linux-kernel - Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

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Date:	Thu, 18 Mar 2010 21:58:55 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrea Righi <arighi@...eler.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Vivek Goyal <vgoyal@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Greg Thelen <gthelen@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-03-18 13:35:27]:

> On Thu, 18 Mar 2010 09:49:44 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-03-18 08:54:11]:
> > 
> > > On Wed, 17 Mar 2010 17:28:55 +0530
> > > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > > 
> > > > * Andrea Righi <arighi@...eler.com> [2010-03-15 00:26:38]:
> > > > 
> > > > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > > > > 
> > > > > Now, file-mapped is maintaiend. But more generic update function
> > > > > will be needed for dirty page accounting.
> > > > > 
> > > > > For accountig page status, we have to guarantee lock_page_cgroup()
> > > > > will be never called under tree_lock held.
> > > > > To guarantee that, we use trylock at updating status.
> > > > > By this, we do fuzzy accounting, but in almost all case, it's correct.
> > > > >
> > > > 
> > > > I don't like this at all, but in almost all cases is not acceptable
> > > > for statistics, since decisions will be made on them and having them
> > > > incorrect is really bad. Could we do a form of deferred statistics and
> > > > fix this.
> > > > 
> > > 
> > > plz show your implementation which has no performance regresssion.
> > > For me, I don't neee file_mapped accounting, at all. If we can remove that,
> > > we can add simple migration lock.
> > 
> > That doesn't matter, if you need it, I think the larger user base
> > matters. Unmapped and mapped page cache is critical and I use it
> > almost daily.
> > 
> > > file_mapped is a feattue you added. please improve it.
> > >
> > 
> > I will, but please don't break it silently
> > 
> Andrea, could you go in following way ?
> 
> 	- don't touch FILE_MAPPED stuff.
> 	- add new functions for other dirty accounting stuff as in this series.
> 	  (using trylock is ok.)
> 
> Then, no probelm. It's ok to add mem_cgroup_udpate_stat() indpendent from
> mem_cgroup_update_file_mapped(). The look may be messy but it's not your
> fault. But please write "why add new function" to patch description.
> 
> I'm sorry for wasting your time.

Do we need to go down this route? We could check the stat and do the
correct thing. In case of FILE_MAPPED, always grab page_cgroup_lock
and for others potentially look at trylock. It is OK for different
stats to be protected via different locks.

/me takes a look at the code again.

-- 
	Three Cheers,
	Balbir
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