linux-kernel - Re: [RFC][PATCH 1/4] cgroup: support per cgroup subsys state ID (CSS ID)

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Message-Id: <20090115151203.e79271c0.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 15 Jan 2009 15:12:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"menage@...gle.com" <menage@...gle.com>
Subject: Re: [RFC][PATCH 1/4] cgroup: support per cgroup subsys state ID
 (CSS ID)

Sorry for delayed reply.


On Mon, 12 Jan 2009 12:51:48 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-01-08 18:28:17]:
> > There are several reasons to develop this.
> > 	- Saving space .... For example, memcg's swap_cgroup is array of
> > 	  pointers to cgroup. But it is not necessary to be very fast.
> > 	  By replacing pointers(8bytes per ent) to ID (2byes per ent), we can
> > 	  reduce much amount of memory usage.
> 
> 2 bytes per entry means that we restrict the entries to 2^16-1 for
> number of cgroups, using a pointer introduces no such restriction.
> 2^16-1 seems a reasonable number for now.
> 
yes.


> >  /* bits in struct cgroup_subsys_state flags field */
> > @@ -363,6 +367,11 @@ struct cgroup_subsys {
> >  	int active;
> >  	int disabled;
> >  	int early_init;
> > +	/*
> > +	 * True if this subsys uses ID. ID is not available before cgroup_init()
> > +	 * (not available in early_init time.)
>                                             ^ period should come later
sure.

;
> > +	/*
> > +	 * Hierarchy of CSS ID belongs to.
> > +	 */
> > +	unsigned short  stack[0]; /* Array of Length (depth+1) */
> 
> By maintaining the path up to the root here, is that how we avoid walking
> through cgroups links?
> 
yes. we can check this css is under hierarchy by

   css->stack[root->depth] == root->id.


> > +/**
> >   * idr_replace - replace pointer for given id
> >   * @idp: idr handle
> >   * @ptr: pointer you want associated with the id
> >
> 
> Overall, I've taken a quick look and the patches seem OK.
> 
thx,

-Kame

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