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Date: Wed, 28 Jul 2010 00:09:21 -0700
From: Greg Thelen <gthelen@...gle.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: "linux-mm\@kvack.org" <linux-mm@...ck.org>,
"nishimura\@mxp.nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
"balbir\@linux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
m-ikeda@...jp.nec.com,
"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 5/7][memcg] memcg lockless update of file mapped
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> 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.
>
> Note: update_file_mapped is safe against charge/uncharge even if it's
> not under address_space->tree_lock or lock_page(). Because it's under
> page_table_lock(), anyone can't unmap it...then, anyone can't uncharge().
>
>
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> mm/memcontrol.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 60 insertions(+), 4 deletions(-)
>
> Index: mmotm-0719/mm/memcontrol.c
> ===================================================================
> --- mmotm-0719.orig/mm/memcontrol.c
> +++ mmotm-0719/mm/memcontrol.c
> @@ -89,6 +89,7 @@ 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,
> };
> @@ -1071,7 +1072,48 @@ 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);
> + 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)
> +{
Could we add an assertion to confirm locking contract is upheld:
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)
> {
> @@ -1470,13 +1512,21 @@ 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_mem(pc->mem_cgroup);
> + if (!mem)
> + goto done;
> + need_lock = mem_cgroup_is_moved(mem);
> + if (need_lock) {
> + /* need to serialize with move_account */
> + lock_page_cgroup(pc);
> + mem = id_to_mem(pc->mem_cgroup);
> + }
> if (!mem || !PageCgroupUsed(pc))
> goto done;
Could we add a preemption() check here to ensure that the
__this_cpu_xxx() is safe to use?
/*
* Preemption is already disabled. We can use __this_cpu_xxx
*/
+ VM_BUG_ON(preemptible());
> @@ -1492,7 +1542,9 @@ void mem_cgroup_update_file_mapped(struc
> }
>
> done:
> - unlock_page_cgroup(pc);
> + if (need_lock)
> + unlock_page_cgroup(pc);
> + rcu_read_unlock();
> }
>
> /*
> @@ -3024,6 +3076,7 @@ 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;
> @@ -3037,6 +3090,7 @@ 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)
> @@ -4503,6 +4557,7 @@ 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);
> @@ -4533,6 +4588,7 @@ 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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