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Date: Thu, 5 Feb 2009 05:17:41 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] Reduce size of swap_cgroup by CSS ID
On Thu, Feb 05, 2009 at 06:59:59PM +0900, KAMEZAWA Hiroyuki wrote:
> !!EXPERIMENTAL!!
>
> against mmotm.
>
> This patch tires to use CSS ID for records in swap_cgroup instead of pointer.
> By this, on 64bit machine, size of swap_cgroup goes down to 2 bytes from 8bytes.
>
> This means, when 2GB of swap is equipped, (assume the page size is 4096bytes)
> From size of swap_cgroup = 2G/4k * 8 = 4Mbytes.
> To size of swap_cgroup = 2G/4k * 2 = 1Mbytes.
> Reduction is large. Of course, there are trade-offs. This CSS ID will add
> overhead to swap-in/swap-out/swap-free.
>
> But in general,
> - swap is a resource which the user tend to avoid use.
> - If swap is never used, swap_cgroup area is not used.
> - Reading traditional manuals, size of swap should be proportional to
> size of memory. Memory size of machine is increasing now. So, reducing
> size of swap_cgroup makes sense.
> Note:
> ID->CSS lookup routine has no locks, it's under RCU-Read-Side.
One question about css_tryget() below.
Thanx, Paul
> This is still under test. Any comments are welcome.
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> include/linux/page_cgroup.h | 9 +++----
> mm/memcontrol.c | 55 +++++++++++++++++++++++++++++++++++---------
> mm/page_cgroup.c | 22 ++++++++---------
> 3 files changed, 59 insertions(+), 27 deletions(-)
>
> Index: mmotm-2.6.29-Feb03/include/linux/page_cgroup.h
> ===================================================================
> --- mmotm-2.6.29-Feb03.orig/include/linux/page_cgroup.h
> +++ mmotm-2.6.29-Feb03/include/linux/page_cgroup.h
> @@ -91,22 +91,21 @@ static inline void page_cgroup_init(void
>
> #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
> #include <linux/swap.h>
> -extern struct mem_cgroup *
> -swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem);
> -extern struct mem_cgroup *lookup_swap_cgroup(swp_entry_t ent);
> +extern unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id);
> +extern unsigned short lookup_swap_cgroup(swp_entry_t ent);
> extern int swap_cgroup_swapon(int type, unsigned long max_pages);
> extern void swap_cgroup_swapoff(int type);
> #else
> #include <linux/swap.h>
>
> static inline
> -struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
> +unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
> {
> return NULL;
> }
>
> static inline
> -struct mem_cgroup *lookup_swap_cgroup(swp_entry_t ent)
> +unsigned short lookup_swap_cgroup(swp_entry_t ent)
> {
> return NULL;
> }
> Index: mmotm-2.6.29-Feb03/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.29-Feb03.orig/mm/memcontrol.c
> +++ mmotm-2.6.29-Feb03/mm/memcontrol.c
> @@ -1001,20 +1001,38 @@ nomem:
> return -ENOMEM;
> }
>
> +/*
> + * A helper function to get mem_cgroup from ID. must be called under
> + * rcu_read_lock(). Because css_tryget() is called under this, css_put
> + * should be called later.
> + */
> +static struct mem_cgroup *mem_cgroup_lookup_get(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 && css_tryget(css))
> + return container_of(css, struct mem_cgroup, css);
So css_tryget(), if successful, prevents the structure referenced by
css from being freed, correct? (If not, the range of the RCU read-side
critical sections surrounding calls to mem_cgroup_lookup_get() must be
extended.)
> + return NULL;
> +}
> +
> static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
> {
> - struct mem_cgroup *mem;
> + unsigned short id;
> + struct mem_cgroup *mem = NULL;
> swp_entry_t ent;
>
> if (!PageSwapCache(page))
> return NULL;
>
> ent.val = page_private(page);
> - mem = lookup_swap_cgroup(ent);
> - if (!mem)
> - return NULL;
> - if (!css_tryget(&mem->css))
> - return NULL;
> + id = lookup_swap_cgroup(ent);
> + rcu_read_lock();
> + mem = mem_cgroup_lookup_get(id);
> + rcu_read_unlock();
> return mem;
> }
>
> @@ -1275,11 +1293,16 @@ int mem_cgroup_cache_charge(struct page
>
> if (do_swap_account && !ret && PageSwapCache(page)) {
> swp_entry_t ent = {.val = page_private(page)};
> + unsigned short id;
> /* avoid double counting */
> - mem = swap_cgroup_record(ent, NULL);
> + id = swap_cgroup_record(ent, 0);
> + rcu_read_lock();
> + mem = mem_cgroup_lookup_get(id);
> + rcu_read_unlock();
> if (mem) {
> res_counter_uncharge(&mem->memsw, PAGE_SIZE);
> mem_cgroup_put(mem);
> + css_put(&mem->css);
> }
> }
> return ret;
> @@ -1345,13 +1368,18 @@ void mem_cgroup_commit_charge_swapin(str
> */
> if (do_swap_account && PageSwapCache(page)) {
> swp_entry_t ent = {.val = page_private(page)};
> + unsigned short id;
> struct mem_cgroup *memcg;
> - memcg = swap_cgroup_record(ent, NULL);
> +
> + id = swap_cgroup_record(ent, 0);
> + rcu_read_lock();
> + memcg = mem_cgroup_lookup_get(id);
> + rcu_read_unlock();
> if (memcg) {
> res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> mem_cgroup_put(memcg);
> + css_put(&memcg->css);
> }
> -
> }
> /* add this page(page_cgroup) to the LRU we want. */
>
> @@ -1472,7 +1500,7 @@ void mem_cgroup_uncharge_swapcache(struc
> MEM_CGROUP_CHARGE_TYPE_SWAPOUT);
> /* record memcg information */
> if (do_swap_account && memcg) {
> - swap_cgroup_record(ent, memcg);
> + swap_cgroup_record(ent, css_id(&memcg->css));
> mem_cgroup_get(memcg);
> }
> if (memcg)
> @@ -1487,14 +1515,19 @@ void mem_cgroup_uncharge_swapcache(struc
> void mem_cgroup_uncharge_swap(swp_entry_t ent)
> {
> struct mem_cgroup *memcg;
> + unsigned short id;
>
> if (!do_swap_account)
> return;
>
> - memcg = swap_cgroup_record(ent, NULL);
> + id = swap_cgroup_record(ent, 0);
> + rcu_read_lock();
> + memcg = mem_cgroup_lookup_get(id);
> + rcu_read_unlock();
> if (memcg) {
> res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> mem_cgroup_put(memcg);
> + css_put(&memcg->css);
> }
> }
> #endif
> Index: mmotm-2.6.29-Feb03/mm/page_cgroup.c
> ===================================================================
> --- mmotm-2.6.29-Feb03.orig/mm/page_cgroup.c
> +++ mmotm-2.6.29-Feb03/mm/page_cgroup.c
> @@ -290,7 +290,7 @@ struct swap_cgroup_ctrl swap_cgroup_ctrl
> * cgroup rather than pointer.
> */
> struct swap_cgroup {
> - struct mem_cgroup *val;
> + unsigned short id;
> };
> #define SC_PER_PAGE (PAGE_SIZE/sizeof(struct swap_cgroup))
> #define SC_POS_MASK (SC_PER_PAGE - 1)
> @@ -345,7 +345,7 @@ not_enough_page:
> * Returns old value at success, NULL at failure.
> * (Of course, old value can be NULL.)
> */
> -struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
> +unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
> {
> int type = swp_type(ent);
> unsigned long offset = swp_offset(ent);
> @@ -354,18 +354,18 @@ struct mem_cgroup *swap_cgroup_record(sw
> struct swap_cgroup_ctrl *ctrl;
> struct page *mappage;
> struct swap_cgroup *sc;
> - struct mem_cgroup *old;
> + unsigned short old;
>
> if (!do_swap_account)
> - return NULL;
> + return 0;
>
> ctrl = &swap_cgroup_ctrl[type];
>
> mappage = ctrl->map[idx];
> sc = page_address(mappage);
> sc += pos;
> - old = sc->val;
> - sc->val = mem;
> + old = sc->id;
> + sc->id = id;
>
> return old;
> }
> @@ -374,9 +374,9 @@ struct mem_cgroup *swap_cgroup_record(sw
> * lookup_swap_cgroup - lookup mem_cgroup tied to swap entry
> * @ent: swap entry to be looked up.
> *
> - * Returns pointer to mem_cgroup at success. NULL at failure.
> + * Returns CSS ID of mem_cgroup at success. NULL at failure.
> */
> -struct mem_cgroup *lookup_swap_cgroup(swp_entry_t ent)
> +unsigned short lookup_swap_cgroup(swp_entry_t ent)
> {
> int type = swp_type(ent);
> unsigned long offset = swp_offset(ent);
> @@ -385,16 +385,16 @@ struct mem_cgroup *lookup_swap_cgroup(sw
> struct swap_cgroup_ctrl *ctrl;
> struct page *mappage;
> struct swap_cgroup *sc;
> - struct mem_cgroup *ret;
> + unsigned short ret;
>
> if (!do_swap_account)
> - return NULL;
> + return 0; /* 0 is invalid ID */
>
> ctrl = &swap_cgroup_ctrl[type];
> mappage = ctrl->map[idx];
> sc = page_address(mappage);
> sc += pos;
> - ret = sc->val;
> + ret = sc->id;
> return ret;
> }
>
>
> --
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