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

On Wed, Aug 24, 2011 at 09:49:59AM +0200, Tejun Heo wrote:

Hello, Frederic.

On Wed, Aug 24, 2011 at 03:14:30AM +0200, Frederic Weisbecker wrote:

quoted

quoted

 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?

I'll try again.

It seems like methods were added to serve the immediate need of the
particular user at the time and that in turn led to addition of
callbacks which were both superflous and incomplete (the bullet points
in the original message list them).  This seems to have happened
because extra interface was added without trying to make the existing
interface complete.

The interface is complicated and cumbersome to use - are
[can_]attach() called first or [can_]attach_task()?  What about
cancelation?  What if a subsys wants to perform operations across
multiple tasks atomically?

In general, iteration-by-callback is painful to use.  Establishing
common context (be it synchronization domain or shared variables)
becomes very cumbersome and implementation becomes fragmented and
difficult to follow.  For example, imagine how it would be like to use
list if we had call_for_each_list_entry(func, list_head) instead of
the control-loop style iterators we have know.

So, using iterators enables making all relevant information to each
stage of attach so that only one callback is required for each step -
the way it should be.  In addition, it makes it far easier for
subsystems to implement more involved logic in their methods.

I tried to make cgroup_freezer behave better which requires better
synchronization against the freezer and, with the current interface,
it's extremely ugly and painful.  The new interface is complete, easy
to understand and use with far less subtleties.

Yeah it's true that the order between [can]attach/[can]attach_task plus
the added mess with pre_attach was not entirely sane. The fact we have
foo and foo_task is already a problem.

I guess we indeed need to sacrifice the iteration from the cgroup core
for that.