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Message-Id: <20100823175015.8d834645.nishimura@mxp.nes.nec.co.jp>
Date: Mon, 23 Aug 2010 17:50:15 +0900
From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: linux-mm@...ck.org,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
gthelen@...gle.com, m-ikeda@...jp.nec.com,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
kamezawa.hiroyuki@...il.com,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 4/5] memcg: lockless update of file_mapped
This patch looks good to me, but I have one question.
Why do we need to acquire sc.lock inside mem_cgroup_(start|end)_move() ?
These functions doesn't access mc.*.
Thanks,
Daisuke Nishimura.
On Fri, 20 Aug 2010 19:02:56 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> No changes from v4.
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> At accounting file events per memory cgroup, we need to find memory cgroup
> via page_cgroup->mem_cgroup. Now, we use lock_page_cgroup().
>
> But, considering the context which page-cgroup for files are accessed,
> we can use alternative light-weight mutual execusion in the most case.
> At handling file-caches, the only race we have to take care of is "moving"
> account, IOW, overwriting page_cgroup->mem_cgroup. Because file status
> update is done while the page-cache is in stable state, we don't have to
> take care of race with charge/uncharge.
>
> Unlike charge/uncharge, "move" happens not so frequently. It happens only when
> rmdir() and task-moving (with a special settings.)
> This patch adds a race-checker for file-cache-status accounting v.s. account
> moving. The new per-cpu-per-memcg counter MEM_CGROUP_ON_MOVE is added.
> The routine for account move
> 1. Increment it before start moving
> 2. Call synchronize_rcu()
> 3. Decrement it after the end of moving.
> By this, file-status-counting routine can check it needs to call
> lock_page_cgroup(). In most case, I doesn't need to call it.
>
>
> Changelog: 20100804
> - added a comment for possible optimization hint.
> Changelog: 20100730
> - some cleanup.
> Changelog: 20100729
> - replaced __this_cpu_xxx() with this_cpu_xxx
> (because we don't call spinlock)
> - added VM_BUG_ON().
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> mm/memcontrol.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 67 insertions(+), 12 deletions(-)
>
> Index: mmotm-0811/mm/memcontrol.c
> ===================================================================
> --- mmotm-0811.orig/mm/memcontrol.c
> +++ mmotm-0811/mm/memcontrol.c
> @@ -90,6 +90,7 @@ enum mem_cgroup_stat_index {
> MEM_CGROUP_STAT_PGPGOUT_COUNT, /* # of pages paged out */
> MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
> MEM_CGROUP_EVENTS, /* incremented at every pagein/pageout */
> + MEM_CGROUP_ON_MOVE, /* A check for locking move account/status */
>
> MEM_CGROUP_STAT_NSTATS,
> };
> @@ -1086,7 +1087,50 @@ static unsigned int get_swappiness(struc
> return swappiness;
> }
>
> -/* A routine for testing mem is not under move_account */
> +static void mem_cgroup_start_move(struct mem_cgroup *mem)
> +{
> + int cpu;
> + /* for fast checking in mem_cgroup_update_file_stat() etc..*/
> + spin_lock(&mc.lock);
> + /* TODO: Can we optimize this by for_each_online_cpu() ? */
> + for_each_possible_cpu(cpu)
> + per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) += 1;
> + spin_unlock(&mc.lock);
> +
> + synchronize_rcu();
> +}
> +
> +static void mem_cgroup_end_move(struct mem_cgroup *mem)
> +{
> + int cpu;
> +
> + if (!mem)
> + return;
> + /* for fast checking in mem_cgroup_update_file_stat() etc..*/
> + spin_lock(&mc.lock);
> + for_each_possible_cpu(cpu)
> + per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) -= 1;
> + spin_unlock(&mc.lock);
> +}
> +
> +/*
> + * mem_cgroup_is_moved -- checking a cgroup is mc.from target or not.
> + * used for avoiding race.
> + * mem_cgroup_under_move -- checking a cgroup is mc.from or mc.to or
> + * under hierarchy of them. used for waiting at
> + * memory pressure.
> + * Result of is_moved can be trusted until the end of rcu_read_unlock().
> + * The caller must do
> + * rcu_read_lock();
> + * result = mem_cgroup_is_moved();
> + * .....make use of result here....
> + * rcu_read_unlock();
> + */
> +static bool mem_cgroup_is_moved(struct mem_cgroup *mem)
> +{
> + VM_BUG_ON(!rcu_read_lock_held());
> + return this_cpu_read(mem->stat->count[MEM_CGROUP_ON_MOVE]) > 0;
> +}
>
> static bool mem_cgroup_under_move(struct mem_cgroup *mem)
> {
> @@ -1502,29 +1546,36 @@ void mem_cgroup_update_file_mapped(struc
> {
> struct mem_cgroup *mem;
> struct page_cgroup *pc;
> + bool need_lock = false;
>
> pc = lookup_page_cgroup(page);
> if (unlikely(!pc))
> return;
> -
> - lock_page_cgroup(pc);
> + rcu_read_lock();
> mem = id_to_memcg(pc->mem_cgroup, true);
> - if (!mem || !PageCgroupUsed(pc))
> + if (likely(mem)) {
> + if (mem_cgroup_is_moved(mem)) {
> + /* need to serialize with move_account */
> + lock_page_cgroup(pc);
> + need_lock = true;
> + mem = id_to_memcg(pc->mem_cgroup, true);
> + if (unlikely(!mem))
> + goto done;
> + }
> + }
> + if (unlikely(!PageCgroupUsed(pc)))
> goto done;
> -
> - /*
> - * Preemption is already disabled. We can use __this_cpu_xxx
> - */
> if (val > 0) {
> - __this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> + this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> SetPageCgroupFileMapped(pc);
> } else {
> - __this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> + this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> ClearPageCgroupFileMapped(pc);
> }
> -
> done:
> - unlock_page_cgroup(pc);
> + if (need_lock)
> + unlock_page_cgroup(pc);
> + rcu_read_unlock();
> }
>
> /*
> @@ -3064,6 +3115,7 @@ move_account:
> lru_add_drain_all();
> drain_all_stock_sync();
> ret = 0;
> + mem_cgroup_start_move(mem);
> for_each_node_state(node, N_HIGH_MEMORY) {
> for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
> enum lru_list l;
> @@ -3077,6 +3129,7 @@ move_account:
> if (ret)
> break;
> }
> + mem_cgroup_end_move(mem);
> memcg_oom_recover(mem);
> /* it seems parent cgroup doesn't have enough mem */
> if (ret == -ENOMEM)
> @@ -4563,6 +4616,7 @@ static void mem_cgroup_clear_mc(void)
> mc.to = NULL;
> mc.moving_task = NULL;
> spin_unlock(&mc.lock);
> + mem_cgroup_end_move(from);
> memcg_oom_recover(from);
> memcg_oom_recover(to);
> wake_up_all(&mc.waitq);
> @@ -4593,6 +4647,7 @@ static int mem_cgroup_can_attach(struct
> VM_BUG_ON(mc.moved_charge);
> VM_BUG_ON(mc.moved_swap);
> VM_BUG_ON(mc.moving_task);
> + mem_cgroup_start_move(from);
> spin_lock(&mc.lock);
> mc.from = from;
> mc.to = mem;
>
--
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