Message-ID: <20110214114142.2ee04f69@jacob-laptop>
Date:	Mon, 14 Feb 2011 11:41:42 -0800
From:	jacob pan <jacob.jun.pan@...ux.intel.com>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	container cgroup <containers@...ts.linux-foundation.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>, akpm@...ux-foundation.org,
	rdunlap@...otime.net, Cedric Le Goater <clg@...t.ibm.com>
Subject: Re: [PATCH 1/1, v7] cgroup/freezer: add per freezer duty ratio
 control

On Sat, 12 Feb 2011 15:29:07 -0800
Matt Helsley <matthltc@...ibm.com> wrote:

> On Fri, Feb 11, 2011 at 11:10:44AM -0800,
> jacob.jun.pan@...ux.intel.com wrote:
> > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > 
> > Freezer subsystem is used to manage batch jobs which can start
> > stop at the same time. However, sometime it is desirable to let
> > the kernel manage the freezer state automatically with a given
> > duty ratio.
> > For example, if we want to reduce the time that backgroup apps
> > are allowed to run we can put them into a freezer subsystem and
> > set the kernel to turn them THAWED/FROZEN at given duty ratio.
> > 
> > This patch introduces two file nodes under cgroup
> > freezer.duty_ratio_pct and freezer.period_sec
> 
> Again: I don't think this is the right approach in the long term.
> It would be better not to add this interface and instead enable the
> cpu cgroup subsystem for non-rt tasks using a similar duty ratio
> concept..
> 
> Nevertheless, I've added some feedback on the code for you here :).
> 
Thanks for the thorough review, really appreciated.
> > 
> > Usage example: set period to be 5 seconds and frozen duty ratio 90%
> > [root@...alhost aoa]# echo 90 > freezer.duty_ratio_pct
> > [root@...alhost aoa]# echo 5000 > freezer.period_ms
> > 
> > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > ---
> >  Documentation/cgroups/freezer-subsystem.txt |   23 ++++
> >  kernel/cgroup_freezer.c                     |  153
> > ++++++++++++++++++++++++++- 2 files changed, 174 insertions(+), 2
> > deletions(-)
> > 
> > diff --git a/Documentation/cgroups/freezer-subsystem.txt
> > b/Documentation/cgroups/freezer-subsystem.txt index
> > 41f37fe..2022b32 100644 ---
> > a/Documentation/cgroups/freezer-subsystem.txt +++
> > b/Documentation/cgroups/freezer-subsystem.txt @@ -100,3 +100,26 @@
> > things happens: and returns EINVAL)
> >  	3) The tasks that blocked the cgroup from entering the
> > "FROZEN" state disappear from the cgroup's set of tasks.
> > +
> > +In embedded systems, it is desirable to manage group of
> > applications +for power saving. E.g. tasks that are not in the
> > foreground may be +frozen and unfrozen periodically to save power
> > without affecting user +experience. In this case, user/management
> > software can attach tasks +into freezer cgroup then specify duty
> > ratio and period that the +managed tasks are allowed to run.
> > +
> > +Usage example:
> > +Assuming freezer cgroup is already mounted, application being
> > managed +are included the "tasks" file node of the given freezer
> > cgroup. +To make the tasks frozen at 90% of the time every 5
> > seconds, do: +
> > +[root@...alhost ]# echo 90 > freezer.duty_ratio_pct
> > +[root@...alhost ]# echo 5000 > freezer.period_ms
> > +
> > +After that, the application in this freezer cgroup will only be
> > +allowed to run at the following pattern.
> > +     __                    __                    __
> > +    |  |<-- 90% frozen -->|  |                  |  |
> > +____|  |__________________|  |__________________|  |_____
> > +
> > +    |<---- 5 seconds ---->|
> 
> So most of the time I've been reviewing this I managed to invert it!
> I imagined "duty" meant the tasks were "on duty" ie runnable ie
> thawed. But according this this documentation it's the opposite...
> 
My logic is that since this is a freezer, so positive logic should be
frozen instead of thaw.
> I've reviewed my review and now my comments are consistent with the
> above. :) However it makes me wonder if there are better names which
> would avoid this confusion.
> 
How about frozen_time_pct?

> > diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> > index e7bebb7..aaa91ca 100644
> > --- a/kernel/cgroup_freezer.c
> > +++ b/kernel/cgroup_freezer.c
> > @@ -21,6 +21,7 @@
> >  #include <linux/uaccess.h>
> >  #include <linux/freezer.h>
> >  #include <linux/seq_file.h>
> > +#include <linux/kthread.h>
> > 
> >  enum freezer_state {
> >  	CGROUP_THAWED = 0,
> > @@ -28,12 +29,35 @@ enum freezer_state {
> >  	CGROUP_FROZEN,
> >  };
> > 
> > +enum duty_ratio_params {
> > +	FREEZER_DUTY_RATIO = 0,
> > +	FREEZER_PERIOD,
> > +};
> > +
> > +struct freezer_toggle {
> > +	unsigned int enabled:1;
> > +	unsigned int freeze_thaw:1; /* 1: freeze 0: thaw */
> > +} __packed;
> > +
> > +struct freezer_duty {
> > +	u32 ratio; /* percentage of time frozen */
> > +	u32 period_pct_ms; /* one percent of the period in
> > miliseconds */ +};
> 
> I'd rather see you merge these two structures -- I don't see the value
> of keeping them separate nor of packing the struct. You could also
> merge the work item in there:
> 
> struct freezer_duty {
> 	struct delayed_work freezer_work; /* work to duty-cycle a
> cgroup */
> 
> 	u32 ratio; /* percentage of time frozen */
> 	u32 period_pct_ms; /* one percent of the period in
> miliseconds */ unsigned int enabled:1;
> 	unsigned int freeze_thaw:1; /* 1: freeze 0: thaw */
> };
> 
> (I'm going to make the rest of my comments without assuming you've
> done this because it'll make them easier to follow given the context)
> 
I don't have strong preference, but it seems more logical to merge all
toggling related structures. I will change that.
> > +
> >  struct freezer {
> >  	struct cgroup_subsys_state css;
> >  	enum freezer_state state;
> > +	struct freezer_duty duty;
> > +	struct delayed_work freezer_work; /* work to duty-cycle a
> > cgroup */
> > +	struct freezer_toggle toggle;
> >  	spinlock_t lock; /* protects _writes_ to state */
> >  };
> > 
> > +static int try_to_freeze_cgroup(struct cgroup *cgroup, struct
> > freezer *freezer); +static void unfreeze_cgroup(struct cgroup
> > *cgroup, struct freezer *freezer); +static int
> > freezer_change_state(struct cgroup *cgroup,
> > +				enum freezer_state goal_state);
> > +
> >  static inline struct freezer *cgroup_freezer(
> >  		struct cgroup *cgroup)
> >  {
> > @@ -63,6 +87,41 @@ int cgroup_freezing_or_frozen(struct task_struct
> > *task) return result;
> >  }
> > 
> > +static DECLARE_WAIT_QUEUE_HEAD(freezer_wait);
> > +
> > +static void freezer_work_fn(struct work_struct *work)
> > +{
> > +	struct freezer *freezer;
> > +	unsigned long delay_jiffies = 0;
> > +	enum freezer_state goal_state;
> > +
> 
> Looking better. There are alot of field accesses here which can race
> with writes to the cgroup's duty ratio and period files. They should
> be protected. Perhaps we can reuse the freezer spin lock. That also
> has the benefit that we can eliminate the toggle.freeze_thaw bit I
> think:
> 
I did think about the race, it does exist. but in practice. My thought
was that since freezer_change_state() holds the spin_lock of the
freezer, the race with writes to params are harmless, it just means the
new period or ratio will take effect in the next period.
In terms of using freezer spin lock to eliminate toggle flag, I am not
sure if i know how to do that. Are you suggesting based on whether the
spin lock is taken or not, we can decide the toggle? but the freeze
spin lock is used by other functions as well not just the delay work
here. I guess I have missed something.

> > +
> > +	freezer = container_of(work, struct freezer,
> > freezer_work.work);
> > +	/* toggle between THAWED and FROZEN state.
> > +	 * thaw if freezer->toggle.freeze_thaw = 0; freeze
> > otherwise
> > +	 * skip the first round if already in the target states.
> > +	 */
> 
> spin_lock(&freezer->lock);
> 
> > +	if ((freezer->toggle.freeze_thaw && freezer->state ==
> > CGROUP_FROZEN) ||
> > +		(!freezer->toggle.freeze_thaw &&
> > +			freezer->state == CGROUP_THAWED)) {
> > +		delay_jiffies = 0;
> 
> This looks wrong. We should never schedule freezer work delayed by 0
> jiffies -- even if the delayed work API allows it. With 0-length
> delays I'd worry that we could peg the CPU in an obscure infinite
> loop.
> 
> I think you can safely eliminate this block and the "exit_toggle"
> label.
> 
Good point. My initial thought was that since the period for targeted
usage is quite long, e.g. 30 sec., we want to start the duty ratio
right away. But that shouldn't matter since we already schedule work
based on the new ratio/period.
> > +		goto exit_toggle;
> > +	} else if (freezer->toggle.freeze_thaw) {
> 
> 	if (freezer->state == CGROUP_THAWED) {
> 
> > +		goal_state = CGROUP_FROZEN;
> > +		delay_jiffies =
> > msecs_to_jiffies(freezer->duty.ratio *
> > +
> > freezer->duty.period_pct_ms);
> > +	} else  {
> > +		goal_state = CGROUP_THAWED;
> > +		delay_jiffies = msecs_to_jiffies((100 -
> > freezer->duty.ratio) *
> > +
> > freezer->duty.period_pct_ms);
> > +	}
> > +	freezer_change_state(freezer->css.cgroup, goal_state);
> 
> __freezer_change_state(freezer->css.cgroup, goal_state);
> spin_unlock(&freezer->lock);
> 
> (where the __freezer_change_state() function expects to already have
> the freezer lock -- you can make that your first patch and this your
> second)
> 
> But you ought to double check the lock ordering, may-sleep, and
> whether the _irq variants are correct.
> 
I agree with the change to deal with race but again, I don't see the
harm of the race other than delaying one period. If the user has to
change period and duty ratio separately, there will always be a window
of unwanted params unless user disable it first.
Can you please explain the problem might be caused by the race.
> > +
> > +exit_toggle:
> > +	schedule_delayed_work(&freezer->freezer_work,
> > delay_jiffies);
> > +	freezer->toggle.freeze_thaw ^= 1;
> 
> This looks wrong. It looks like there could be a race between the next
> scheduled work and the toggling of the freeze_thaw value. This race
> would cause the cgroup to miss one or more duty cycles. You'd have
> to re-order these two lines and probably need an smp barrier of one
> sort or another between them.
> 
I will fix that. good point.


> Of course if you use locking and eliminate the toggle.freeze_thaw
> field as I've suggested then you can ignore this.
> 
same as before, not sure how to reuse the freezer spin lock for this.
can you please explain.
> > +}
> > +
> >  /*
> >   * cgroups_write_string() limits the size of freezer state strings
> > to
> >   * CGROUP_LOCAL_BUFFER_SIZE
> > @@ -150,7 +209,12 @@ static struct cgroup_subsys_state
> > *freezer_create(struct cgroup_subsys *ss, static void
> > freezer_destroy(struct cgroup_subsys *ss, struct cgroup *cgroup)
> >  {
> > -	kfree(cgroup_freezer(cgroup));
> > +	struct freezer *freezer;
> > +
> > +	freezer = cgroup_freezer(cgroup);
> > +	if (freezer->toggle.enabled)
> > +		cancel_delayed_work_sync(&freezer->freezer_work);
> > +	kfree(freezer);
> >  }
> > 
> >  /*
> > @@ -282,6 +346,16 @@ static int freezer_read(struct cgroup *cgroup,
> > struct cftype *cft, return 0;
> >  }
> > 
> > +static u64 freezer_read_duty_ratio(struct cgroup *cgroup, struct
> > cftype *cft) +{
> > +	return cgroup_freezer(cgroup)->duty.ratio;
> > +}
> > +
> > +static u64 freezer_read_period(struct cgroup *cgroup, struct
> > cftype *cft) +{
> > +	return cgroup_freezer(cgroup)->duty.period_pct_ms * 100;
> > +}
> > +
> >  static int try_to_freeze_cgroup(struct cgroup *cgroup, struct
> > freezer *freezer) {
> >  	struct cgroup_iter it;
> > @@ -353,6 +427,7 @@ static int freezer_write(struct cgroup *cgroup,
> >  {
> >  	int retval;
> >  	enum freezer_state goal_state;
> > +	struct freezer *freezer;
> > 
> >  	if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
> >  		goal_state = CGROUP_THAWED;
> > @@ -360,7 +435,18 @@ static int freezer_write(struct cgroup *cgroup,
> >  		goal_state = CGROUP_FROZEN;
> >  	else
> >  		return -EINVAL;
> > -
> > +	/* we should stop duty ratio toggling if user wants to
> > +	 * force change to a valid state.
> > +	 */
> > +	freezer = cgroup_freezer(cgroup);
> > +	if (freezer->duty.period_pct_ms && freezer->duty.ratio <
> > 100) {
> 
> If duty.ratio is 100 then the delayed work should be cancelled too.
> In fact it doesn't matter what the duty.ratio or period_pct_ms are --
> writes to this file should always disable the duty cycle. Thus
> you can omit the above if () and do this:
> 
> > +		if (freezer->toggle.enabled)
> 
agreed, i will fix it.
> 						{
> 
> > +
> > cancel_delayed_work_sync(&freezer->freezer_work);
> > +		freezer->duty.ratio = 0;
> 
> Actually, shouldn't this be 0 if the cgroup is going to be thawed and
> 100 if it's going to be frozen?
> 
I am using 0 as an invalid value when toggle is not enabled. perhaps i
should introduce -1 such that when user override occurs we just do
	freezer->toggle.enabled = 0;
	freezer->duty.ratio = -1;
	freezer->duty.period_pct_ms = -1;
then we can allow and or 100% where both will turn off toggle as well.

> > +		freezer->duty.period_pct_ms = 0;
> 
> I think this should always be a non-zero value -- even when duty
> cycling is disabled. Perhaps:
> 
> 		freezer->duty.period_pct_ms = 1000/100;
> 
> So it's clear the default period is 1000ms and one percent of it is
> 10ms.
> 
> (NOTE: To make it always non-zero you also need to add one line to the
> cgroup initialization code in freezer_create()).
how about -1 as suggested above.

> 
> > +		freezer->toggle.enabled = 0;
> > +		pr_info("freezer state changed by user, stop duty
> > ratio\n");
> 
> nit: I don't think this pr_info() is terribly useful.
> 
I will make it pr_debug instead.
> > +	}
> >  	if (!cgroup_lock_live_group(cgroup))
> >  		return -ENODEV;
> >  	retval = freezer_change_state(cgroup, goal_state);
> > @@ -368,12 +454,75 @@ static int freezer_write(struct cgroup
> > *cgroup, return retval;
> >  }
> > 
> > +#define FREEZER_KH_PREFIX  "freezer_"
> 
> This #define is unused.
fixed in v8
> 
> > +static int freezer_write_param(struct cgroup *cgroup, struct
> > cftype *cft,
> > +		u64 val)
> > +{
> > +	struct freezer *freezer;
> > +	int ret = 0;
> > +
> > +	freezer = cgroup_freezer(cgroup);
> > +
> > +	if (!cgroup_lock_live_group(cgroup))
> > +		return -ENODEV;
> 
> Because the delayed work function does not lock the cgroup this whole
> function can race with the delayed work. So we need to be careful
> about how we set/test all of these new fields -- we probably want to
> reuse the freezer state lock for this so I've sprinkled some
> lock/unlock bits with my comments below.
> 
same as before, I don't see the harm of the race.
> > +
> > +	switch (cft->private) {
> > +	case FREEZER_DUTY_RATIO:
> > +		if (val >= 100) {
> 
> ratio == 100 ought to be allowed too.
Ok, 100% frozen would be equivalent to echo FROZEN > freezer.state. I
will document these corner cases. I think as long as these behaviors
are documented well so that user can get the anticipated results, the
interface does matter that much.

> 
> > +			ret = -EINVAL;
> > +			goto exit;
> > +		}
> 
> Add:
> 
> 		spin_lock_irq(&freezer->lock);
> 
> > +		freezer->duty.ratio = val;
> 
> Because this can race with the delayed work.
> 
> > +		break;
> > +	case FREEZER_PERIOD:
> > +		do_div(val, 100);
> > +		freezer->duty.period_pct_ms = val;
> 
> This can race with the delayed work. Also I think that a 0ms
> period_pct_ms should be disallowed. Otherwise all the work delays go
> to zero and we'll probably peg the CPU so that it's just spinning the
> freezer state between FROZEN and THAWED and doing nothing else.
> 
0 or low number of period is dangerous for reason as you mentioned,
I think I should move back to one second resolution. Especially, we are
using common workqueue now.

> Finally, I wonder if a 64-bit value is really necessary -- is a period
> longer than roughly 50 days really necessary?
> 
> In summary, couldn't you just do something like:
> 
> 		if ((val < 100) || (val > UINT_MAX)) {
> 			/* Too much time or may have rounded down to
> 0 */ ret = -EINVAL;
> 			goto exit;
> 		}
> 		val = (unsigned int)val / 100;
> 		spin_lock_irq(&freezer->lock);
> 		freezer->duty.period_pct_ms = val;
> 
u64 is not needed. I will fix that.

> > +		break;
> > +	default:
> > +		BUG();
> > +	}
> > +
> > +	/* only use delayed work when valid params are given. */
> > +	if (freezer->duty.ratio && freezer->duty.period_pct_ms &&
> > +		!freezer->toggle.enabled) {
> > +		pr_debug("starting duty ratio mode\n");
> > +		INIT_DELAYED_WORK(&freezer->freezer_work,
> > freezer_work_fn);
> > +		freezer->toggle.enabled = 1;
> > +		schedule_delayed_work(&freezer->freezer_work, 0);
> > +	} else if ((!freezer->duty.ratio
> > || !freezer->duty.period_pct_ms) &&
> > +		freezer->toggle.enabled) {
> > +		pr_debug("invalid param, stop duty ratio mode
> > %p\n",
> > +			freezer->freezer_work.work.func);
> > +		cancel_delayed_work_sync(&freezer->freezer_work);
> > +		freezer->toggle.enabled = 0;
> > +		/* thaw the cgroup if we are not toggling */
> > +		freezer_change_state(freezer->css.cgroup,
> > CGROUP_THAWED); +
> > +	}
> 
> I don't think this is as readable as (assuming the change above to
> disallow setting period_pct_ms to 0):
> 
> 	if (freezer->duty.ratio == 100) {
> 		freezer_disable_duty_cycling(freezer); /* see helper
> below */ __freezer_change_state(freezer->css.cgroup, CGROUP_FROZEN);
> 	} else if (freezer->duty.ratio == 0) {
> 		freezer_disable_duty_cycling(freezer);
> 		__freezer_change_state(freezer->css.cgroup,
> CGROUP_THAWED); } else {
> 		if (freezer->toggle.enabled)
> 			goto exit; /* Already enabled */
> 		INIT_DELAYED_WORK(&freezer->freezer_work,
> freezer_work_fn); freezer->toggle.enabled = 1;
> 		schedule_delayed_work(&freezer->freezer_work, 0);
> 	}
> 	spin_unlock_irq(&freezer->lock);
> 
> Where the helper is:
> 
> static void freezer_disable_duty_cycling(struct freezer *freezer)
> {
> 	if (freezer->toggle.enabled) {
> 		cancel_delayed_work_sync(&freezer->freezer_work);
> 		freezer->toggle.enabled = 0;
> 	}
> }
> 
> which could also be called from freezer_write() I think.
>
Other than my doubt on spin lock i.e. __freezer_change_state(), i think
the helper function does make it cleaner.

Thanks again,

Jacob
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