERIMENTAL && CGROUPS
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From: KAMEZAWA Hiroyuki on 6 Aug 2010 00:20
On Thu, 05 Aug 2010 21:12:50 -0700
Greg Thelen <gthelen(a)google.com> wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> writes:
>
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.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(a)jp.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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From: Greg Thelen on 6 Aug 2010 00:40
On Thu, Aug 5, 2010 at 9:10 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu(a)jp.fujitsu.com> wrote:
> On Thu, 05 Aug 2010 21:12:50 -0700
> Greg Thelen <gthelen(a)google.com> wrote:
>
>> KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> writes:
>>
>> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.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(a)jp.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
>
>

I see that now. Thank you clarification. I am doing some testing on
the patches now.
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