Re: [PATCH 4/5] memcg: lockless update of file stat withmove-account safe method

[Daisuke Nishimura]

On Wed, 25 Aug 2010 17:10:50 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> 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: 20100825
>  - added a comment about mc.lock
>  - fixed bad lock.
> 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@xxxxxxxxxxxxxx>

(snip)

> @@ -1505,29 +1551,36 @@ 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);
It doesn't cause any problem, but I think it would be better to change this to
"id_to_memcg(..., false)". It's just under rcu_read_lock(), not under page_cgroup
lock anymore.

Otherwise, it looks good to me.

Reviewed-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

Thanks,
Daisuke Nishimura.
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