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SubjectRe: [PATCH 1/4 -mm][memcg] quick ID lookup in memcg
Date
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@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@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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