linux-kernel - Re: [RFC PATCH 1/2] New cgroup subsystem API (->initialize())

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Date:	Thu, 26 Feb 2009 15:42:13 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, Balaji Rao <balajirrao@...il.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC PATCH 1/2] New cgroup subsystem API (->initialize())

On Thu, Feb 26, 2009 at 04:11:36PM +0800, Li Zefan wrote:
> Bharata B Rao wrote:
> > On Thu, Feb 26, 2009 at 10:55:54AM +0800, Li Zefan wrote:
> >> Bharata B Rao wrote:
> >>> From: Balaji Rao <balajirrao@...il.com>
> >>>
> >>> cgroup: Add ->initialize() to cgroup_subsys structure
> >>>
> >>> Some cgroup subsystems (like cpu controller) would need subsystem
> >>> specific initialization. Such subsystems can define ->initialize()
> >>> which gets called during cgroup_init() (and not cgroup_init_early()).
> >>>
> >> I think it's better to avoid adding this.
> >>
> >> It would be best if we can add a hook to initialize init_task_group.stat where
> >> kmalloc is available but acount_xxx_time() hasn't been called. Otherwise, we
> >> have to check (tg->stat == NULL) in account_task_group_time(), then why not add
> >> a hook in smp_init_smp() to do initialization?
> > 
> > account_xxx_time() is called from scheduler ticks and AFAICS they end up
> > getting called much before kmalloc is available. In any case, I would think
> > any hook to just initialize stats for init_task_group would be
> > very very (cpu controller) subsytem specific. Isn't that bad ?
> > 
> 
> Since it's very very cpu subsystem specific, so it's better to use it's own hook.
> (and because the initialize() API is not so elegant..)
> 
> > Another solution I see which can prevent all this is not to collect
> > stats for init_task_group at all with the understanding that system wide
> 
> This came to my mind too. ;)
> 
> > stime/utime accounting (which is already present) is essentially the
> > accounting for init_task_group because init_task_group comprises of all
> > the tasks in the system. But this would necessiate us to make collection
> > of cpu controller stats hierarchial. This was one of the questions I asked
> > in my 0/2 thread. Shouldn't we be doing hierarchial accounting for
> > cpu controller ?
> > 
> 
> Don't know. I have no strong opinion about this. I'm a bit doubt how useful
> this is.
> 
> > Another thing that could be done is to enhance already existing
> > cpuacct controller to do stime/utime accouting also. cpuacct controller
> > exists precisely for doing per-cgroup accounting and is there any reason
> > why these stats shouldn't be part of cpuacct controller. If we do this,
> > users would be forced to use cpu controller and cpuacct controller
> > together. Is that a problem ?
> > 
> 
> I wondered why these stats is part of cpu subsystem but not cpuacct.
> And I don't see any problem to use these 2 subsystems together. Actually
> this is one of the advantage of cgroup.

Ok, so if there are no objections, my next version would move these
stats to cpuacct subsystem.

Regards,
Bharata.
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