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

[Daniel Lezcano]

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@xxxxxxxxx> 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;
}


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