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

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Date:	Wed, 28 Jan 2015 08:17:50 +0800
From:	"Li, Aubrey" <aubrey.li@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	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 2015/1/27 23:10, Rafael J. Wysocki wrote:
> On Tuesday, January 27, 2015 04:03:29 PM Li, Aubrey wrote:
>> 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.
> 
> No, we don't.
> 
> I think this needs to be addressed in a different way overall.  If you don't
> mind, I'd like to prepare my own version of the patch at this point.  That
> likely will be simpler than trying to explain what I'd like to do and I guess
> I'll need a few iterations to get something acceptable anyway.

Sure, please go ahead and just keep me posted.

Thanks,
-Aubrey
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