Date:	Mon, 26 Jan 2015 15:41:44 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Li, Aubrey" <aubrey.li@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Brown, Len" <len.brown@...el.com>,
	"alan@...ux.intel.com" <alan@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v3]PM/Sleep: Timer quiesce in freeze state

On Monday, January 26, 2015 10:40:24 AM Thomas Gleixner wrote:
> On Mon, 26 Jan 2015, Li, Aubrey wrote:
> > On 2015/1/22 18:15, Thomas Gleixner wrote:

[...]

> > >> +		/*
> > >> +		 * cpuidle_enter will return with interrupt enabled
> > >> +		 */
> > >> +		cpuidle_enter(drv, dev, next_state);
> > > 
> > > How is that supposed to work?
> > > 
> > > If timekeeping is not yet unfrozen, then any interrupt handling code
> > > which calls anything time related is going to hit lala land.
> > > 
> > > You must guarantee that timekeeping is unfrozen before any interrupt
> > > is handled. If you cannot guarantee that, you cannot freeze
> > > timekeeping ever.
> > > 
> > > The cpu local tick device is less critical, but it happens to work by
> > > chance, not by design.
> > 
> > There are two way to guarantee this: the first way is, disable interrupt
> > before timekeeping frozen and enable interrupt after timekeeping is
> > unfrozen. However, we need to handle wakeup handler before unfreeze
> > timekeeping to wake freeze task up from wait queue.
> > 
> > So we have to go the other way, the other way is, we ignore time related
> > calls during freeze, like what I added in irq_enter below.
> 
> Groan. You just do not call in irq_enter/exit(), but what prevents any
> interrupt handler or whatever to call into the time/timer code after
> interrupts got reenabled?
> 
> Nothing. 
> 
> > Or, we need to re-implement freeze wait and wake up mechanism?
> 
> You need to make sure in the low level idle implementation that this
> cannot happen.
> 
> tick_freeze()
> {
> 	raw_spin_lock(&tick_freeze_lock);
> 	tick_frozen++;
> 	if (tick_frozen == num_online_cpus())
> 		timekeeping_suspend();
> 	else
> 		tick_suspend_local();
> 	raw_spin_unlock(&tick_freeze_lock);
> }
> 
> tick_unfreeze()
> {
> 	raw_spin_lock(&tick_freeze_lock);
> 	if (tick_frozen == num_online_cpus())
> 		timekeeping_resume();
> 	else
> 		tick_resume_local();
> 	tick_frozen--;
> 	raw_spin_unlock(&tick_freeze_lock);
> }
> 
> idle_freeze()
> {
> 	local_irq_disable();
> 
> 	tick_freeze();
> 
> 	/* Must keep interrupts disabled! */
>        	go_deep_idle()
> 
> 	tick_unfreeze();
> 
> 	local_irq_enable();
> }
> 
> That's the only way you can do it proper, everything else will just be
> a horrible mess of bandaids and duct tape.
> 
> So that does not need any of the irq_enter/exit conditionals, it does
> not need the real_handler hack. It just works.

As long as go_deep_idle() above does not enable interrupts.  This means we won't
be able to use some C-states for suspend-to-idle (hald-induced C1 on some x86
for one example), but that's not a very big deal.

> The only remaining issue might be a NMI calling into
> ktime_get_mono_fast_ns() before timekeeping is resumed. Its probably a
> non issue on x86/tsc, but it might be a problem on other platforms
> which turn off devices, clocks, It's not rocket science to prevent
> that.

I don't see any users of ktime_get_mono_fast_ns() at all, unless some non-trivial
macros are involved.  At least grepping for it only returns the definition,
declarations and the line in trace.c.

Rafael

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