[Devel] Re: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v6)

[Vivek Goyal]

On Fri, Mar 12, 2010 at 11:24:33AM +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 12 Mar 2010 10:14:11 +0900
> Daisuke Nishimura <nishimura at mxp.nes.nec.co.jp> wrote:
> 
> > On Thu, 11 Mar 2010 18:42:44 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> > > On Thu, 11 Mar 2010 18:25:00 +0900
> > > KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> > > > Then, it's not problem that check pc->mem_cgroup is root cgroup or not
> > > > without spinlock.
> > > > ==
> > > > void mem_cgroup_update_stat(struct page *page, int idx, bool charge)
> > > > {
> > > > 	pc = lookup_page_cgroup(page);
> > > > 	if (unlikely(!pc) || mem_cgroup_is_root(pc->mem_cgroup))
> > > > 		return;	
> > > > 	...
> > > > }
> > > > ==
> > > > This can be handle in the same logic of "lock failure" path.
> > > > And we just do ignore accounting.
> > > > 
> > > > There are will be no spinlocks....to do more than this,
> > > > I think we have to use "struct page" rather than "struct page_cgroup".
> > > > 
> > > Hmm..like this ? The bad point of this patch is that this will corrupt FILE_MAPPED
> > > status in root cgroup. This kind of change is not very good.
> > > So, one way is to use this kind of function only for new parameters. Hmm.
> > IMHO, if we disable accounting file stats in root cgroup, it would be better
> > not to show them in memory.stat to avoid confusing users.
> agreed.
> 
> > But, hmm, I think accounting them in root cgroup isn't so meaningless.
> > Isn't making mem_cgroup_has_dirty_limit() return false in case of root cgroup enough?
> > 
> The problem is spinlock overhead.
> 
> IMHO, there are 2 excuse for "not accounting" in root cgroup
>  1. Low overhead is always appreciated.
>  2. Root's statistics can be obtained by "total - sum of children".
> 

IIUC, Total sum of children works only if user_hierarchy=1? At the same time
it does not work if there tasks in root cgroup and not in children group.

Vivek
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