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

[Matt Helsley <matthltc@us.ibm.com>]

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
it with:

		tset cgroup_taskset_from_cgroup(cgroup)

which would grab the css set lock to construct the tset from the given
cgroup -- essentially inlining the current iterator code into a single
function.

Of course this would eliminate the restriction that all the tasks in the
taskset are part of the same thread group; that would only be true for
those passed as parameters to the subsystem methods.

Cheers,
	-Matt Helsley