linux-kernel - Re: [PATCH 0/4] Memory controller soft limit patches (v3)

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Date:	Mon, 2 Mar 2009 15:21:28 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	linux-mm@...ck.org, Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	Bharata B Rao <bharata@...ibm.com>,
	Paul Menage <menage@...gle.com>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] Memory controller soft limit patches (v3)

On Mon, 2 Mar 2009 11:35:19 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-02 14:32:50]:
> 
> > On Mon, 2 Mar 2009 10:10:43 +0530
> > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > 
> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-02 09:24:04]:
> > > 
> > > > On Sun, 01 Mar 2009 11:59:59 +0530
> > > > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > > > 
> > > > > 
> > > > > From: Balbir Singh <balbir@...ux.vnet.ibm.com>
> > 
> > > > 
> > > > At first, it's said "When cgroup people adds something, the kernel gets slow".
> > > > This is my start point of reviewing. Below is comments to this version of patch.
> > > > 
> > > >  1. I think it's bad to add more hooks to res_counter. It's enough slow to give up
> > > >     adding more fancy things..
> > > 
> > > res_counters was desgined to be extensible, why is adding anything to
> > > it going to make it slow, unless we turn on soft_limits?
> > > 
> > You inserted new "if" logic in the core loop.
> > (What I want to say here is not that this is definitely bad but that "isn't there
> >  any alternatives which is less overhead.)
> > 
> > 
> > > > 
> > > >  2. please avoid to add hooks to hot-path. In your patch, especially a hook to
> > > >     mem_cgroup_uncharge_common() is annoying me.
> > > 
> > > If soft limits are not enabled, the function does a small check and
> > > leaves. 
> > > 
> > &soft_fail_res is passed always even if memory.soft_limit==ULONG_MAX
> > res_counter_soft_limit_excess() adds one more function call and spinlock, and irq-off.
> >
> 
> OK, I see that overhead.. I'll figure out a way to work around it.
>  
> > > > 
> > > >  3. please avoid to use global spinlock more. 
> > > >     no lock is best. mutex is better, maybe.
> > > > 
> > > 
> > > No lock to update a tree which is update concurrently?
> > > 
> > Using tree/sort itself is nonsense, I believe.
> > 
> 
> I tried using prio trees in the past, but they are not easy to update
> either. I won't mind asking for suggestions for a data structure that
> can scaled well, allow quick insert/delete and search.
> 
Now, because the routine is called by kswapd() not by try_to_free.....

It's not necessary to be very very fast. That's my point.

Thanks,
-Kame





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