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Subject[patch 0/4] Container Freezer: Reuse Suspend Freezer

This patchset reuses the container infrastructure and the swsusp freezer to
freeze a group of tasks.

The freezer subsystem in the container filesystem defines a file named
freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the
cgroup. Subsequently writing "RUNNING" will unfreeze the tasks in the cgroup.
Reading will return the current state.

* Examples of usage :

# mkdir /containers/freezer
# mount -t cgroup -ofreezer,signal freezer /containers
# mkdir /containers/0
# echo $some_pid > /containers/0/tasks

to get status of the freezer subsystem :

# cat /containers/0/freezer.state

to freeze all tasks in the container :

# echo FROZEN > /containers/0/freezer.state
# cat /containers/0/freezer.state
# cat /containers/0/freezer.state

to unfreeze all tasks in the container :

# echo RUNNING > /containers/0/freezer.state
# cat /containers/0/freezer.state

to kill all tasks in the container :

# echo 9 > /containers/0/signal.kill

I've taken Cedric's patches, forward-ported them to 2.6.26-rc5-mm2 + Rafael's
NOSIG patches.

Paul, Pavel asked me to send these to Rafael next. They are patches to make
the freezer useful for checkpoint/restart using cgroups so it would be nice
to get an explicit [N]Ack from you first.

Rafael, if Paul agrees, please consider applying these patches.

Changes since v2:
Ported to 2.6.26-rc5-mm2 with Rafael's freezer patches
Tested on 24 combinations of 3 architectures (x86, x86_64, ppc64)
with 8 different kernel configs varying power management
and cgroup config variables. Each patch builds and boots
in these 24 combinations.
Passes functional testing.
v2 (roughly patches 3 and 5):
Moved the "kill" file into a separate cgroup subsystem (signal) and
it's own patch.
Changed the name of the file from freezer.freeze to freezer.state.
Switched from taking 1 and 0 as input to the strings "FROZEN" and
"RUNNING", respectively. This helps keep the interface
human-usable if/when we need to more states.
Checked that stopped or interrupted is "frozen enough"
Since try_to_freeze() is called upon wakeup of these tasks
this should be fine. This idea comes from recent changes to
the freezer.
Checked that if (task == current) whilst freezing cgroup we're ok
Fixed bug where -EBUSY would always be returned when freezing
Added code to handle userspace retries for any remaining -EBUSY

-Matt Helsley


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