[Devel] Re: [PATCH 1/1, v6] cgroup/freezer: add per freezer duty ratio control

[Matt Helsley]

On Thu, Feb 10, 2011 at 11:15:22AM +0200, Kirill A. Shutemov wrote:
> On Wed, Feb 09, 2011 at 07:04:42PM -0800, Matt Helsley wrote:
> > > +{
> > > +	struct cgroup *cgroup = (struct cgroup *)data;
> > > +	struct freezer *freezer = cgroup_freezer(cgroup);
> > > +
> > > +	do {
> > > +		if (freezer->duty.ratio < 100 && freezer->duty.ratio > 0 &&
> > > +			freezer->duty.period_pct_ms) {
> > > +			if (try_to_freeze_cgroup(cgroup, freezer))
> > > +				pr_info("cannot freeze\n");
> > > +			msleep(freezer->duty.period_pct_ms *
> > > +				freezer->duty.ratio);
> > > +			unfreeze_cgroup(cgroup, freezer);
> > > +			msleep(freezer->duty.period_pct_ms *
> > > +				(100 - freezer->duty.ratio));
> > > +		} else {
> > > +			sleep_on(&freezer_wait);
> > > +			pr_debug("freezer thread wake up\n");
> > > +		}
> > > +	} while (!kthread_should_stop());
> > > +	return 0;
> > > +}
> > 
> > Seems to me you could avoid the thread-per-cgroup overhead and the
> > sleep-loop code by using one timer-per-cgroup. When the timer expires
> > you freeze/thaw the cgroup associated with the timer, setup the next
> > wakeup timer, and use only one kernel thread to do it all. If you
> > use workqueues you might even avoid the single kernel thread.
> > 
> > Seems to me like that'd be a good fit for embedded devices.
> 
> I proposed to use delayed workqueues (schedule_delayed_work()).

Even better.

> 
> > > +#define FREEZER_KH_PREFIX  "freezer_"
> > > +static int freezer_write_param(struct cgroup *cgroup, struct cftype *cft,
> > > +		u64 val)
> > > +{
> > > +	struct freezer *freezer;
> > > +	char thread_name[32];
> > > +	int ret = 0;
> > > +
> > > +	freezer = cgroup_freezer(cgroup);
> > > +
> > > +	if (!cgroup_lock_live_group(cgroup))
> > > +		return -ENODEV;
> > > +
> > > +	switch (cft->private) {
> > > +	case FREEZER_DUTY_RATIO:
> > > +		if (val >= 100 || val < 0) {
> > > +			ret = -EINVAL;
> > > +			goto exit;
> > > +		}
> > > +		freezer->duty.ratio = val;
> > 
> > Why can't val == 100? At that point it's always THAWED and no kernel
> > thread is necessary (just like at 0 it's always FROZEN and no kernel
> > thread is necessary).
> 
> val == 100 is interface abuse, I think. I just turn off the feature, if
> you want.

And how is userspace supposed to do that at runtime if we can't disable
it by writing to the state file (see below)? Then I don't see anyway
to get rid of the duty cycling unless you clear out the cgroup and
recreate it.

Frankly, I think 0 and 100 percent aren't interface abuse. Anybody
who knows it's a percent value will naturally try to put 0 or 100
there.

> > >  static struct cftype files[] = {
> > >  	{
> > >  		.name = "state",
> > >  		.read_seq_string = freezer_read,
> > >  		.write_string = freezer_write,
> > 
> > It's not clear what should happen when userspace writes the state
> > file after writing a duty_ratio_pct.
> 
> It should return -EBUSY, I think.

Ahh, that is another solution I hadn't considered. That further proves my
point though :). It's not obvious what should happen and that's a red-flag
that we're defining policy and should be careful which solution we select.

> 
> > >  	},
> > > +	{
> > > +		.name = "duty_ratio_pct",
> > > +		.private = FREEZER_DUTY_RATIO,
> > > +		.read_u64 = freezer_read_duty_ratio,
> > > +		.write_u64 = freezer_write_param,
> > > +	},
> > 
> > nit: Why use a u64 for a value that can only be 0-100? (or perhaps
> > 0-1000 if you wanted sub-1% granularity...)
> 
> .read_u64/.write_64 is a standard cgroup's interface.

Oops -- I was thinking there was a smaller variant of these.

Cheers,
	-Matt Helsley
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