[PATCHSET cgroup/for-3.11] cgroup: convert cgroup_subsys_state refcnt to percpu_ref

[Tejun Heo]

Hello,

This patchset does a lot of cleanup and then updates css
(cgroup_subsys_state) refcnt to use the new percpu reference counter.
A css (cgroup_subsys_state) is how each cgroup is represented to a
controller.  As such, it can be used in hot paths across the various
subsystems different controllers are associated with.

One of the common operations is reference counting, which up until now
has been implemented using a global atomic counter and can have
significant adverse impact on scalability.  For example, css refcnt
can be gotten and put multiple times by blkcg for each IO request.
For highops configurations which try to do as much per-cpu as
possible, the global frequent refcnting can be very expensive.

In general, given the various hugely diverse paths css's end up being
used from, we need to make it cheap and highly scalable.  In its
usage, css refcnting isn't very different from module refcnting.

This patchset contains the following 11 patches.

 0001-cgroup-remove-now-unused-css_depth.patch
 0002-cgroup-consistently-use-cset-for-struct-css_set-vari.patch
 0003-cgroup-bring-some-sanity-to-naming-around-cg_cgroup_.patch
 0004-cgroup-use-kzalloc-and-list_del_init.patch
 0005-cgroup-clean-up-css_-try-get-and-css_put.patch
 0006-cgroup-rename-CGRP_REMOVED-to-CGRP_DEAD.patch
 0007-cgroup-drop-unnecessary-RCU-dancing-from-__put_css_s.patch
 0008-cgroup-remove-cgroup-count-and-use.patch
 0009-cgroup-reorder-the-operations-in-cgroup_destroy_lock.patch
 0010-cgroup-split-cgroup-destruction-into-two-steps.patch
 0011-cgroup-use-percpu-refcnt-for-cgroup_subsys_states.patch

0001-0007 are cleanups, many of them cosmetic.  They clean up the code
paths which are related with the refcnting changes.  If you're only
interested in the precpu usage, you can probably skip these.

0008-0010 updates the cgroup destruction path so that percpu refcnting
can be used.

0011 updates css refcnting to use percpu_ref.

This patchset is on top of

  cgroup/for-3.11 d5c56ced77 ("cgroup: clean up the cftype array for the base cgroup files")
+ [1] percpu/review-percpu-ref-tryget

and available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-css-percpu-ref

diffstat follows.  Thanks.

 include/linux/cgroup.h |   87 ++----
 kernel/cgroup.c        |  710 +++++++++++++++++++++++++------------------------
 2 files changed, 406 insertions(+), 391 deletions(-)

--
tejun

[1] http://thread.gmane.org/gmane.linux.kernel/1507258