linux-kernel - Re: [PATCH v3]PM/Sleep: Timer quiesce in freeze state

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Date:	Tue, 27 Jan 2015 16:03:29 +0800
From:	"Li, Aubrey" <aubrey.li@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	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 2015/1/26 22:41, Rafael J. Wysocki wrote:
> 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.

Does the legacy ACPI system IO method to enter C2/C3 need interrupt
enabled as well?

Do we need some platform ops to cover those legacy platforms? Different
platform go different branch here.

Thanks,
-Aubrey

> 
>> 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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