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SubjectRe: [PATCH 2/5] memcg: quick memcg lookup array
On Tue, 31 Aug 2010 12:44:14 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-08-25 17:07:41]:
>
> > 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: 20100825
> > - applied comments.
> >
> > Changelog: 20100811
> > - adjusted onto mmotm-2010-08-11
> > - fixed RCU related parts.
> > - use attach_id() callback.
> >
> > 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 | 75 +++++++++++++++++++++++++++++++++++++++++---------------
> > 2 files changed, 65 insertions(+), 20 deletions(-)
> >
> > Index: mmotm-0811/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0811.orig/mm/memcontrol.c
> > +++ mmotm-0811/mm/memcontrol.c
> > @@ -195,6 +195,7 @@ static void mem_cgroup_oom_notify(struct
> > */
> > struct mem_cgroup {
> > struct cgroup_subsys_state css;
> > + int valid; /* for checking validness under RCU access.*/
> > /*
> > * the counter to account for memory usage
> > */
> > @@ -294,6 +295,29 @@ 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;
> > +
> > +/* Must be called under rcu_read_lock */
> > +static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +{
> > + struct mem_cgroup *mem;
> > + /* see mem_cgroup_free() */
> > + mem = rcu_dereference_check(mem_cgroups[id], rcu_read_lock_held());
> > + if (likely(mem && mem->valid))
> > + return mem;
> > + return NULL;
> > +}
> > +
> > +static void register_memcg_id(struct mem_cgroup *mem)
> > +{
> > + int id = css_id(&mem->css);
> > + rcu_assign_pointer(mem_cgroups[id], mem);
> > + VM_BUG_ON(!mem->valid);
> > +}
> > +
> > /*
> > * Maximum loops in mem_cgroup_hierarchical_reclaim(), used for soft
> > * limit reclaim to prevent infinite loops, if they ever occur.
> > @@ -1847,18 +1871,7 @@ 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)
> > {
> > @@ -1879,7 +1892,7 @@ 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();
> > @@ -2231,7 +2244,7 @@ __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
> > @@ -2240,9 +2253,10 @@ __mem_cgroup_commit_charge_swapin(struct
> > if (!mem_cgroup_is_root(memcg))
> > res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> > mem_cgroup_swap_statistics(memcg, false);
> > + rcu_read_unlock();
> > mem_cgroup_put(memcg);
> > - }
> > - rcu_read_unlock();
> > + } else
> > + rcu_read_unlock();
> > }
> > /*
> > * At swapin, we may charge account against cgroup which has no tasks.
> > @@ -2495,7 +2509,7 @@ 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.
> > @@ -2504,9 +2518,10 @@ void mem_cgroup_uncharge_swap(swp_entry_
> > if (!mem_cgroup_is_root(memcg))
> > res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> > mem_cgroup_swap_statistics(memcg, false);
> > + rcu_read_unlock();
> > mem_cgroup_put(memcg);
> > - }
> > - rcu_read_unlock();
> > + } else
> > + rcu_read_unlock();
> > }
> >
> > /**
> > @@ -4010,6 +4025,9 @@ 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;
> > +
> > /* Can be very big if MAX_NUMNODES is very big */
> > if (size < PAGE_SIZE)
> > mem = kmalloc(size, GFP_KERNEL);
> > @@ -4020,6 +4038,7 @@ static struct mem_cgroup *mem_cgroup_all
> > return NULL;
> >
> > memset(mem, 0, size);
> > + mem->valid = 1;
> > mem->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
> > if (!mem->stat) {
> > if (size < PAGE_SIZE)
> > @@ -4049,6 +4068,7 @@ static void __mem_cgroup_free(struct mem
> > mem_cgroup_remove_from_trees(mem);
> > 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);
> >
> > @@ -4059,6 +4079,19 @@ static void __mem_cgroup_free(struct mem
> > vfree(mem);
> > }
> >
> > +static void mem_cgroup_free(struct mem_cgroup *mem)
> > +{
> > + /* No more lookup */
> > + mem->valid = 0;
> > + rcu_assign_pointer(mem_cgroups[css_id(&mem->css)], NULL);
> > + /*
> > + * Because we call vfree() etc...use synchronize_rcu() rather than
> > + * call_rcu();
> > + */
> > + synchronize_rcu();
> > + __mem_cgroup_free(mem);
> > +}
> > +
> > static void mem_cgroup_get(struct mem_cgroup *mem)
> > {
> > atomic_inc(&mem->refcnt);
> > @@ -4068,7 +4101,7 @@ static void __mem_cgroup_put(struct mem_
> > {
> > if (atomic_sub_and_test(count, &mem->refcnt)) {
> > struct mem_cgroup *parent = parent_mem_cgroup(mem);
> > - __mem_cgroup_free(mem);
> > + mem_cgroup_free(mem);
> > if (parent)
> > mem_cgroup_put(parent);
> > }
> > @@ -4189,9 +4222,11 @@ 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);
> > + register_memcg_id(mem);
> > return &mem->css;
> > free_out:
> > - __mem_cgroup_free(mem);
> > + mem_cgroup_free(mem);
> > root_mem_cgroup = NULL;
> > return ERR_PTR(error);
> > }
> > Index: mmotm-0811/init/Kconfig
> > ===================================================================
> > --- mmotm-0811.orig/init/Kconfig
> > +++ mmotm-0811/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.
> > +
>
> Looks good, quick thought - should we expost memcg->id to user space
> through a config file? I don't see any reason at this point, unless we
> do it for all controllers.
>

I wonder....showing whether 2 interfaces as "path name" and "id" to users is a
good thing or not. Yes, it's convenient to developpers as me and you, others,
but I don't think it's useful to users, novice people.

I'd like to avoid showing show memcg->id to users as much as possible. I don't
want to say but memcg is enough complicated.


Thanks,
-Kame




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