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Date:	Mon, 7 Feb 2011 15:36:24 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	"Kirill A. Shutsemov" <kirill@...temov.name>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	jacob.jun.pan@...ux.intel.com,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Matt Helsley <matthltc@...ibm.com>
Subject: Re: [PATCH, v5 2/2] cgroups: introduce timer slack subsystem

* Kirill A. Shutsemov <kirill@...temov.name> [2011-02-07 11:46:02]:

> From: Kirill A. Shutemov <kirill@...temov.name>
> 
> Provides a way of tasks grouping by timer slack value. Introduces per
> cgroup max and min timer slack value. When a task attaches to a cgroup,
> its timer slack value adjusts (if needed) to fit min-max range.
> 
> It also provides a way to set timer slack value for all tasks in the
> cgroup at once.
> 
> This functionality is useful in mobile devices where certain background
> apps are attached to a cgroup and minimum wakeups are desired.
> 
> Idea-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill@...temov.name>
> ---
>  include/linux/cgroup_subsys.h |    6 +
>  include/linux/init_task.h     |    4 +-
>  init/Kconfig                  |   10 ++
>  kernel/Makefile               |    1 +
>  kernel/cgroup_timer_slack.c   |  281 +++++++++++++++++++++++++++++++++++++++++
>  kernel/sys.c                  |   19 ++-
>  6 files changed, 314 insertions(+), 7 deletions(-)
>  create mode 100644 kernel/cgroup_timer_slack.c
> 
> diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
> index ccefff0..e399228 100644
> --- a/include/linux/cgroup_subsys.h
> +++ b/include/linux/cgroup_subsys.h
> @@ -66,3 +66,9 @@ SUBSYS(blkio)
>  #endif
> 
>  /* */
> +
> +#ifdef CONFIG_CGROUP_TIMER_SLACK
> +SUBSYS(timer_slack)
> +#endif
> +
> +/* */
> diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> index caa151f..48eca8f 100644
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -124,6 +124,8 @@ extern struct cred init_cred;
>  # define INIT_PERF_EVENTS(tsk)
>  #endif
> 
> +#define TIMER_SLACK_NS_DEFAULT 50000
> +
>  /*
>   *  INIT_TASK is used to set up the first task table, touch at
>   * your own risk!. Base=0, limit=0x1fffff (=2MB)
> @@ -177,7 +179,7 @@ extern struct cred init_cred;
>  	.cpu_timers	= INIT_CPU_TIMERS(tsk.cpu_timers),		\
>  	.fs_excl	= ATOMIC_INIT(0),				\
>  	.pi_lock	= __RAW_SPIN_LOCK_UNLOCKED(tsk.pi_lock),	\
> -	.timer_slack_ns = 50000, /* 50 usec default slack */		\
> +	.timer_slack_ns = TIMER_SLACK_NS_DEFAULT,			\
>  	.pids = {							\
>  		[PIDTYPE_PID]  = INIT_PID_LINK(PIDTYPE_PID),		\
>  		[PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID),		\
> diff --git a/init/Kconfig b/init/Kconfig
> index be788c0..6cf465f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -596,6 +596,16 @@ config CGROUP_FREEZER
>  	  Provides a way to freeze and unfreeze all tasks in a
>  	  cgroup.
> 
> +config CGROUP_TIMER_SLACK
> +	tristate "Timer slack cgroup subsystem"
> +	help
> +	  Provides a way of tasks grouping by timer slack value.
> +	  Introduces per cgroup timer slack value which will override
> +	  the default timer slack value once a task is attached to a
> +	  cgroup.
> +	  It's useful in mobile devices where certain background apps
> +	  are attached to a cgroup and combined wakeups are desired.
> +
>  config CGROUP_DEVICE
>  	bool "Device controller for cgroups"
>  	help
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 353d3fe..0b60239 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -61,6 +61,7 @@ obj-$(CONFIG_BACKTRACE_SELF_TEST) += backtracetest.o
>  obj-$(CONFIG_COMPAT) += compat.o
>  obj-$(CONFIG_CGROUPS) += cgroup.o
>  obj-$(CONFIG_CGROUP_FREEZER) += cgroup_freezer.o
> +obj-$(CONFIG_CGROUP_TIMER_SLACK) += cgroup_timer_slack.o
>  obj-$(CONFIG_CPUSETS) += cpuset.o
>  obj-$(CONFIG_CGROUP_NS) += ns_cgroup.o
>  obj-$(CONFIG_UTS_NS) += utsname.o
> diff --git a/kernel/cgroup_timer_slack.c b/kernel/cgroup_timer_slack.c
> new file mode 100644
> index 0000000..609e1f1
> --- /dev/null
> +++ b/kernel/cgroup_timer_slack.c
> @@ -0,0 +1,281 @@
> +/*
> + * cgroup_timer_slack.c - control group timer slack subsystem
> + *
> + * Copyright Nokia Corparation, 2011
> + * Author: Kirill A. Shutemov
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + */
> +#include <linux/cgroup.h>
> +#include <linux/init_task.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/rcupdate.h>
> +
> +struct cgroup_subsys timer_slack_subsys;
> +struct timer_slack_cgroup {
> +	struct cgroup_subsys_state css;
> +	unsigned long min_slack_ns;
> +	unsigned long max_slack_ns;
> +};
> +
> +enum {
> +	TIMER_SLACK_MIN,
> +	TIMER_SLACK_MAX,
> +};
> +
> +extern int (*timer_slack_check)(struct task_struct *task,
> +		unsigned long slack_ns);
> +
> +static struct timer_slack_cgroup *cgroup_to_tslack_cgroup(struct cgroup *cgroup)
> +{
> +	struct cgroup_subsys_state *css;
> +
> +	css = cgroup_subsys_state(cgroup, timer_slack_subsys.subsys_id);
> +	return container_of(css, struct timer_slack_cgroup, css);
> +}
> +
> +static int is_timer_slack_allowed(struct timer_slack_cgroup *tslack_cgroup,
> +		unsigned long slack_ns)
> +{
> +	if (slack_ns < tslack_cgroup->min_slack_ns ||
> +			slack_ns > tslack_cgroup->max_slack_ns)
> +		return false;
> +	return true;
> +}
> +
> +static int cgroup_timer_slack_check(struct task_struct *task,
> +		unsigned long slack_ns)
> +{
> +	struct cgroup_subsys_state *css;
> +	struct timer_slack_cgroup *tslack_cgroup;
> +
> +	/* XXX: lockdep false positive? */
> +	rcu_read_lock();
> +	css = task_subsys_state(task, timer_slack_subsys.subsys_id);
> +	tslack_cgroup = container_of(css, struct timer_slack_cgroup, css);

Shouldn't the rcu_read_unlock() be after the check of is_timer_slack_allowed()?

> +	rcu_read_unlock();
> +
> +	if (!is_timer_slack_allowed(tslack_cgroup, slack_ns))
> +		return -EPERM;
> +	return 0;
> +}
> +
> +static struct cgroup_subsys_state *
> +tslack_cgroup_create(struct cgroup_subsys *subsys, struct cgroup *cgroup)
> +{
> +	struct timer_slack_cgroup *tslack_cgroup;
> +
> +	tslack_cgroup = kmalloc(sizeof(*tslack_cgroup), GFP_KERNEL);
> +	if (!tslack_cgroup)
> +		return ERR_PTR(-ENOMEM);
> +
> +	if (cgroup->parent) {
> +		struct timer_slack_cgroup *parent;
> +		parent = cgroup_to_tslack_cgroup(cgroup->parent);
> +		tslack_cgroup->min_slack_ns = parent->min_slack_ns;
> +		tslack_cgroup->max_slack_ns = parent->max_slack_ns;
> +	} else {
> +		tslack_cgroup->min_slack_ns = 0UL;
> +		tslack_cgroup->max_slack_ns = ULONG_MAX;
> +	}
> +
> +	return &tslack_cgroup->css;
> +}
> +
> +static void tslack_cgroup_destroy(struct cgroup_subsys *subsys,
> +		struct cgroup *cgroup)
> +{
> +	kfree(cgroup_to_tslack_cgroup(cgroup));
> +}
> +
> +/*
> + * Adjust ->timer_slack_ns and ->default_max_slack_ns of the task to fit
> + * limits of the cgroup.
> + */
> +static void tslack_adjust_task(struct timer_slack_cgroup *tslack_cgroup,
> +		struct task_struct *tsk)
> +{
> +	if (tslack_cgroup->min_slack_ns > tsk->timer_slack_ns)
> +		tsk->timer_slack_ns = tslack_cgroup->min_slack_ns;
> +	else if (tslack_cgroup->max_slack_ns < tsk->timer_slack_ns)
> +		tsk->timer_slack_ns = tslack_cgroup->max_slack_ns;
> +
> +	if (tslack_cgroup->min_slack_ns > tsk->default_timer_slack_ns)
> +		tsk->default_timer_slack_ns = tslack_cgroup->min_slack_ns;
> +	else if (tslack_cgroup->max_slack_ns < tsk->default_timer_slack_ns)
> +		tsk->default_timer_slack_ns = tslack_cgroup->max_slack_ns;
> +}
> +
> +static void tslack_cgroup_attach(struct cgroup_subsys *subsys,
> +		struct cgroup *cgroup, struct cgroup *prev,
> +		struct task_struct *tsk, bool threadgroup)
> +{
> +	tslack_adjust_task(cgroup_to_tslack_cgroup(cgroup), tsk);
> +}
> +
> +static int tslack_write_set_slack_ns(struct cgroup *cgroup, struct cftype *cft,
> +		u64 val)
> +{
> +	struct timer_slack_cgroup *tslack_cgroup;
> +	struct cgroup_iter it;
> +	struct task_struct *task;
> +
> +	tslack_cgroup = cgroup_to_tslack_cgroup(cgroup);
> +	if (!is_timer_slack_allowed(cgroup_to_tslack_cgroup(cgroup), val))
> +		return -EPERM;
> +
> +	/* Change timer slack value for all tasks in the cgroup */
> +	cgroup_iter_start(cgroup, &it);
> +	while ((task = cgroup_iter_next(cgroup, &it)))
> +		task->timer_slack_ns = val;
> +	cgroup_iter_end(cgroup, &it);

OK, I've got questions, are the following possible

1. Assign a task to a cgroup based on timer slack, it is accepted
based on min and max values
2. We update the slack

Can't this be bad for some tasks, is there a way for a task to prevent
the max slack from being updated?

2. Can there be an overlap in the ranges?


-- 
	Three Cheers,
	Balbir
--
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