linux-kernel - Re: [RFC][PATCH -mm 0/7] memcg: lockless page

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Date:	Thu, 21 Aug 2008 12:54:18 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	nishimura@....nes.nec.co.jp, LKML <linux-kernel@...r.kernel.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"yamamoto@...inux.co.jp" <yamamoto@...inux.co.jp>,
	ryov@...inux.co.jp, "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH -mm 0/7] memcg: lockless page_cgroup v1

On Thu, 21 Aug 2008 11:17:40 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> On Wed, 20 Aug 2008 20:00:06 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > On Wed, 20 Aug 2008 19:41:08 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > 
> > > On Wed, 20 Aug 2008 18:53:06 +0900
> > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > > 
> > > > Hi, this is a patch set for lockless page_cgroup.
> > > > 
> > > > dropped patches related to mem+swap controller for easy review.
> > > > (I'm rewriting it, too.)
> > > > 
> > > > Changes from current -mm is.
> > > >   - page_cgroup->flags operations is set to be atomic.
> > > >   - lock_page_cgroup() is removed.
> > > >   - page->page_cgroup is changed from unsigned long to struct page_cgroup*
> > > >   - page_cgroup is freed by RCU.
> > > >   - For avoiding race, charge/uncharge against mm/memory.c::insert_page() is
> > > >     omitted. This is ususally used for mapping device's page. (I think...)
> > > > 
> > > > In my quick test, perfomance is improved a little. But the benefit of this
> > > > patch is to allow access page_cgroup without lock. I think this is good 
> > > > for Yamamoto's Dirty page tracking for memcg.
> > > > For I/O tracking people, I added a header file for allowing access to
> > > > page_cgroup from out of memcontrol.c
> > > > 
> > > > The base kernel is recent mmtom. Any comments are welcome.
> > > > This is still under test. I have to do long-run test before removing "RFC".
> > > > 
> > > Known problem: force_emtpy is broken...so rmdir will struck into nightmare.
> > > It's because of patch 2/7.
> > > will be fixed in the next version.
> > > 
> > 
> > This is a quick fix but I think I can find some better solution..
> > ==
> > Because removal from LRU is delayed, mz->lru will never be empty until
> > someone kick drain. This patch rotate LRU while force_empty and makes
> > page_cgroup will be freed.
> > 
> 
> I'd like to rewrite force_empty to move all usage to "default" cgroup.
> There are some reasons.
> 
> 1. current force_empty creates an alive page which has no page_cgroup.
>    This is bad for routine which want to access page_cgroup from page.
>    And this behavior will be an issue of race condition in future.    
I agree that current force_empty is not good in this point.

> 2. We can see amount of out-of-control usage in default cgroup.
> 
> But to do this, I'll have to avoid "hitting limit" in default cgroup.
> I'm now wondering to make it impossible to set limit to default cgroup.
> (will show as a patch in the next version of series.) 
> Does anyone have an idea ?
> 
I don't have a strong objection about setting default cgroup unlimited
and moving usages to default cgroup.

But I think this is related to hierarchy support as Balbir-san says.
And, setting default cgroup unlimited would not be so strange if
hierarchy is supported.


Thanks,
Daisuke Nishimura.
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