cgroup: implement eventfd-based generic API for notifications

This patchset introduces eventfd-based API for notifications in cgroups
and implements memory notifications on top of it.

It uses statistics in memory controler to track memory usage.

Output of time(1) on building kernel on tmpfs:

Root cgroup before changes:
	make -j2  506.37 user 60.93s system 193% cpu 4:52.77 total
Non-root cgroup before changes:
	make -j2  507.14 user 62.66s system 193% cpu 4:54.74 total
Root cgroup after changes (0 thresholds):
	make -j2  507.13 user 62.20s system 193% cpu 4:53.55 total
Non-root cgroup after changes (0 thresholds):
	make -j2  507.70 user 64.20s system 193% cpu 4:55.70 total
Root cgroup after changes (1 thresholds, never crossed):
	make -j2  506.97 user 62.20s system 193% cpu 4:53.90 total
Non-root cgroup after changes (1 thresholds, never crossed):
	make -j2  507.55 user 64.08s system 193% cpu 4:55.63 total

This patch:

Introduce the write-only file "cgroup.event_control" in every cgroup.

To register new notification handler you need:
- create an eventfd;
- open a control file to be monitored. Callbacks register_event() and
  unregister_event() must be defined for the control file;
- write "<event_fd> <control_fd> <args>" to cgroup.event_control.
  Interpretation of args is defined by control file implementation;

eventfd will be woken up by control file implementation or when the
cgroup is removed.

To unregister notification handler just close eventfd.

If you need notification functionality for a control file you have to
implement callbacks register_event() and unregister_event() in the
struct cftype.

[[email protected]: Kconfig fix]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Paul Menage <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Dan Malek <[email protected]>
Cc: Vladislav Buzov <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Davide Libenzi <[email protected]>
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions