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

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Message-ID: <20090226084814.GA2739@linux.vnet.ibm.com>
Date:	Thu, 26 Feb 2009 14:18:15 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Cc:	Li Zefan <lizf@...fujitsu.com>, linux-kernel@...r.kernel.org,
	Balaji Rao <balajirrao@...il.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 01:22:59PM +0530, 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 ?
> 
> 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
> 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 ?
> 

I believe hierarchical accounting is natural. Since the CPU controller
divides its bandwidth as per hierarchy, it makes sense for the
accounting to also be hierarchical. I guess this is a reasonable
compromise. But how do you still check if the group you are accounting
for is the root group or not? (Unless I am missing something obvious)

thanks,
-- 
regards,
Dhaval
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