[Devel] Re: [PATCH 3/6] Container Freezer: Implement freezer cgroup subsystem

[Rafael J. Wysocki]

On Friday, 1 of August 2008, Matt Helsley wrote:
> This patch implements a new freezer subsystem in the control groups framework.
> It provides a way to stop and resume execution of all tasks in a cgroup by
> writing in the cgroup filesystem.
> 
> 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 freezer  /containers
>    # mkdir /containers/0
>    # echo $some_pid > /containers/0/tasks
> 
> to get status of the freezer subsystem :
> 
>    # cat /containers/0/freezer.state
>    RUNNING
> 
> to freeze all tasks in the container :
> 
>    # echo FROZEN > /containers/0/freezer.state
>    # cat /containers/0/freezer.state
>    FREEZING
>    # cat /containers/0/freezer.state
>    FROZEN
> 
> to unfreeze all tasks in the container :
> 
>    # echo RUNNING > /containers/0/freezer.state
>    # cat /containers/0/freezer.state
>    RUNNING
> 
> This is the basic mechanism which should do the right thing for user space task
> in a simple scenario.
> 
> It's important to note that freezing can be incomplete. In that case we return
> EBUSY. This means that some tasks in the cgroup are busy doing something that
> prevents us from completely freezing the cgroup at this time. After EBUSY,
> the cgroup will remain partially frozen -- reflected by freezer.state reporting
> "FREEZING" when read. The state will remain "FREEZING" until one of these
> things happens:
> 
> 	1) Userspace cancels the freezing operation by writing "RUNNING" to
> 		the freezer.state file
> 	2) Userspace retries the freezing operation by writing "FROZEN" to
> 		the freezer.state file (writing "FREEZING" is not legal
> 		and returns EIO)
> 	3) The tasks that blocked the cgroup from entering the "FROZEN"
> 		state disappear from the cgroup's set of tasks.
> 
> Signed-off-by: Cedric Le Goater <clg at fr.ibm.com>
> Signed-off-by: Matt Helsley <matthltc at us.ibm.com>
> Acked-by: Serge E. Hallyn <serue at us.ibm.com>
> Tested-by: Matt Helsley <matthltc at us.ibm.com>
> ---
>  include/linux/cgroup_freezer.h |   71 ++++++++
>  include/linux/cgroup_subsys.h  |    6 
>  include/linux/freezer.h        |   16 +-
>  init/Kconfig                   |    7 
>  kernel/Makefile                |    1 
>  kernel/cgroup_freezer.c        |  328 +++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 425 insertions(+), 4 deletions(-)
>  create mode 100644 include/linux/cgroup_freezer.h
>  create mode 100644 kernel/cgroup_freezer.c
> 
> Index: linux-2.6.27-rc1-mm1/include/linux/cgroup_freezer.h
> ===================================================================
> --- /dev/null
> +++ linux-2.6.27-rc1-mm1/include/linux/cgroup_freezer.h
> @@ -0,0 +1,71 @@
> +#ifndef _LINUX_CGROUP_FREEZER_H
> +#define _LINUX_CGROUP_FREEZER_H
> +/*
> + * cgroup_freezer.h -  control group freezer subsystem interface
> + *
> + * Copyright IBM Corporation, 2007
> + *
> + * Author : Cedric Le Goater <clg at fr.ibm.com>
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of version 2.1 of the GNU Lesser General Public License
> + * as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it would be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> + */
> +
> +#include <linux/cgroup.h>
> +
> +#ifdef CONFIG_CGROUP_FREEZER
> +
> +enum freezer_state {
> +	STATE_RUNNING = 0,
> +	STATE_FREEZING,
> +	STATE_FROZEN,
> +};
> +
> +struct freezer {
> +	struct cgroup_subsys_state css;
> +	enum freezer_state state;
> +	spinlock_t lock; /* protects _writes_ to state */
> +};
> +
> +static inline struct freezer *cgroup_freezer(
> +		struct cgroup *cgroup)
> +{
> +	return container_of(
> +		cgroup_subsys_state(cgroup, freezer_subsys_id),
> +		struct freezer, css);
> +}
> +
> +static inline struct freezer *task_freezer(struct task_struct *task)
> +{
> +	return container_of(task_subsys_state(task, freezer_subsys_id),
> +			    struct freezer, css);
> +}
> +
> +static inline int cgroup_frozen(struct task_struct *task)
> +{
> +	struct freezer *freezer;
> +	enum freezer_state state;
> +
> +	task_lock(task);
> +	freezer = task_freezer(task);
> +	state = freezer->state;
> +	task_unlock(task);
> +
> +	return state == STATE_FROZEN;
> +}
> +
> +#else /* !CONFIG_CGROUP_FREEZER */
> +
> +static inline int cgroup_frozen(struct task_struct *task)
> +{
> +	return 0;
> +}
> +
> +#endif /* !CONFIG_CGROUP_FREEZER */
> +
> +#endif /* _LINUX_CGROUP_FREEZER_H */

Hmm.  I wonder if we really need a separate file for this.  I'd prefer it to be
in freezer.h, unless there's a good reason not to place it in there.

Thanks,
Rafael
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