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

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Message-ID: <20110826024822.GA1919@count0.beaverton.ibm.com>
Date:	Thu, 25 Aug 2011 19:48:22 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Tejun Heo <tj@...nel.org>, rjw@...k.pl, paul@...lmenage.org,
	lizf@...fujitsu.com, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, fweisbec@...il.com,
	kamezawa.hiroyu@...fujitsu.com,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	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()

On Thu, Aug 25, 2011 at 07:38:18PM -0700, Matt Helsley wrote:
> On Fri, Aug 26, 2011 at 12:43:09AM +0200, Tejun Heo wrote:
> > Currently, there's no way to pass multiple tasks to cgroup_subsys
> > methods necessitating the need for separate per-process and per-task
> > methods.  This patch introduces cgroup_taskset which can be used to
> > pass multiple tasks and their associated cgroups to cgroup_subsys
> > methods.
> 
> This will be the third iterator-ish pattern in the cgroup code.
> It's not your fault but it does seem a bit much to have:
> 
> 	1) When we need to iterate over all tasks in the cgroup and
> 	don't mind holding the css set lock:
> 		void cgroup_iter_start(cgroup, iterator)
> 		task cgroup_iter_next(cgroup, iterator)
> 		void cgroup_iter_end(cgroup, iterator)
> 
> 	2) For subsystem methods when we're iterating over a subset of
> 	tasks that may or may not be in the cgroup (e.g. for can_attach)
> 	-- use cgroup_tasksets:
> 		task cgroup_taskset_first(tset)
> 		task cgroup_taskset_next(tset)
> 
> 	3) An iterator over all the tasks which doesn't hold the css set
> 	lock:
> 		struct cgroup_scanner {
> 			struct cgroup *cg;
> 			int (*test_task)(struct task_struct *p,
> 					struct cgroup_scanner *scan);
> 			void (*process_task)(struct task_struct *p,
> 					struct cgroup_scanner *scan);
> 			struct ptr_heap *heap;
> 			void *data;
> 		};
> 	This is only used in cpuset code so far. Are other cgroup
> 	patches planning on making use of it?
> 
> Is there a sane way to merge all this?
> 
> Perhaps we could drop the iterator interfaces in 1) and 2) and replace

Oops! I meant to suggest dropping interfaces 1) and 3).

Cheers,
	-Matt
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