linux-kernel - Re: [PATCH 3/6] cgroup: introduce cgroup_taskset and use it in subsys->can_attach(), cancel

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Message-ID: <20110825092045.GG3286@htj.dyndns.org>
Date:	Thu, 25 Aug 2011 11:20:45 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Paul Menage <paul@...lmenage.org>
Cc:	rjw@...k.pl, lizf@...fujitsu.com,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	James Morris <jmorris@...ei.org>
Subject: Re: [PATCH 3/6] cgroup: introduce cgroup_taskset and use it in
 subsys->can_attach(), cancel_attach() and attach()

Hello,

On Thu, Aug 25, 2011 at 02:14:12AM -0700, Paul Menage wrote:
> The general idea of passing consistent information to all *attach
> methods seems good, but isn't it simpler to just fix up the various
> method signatures?

I think having separate ->attach() and ->attach_task() is inherently
broken.  Look at the memcg discussion I had in this thread for
reference and as soon as we need to do something across the tasks
being migrated, iteration-by-callback becomes very painful.
e.g. let's say memcg wants to find the mm->owner and wants to print
warning or fail if that doesn't work out.  How would that be
implemented if it's iterating by callback.

> The whole point of having *attach() and *attach_task() was to minimize
> the amount of boilerplate (in this case, iterating across a new
> cgroup_taskset abstraction) in the subsystems, leaving that to the
> cgroups framework.

Yeah, I agree with making things easier for subsystems but I violently
disagree that iteration-by-callback is helpful in any way.  If
control-loop style iterator is at all possible, it's almost always
better to go that way.

Thanks.

-- 
tejun
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