Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS back to DEFAULT

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 08/14/2014 02:41 PM, Peter Zijlstra wrote:
> On Thu, Aug 14, 2014 at 01:14:49PM +0200, Daniel Lezcano wrote:
>> On 08/14/2014 01:00 PM, Peter Zijlstra wrote:
>>> On Thu, Aug 14, 2014 at 12:29:32PM +0200, Daniel Lezcano wrote:
>>>> Hi Chuansheng,
>>>>
>>>> On 14 August 2014 04:11, Chuansheng Liu <chuansheng.liu@intel.com> wrote:
>>>>
>>>>> We found sometimes even after we let PM_QOS back to DEFAULT,
>>>>> the CPU still stuck at C0 for 2-3s, don't do the new suitable C-state
>>>>> selection immediately after received the IPI interrupt.
>>>>>
>>>>> The code model is simply like below:
>>>>> {
>>>>>          pm_qos_update_request(&pm_qos, C1 - 1);
>>>>>                  < == Here keep all cores at C0
>>>>>          ...;
>>>>>          pm_qos_update_request(&pm_qos, PM_QOS_DEFAULT_VALUE);
>>>>>                  < == Here some cores still stuck at C0 for 2-3s
>>>>> }
>>>>>
>>>>> The reason is when pm_qos come back to DEFAULT, there is IPI interrupt to
>>>>> wake up the core, but when core is in poll idle state, the IPI interrupt
>>>>> can not break the polling loop.
>>>
>>> So seeing how you're from @intel.com I'm assuming you're using x86 here.
>>>
>>> I'm not seeing how this can be possible, MWAIT is interrupted by IPIs
>>> just fine, which means we'll fall out of the cpuidle_enter(), which
>>> means we'll cpuidle_reflect(), and then leave cpuidle_idle_call().
>>>
>>> It will indeed not leave the cpu_idle_loop() function and go right back
>>> into cpuidle_idle_call(), but that will then call cpuidle_select() which
>>> should pick a new C state.
>>>
>>> So the interrupt _should_ work. If it doesn't you need to explain why.
>>
>> I think the issue is related to the poll_idle state, in
>> drivers/cpuidle/driver.c. This state is x86 specific and inserted in the
>> cpuidle table as the state 0 (POLL). There is no mwait for this state. It is
>> a bit confusing because this state is not listed in the acpi / intel idle
>> driver but inserted implicitly at the beginning of the idle table by the
>> cpuidle framework when the driver is registered.
>>
>> static int poll_idle(struct cpuidle_device *dev,
>>                  struct cpuidle_driver *drv, int index)
>> {
>>          local_irq_enable();
>>          if (!current_set_polling_and_test()) {
>>                  while (!need_resched())
>>                          cpu_relax();
>>          }
>>          current_clr_polling();
>>
>>          return index;
>> }
>
> Ah, well, in that case there's a ton more broken than just this.
> kick_all_cpus_sync() won't work either, and cpuidle_reflect() pretty
> much expects to be called after each interrupt.

Agree.

> Then again, not reflecting properly isn't really a problem, its not like
> not accounting interrupts is going to safe power much.

I think the main issue here is to exit the poll_idle loop when an IPI is 
received. IIUC, there is a pm_qos user, perhaps a driver (Chuansheng can 
give more details), setting a very short latency, so the cpuidle 
framework choose a shallow state like the poll_idle and then the driver 
sets a bigger latency, leading to the IPI to wake all the cpus. As the 
CPUs are in the poll_idle, they don't exit until an event make them to 
exit the need_resched() loop (reschedule or whatever). This situation 
can let the CPUs to stand in the infinite loop several seconds while we 
are expecting them to exit the poll_idle and enter a deeper idle state, 
thus with an extra energy consumption.


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