Re: [RFC][PATCH] freezer,sched: Rewrite core freezer logic

[Will Deacon]

On Thu, Jun 03, 2021 at 12:35:22PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 02, 2021 at 01:54:53PM +0100, Will Deacon wrote:
> 
> > There's also Documentation/power/freezing-of-tasks.rst to update. I'm not
> 
> Since it's .rst, the only update I'm willing to do is delete it
> outright.

Hah! Well, don't do that.

> > sure if fs/proc/array.c should be updated to display frozen tasks; I
> > couldn't see how that was useful, but thought I'd mention it anyway.
> 
> Yeah, I considered it too, but I figured that if we're all frozen
> there's noone left to observe us being frozen, so I didn't bother.

Agreed.

> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index 2982cfab1ae9..bfadc1dbcf24 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -95,7 +95,12 @@ struct task_group;
> > >  #define TASK_WAKING			0x0200
> > >  #define TASK_NOLOAD			0x0400
> > >  #define TASK_NEW			0x0800
> > > -#define TASK_STATE_MAX			0x1000
> > > +#define TASK_FREEZABLE			0x1000
> > > +#define __TASK_FREEZABLE_UNSAFE		0x2000
> > 
> > Give that this is only needed to avoid lockdep checks, maybe we should avoid
> > allocating the bit if lockdep is not enabled? Otherwise, people might start
> > to use it for other things.
> 
> Something like
> 
> #define __TASK_FREEZABLE_UNSAFE			(0x2000 * IS_ENABLED(CONFIG_LOCKDEP))
> 
> ?

Yup.

> > > +#define TASK_FROZEN			0x4000
> > > +#define TASK_STATE_MAX			0x8000
> > > +
> > > +#define TASK_FREEZABLE_UNSAFE		(TASK_FREEZABLE | __TASK_FREEZABLE_UNSAFE)
> > 
> > We probably want to preserve the "DO NOT ADD ANY NEW CALLERS OF THIS STATE"
> > comment for the unsafe stuff.
> 
> Done.

Thanks.

> > > +/* Recursion relies on tail-call optimization to not blow away the stack */
> > > +static bool __frozen(struct task_struct *p)
> > > +{
> > > +	if (p->state == TASK_FROZEN)
> > > +		return true;
> > 
> > READ_ONCE()?
> 
> task_struct::state is volatile -- for now. I've got other patches to
> deal with that.

Thanks, I missed that and have since reviewed your other series.

> > > @@ -116,20 +173,8 @@ bool freeze_task(struct task_struct *p)
> > >  {
> > >  	unsigned long flags;
> > >  
> > >  	spin_lock_irqsave(&freezer_lock, flags);
> > > +	if (!freezing(p) || frozen(p) || __freeze_task(p)) {
> > >  		spin_unlock_irqrestore(&freezer_lock, flags);
> > >  		return false;
> > >  	}
> > 
> > I've been trying to figure out how this serialises with ttwu(), given that
> > frozen(p) will go and read p->state. I suppose it works out because only the
> > freezer can wake up tasks from the FROZEN state, but it feels a bit brittle.
> 
> p->pi_lock; both ttwu() and __freeze_task() (which is essentially a
> variant of set_special_state()) take ->pi_lock. I'll put in a comment.

The part I struggled with was freeze_task(), which doesn't take ->pi_lock
yet calls frozen(p).

> 
> > > @@ -137,7 +182,7 @@ bool freeze_task(struct task_struct *p)
> > >  	if (!(p->flags & PF_KTHREAD))
> > >  		fake_signal_wake_up(p);
> > >  	else
> > > -		wake_up_state(p, TASK_INTERRUPTIBLE);
> > > +		wake_up_state(p, TASK_INTERRUPTIBLE); // TASK_NORMAL ?!?
> > >  
> > >  	spin_unlock_irqrestore(&freezer_lock, flags);
> > >  	return true;
> > > @@ -148,8 +193,8 @@ void __thaw_task(struct task_struct *p)
> > >  	unsigned long flags;
> > >  
> > >  	spin_lock_irqsave(&freezer_lock, flags);
> > > -	if (frozen(p))
> > > -		wake_up_process(p);
> > > +	WARN_ON_ONCE(freezing(p));
> > > +	wake_up_state(p, TASK_FROZEN | TASK_NORMAL);
> > 
> > Why do we need TASK_NORMAL here?
> 
> It's a left-over from hacking, but I left it in because anything
> TASK_NORMAL should be able to deal with spuriuos wakeups, something
> try_to_freeze() now also relies on.

I just worry that it might hide bugs if TASK_FROZEN is supposed to be
sufficient, as it would imply that we have some unfrozen tasks kicking
around. I dunno, maybe just a comment saying that everything _should_ be
FROZEN at this point?

Will