Re: [PATCHSET] cgroup: introduce cgroup_taskset and consolidate subsys methods

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Aug 24, 2011 at 12:19:54AM +0200, Tejun Heo wrote:
> Hello,
> 
> cgroup has grown quite some number of subsys methods.  Some of them
> are overlapping, inconsistent with each other and called under
> different conditions depending on whether they're called for a single
> task or whole process.  Unfortunately, these callbacks are complicated
> and incomplete at the same time.
> 
> * ->attach_task() is called after migration for task attach but before
>   for process.
> 
> * Ditto for ->pre_attach().
> 
> * ->can_attach_task() is called for every task in the thread group but
>   ->attach_task() skips the ones which don't actually change cgroups.
> 
> * Task attach becomes noop if the task isn't actually moving.  Process
>   attach is always performed.
> 
> * ->attach_task() doesn't (or at least aren't supposed to) have access
>   to the old cgroup.
> 
> * During cancel, there's no way to access the affected tasks.
> 
> This patchset introduces cgroup_taskset along with some accessors and
> iterator, updates methods to use it, consolidates usages and drops
> superflous methods.
> 
> It contains the following six patches.
> 
>  0001-cgroup-subsys-attach_task-should-be-called-after-mig.patch
>  0002-cgroup-improve-old-cgroup-handling-in-cgroup_attach_.patch
>  0003-cgroup-introduce-cgroup_taskset-and-use-it-in-subsys.patch
>  0004-cgroup-don-t-use-subsys-can_attach_task-or-attach_ta.patch
>  0005-cgroup-cpuset-don-t-use-ss-pre_attach.patch
>  0006-cgroup-kill-subsys-can_attach_task-pre_attach-and-at.patch

I don't understand the point on patches 3,4,5,6

Why pushing the task iterations down to the subsystems?