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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
