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

[Peter Zijlstra <peterz@infradead.org>]

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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 here in the IPI callback interrupt, when currently the idle task is
> > running, we need to forcedly set reschedule bit to break the polling loop,
> > as for other non-polling idle state, IPI interrupt can break them directly,
> > and setting reschedule bit has no harm for them too.
> >
> > With this fix, we saved about 30mV power in our android platform.
> >
> > Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
> > ---
> >  drivers/cpuidle/cpuidle.c |    8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> > index ee9df5e..9e28a13 100644
> > --- a/drivers/cpuidle/cpuidle.c
> > +++ b/drivers/cpuidle/cpuidle.c
> > @@ -532,7 +532,13 @@ EXPORT_SYMBOL_GPL(cpuidle_register);
> >
> >  static void smp_callback(void *v)
> >  {
> > -       /* we already woke the CPU up, nothing more to do */
> > +       /* we already woke the CPU up, and when the corresponding
> > +        * CPU is at polling idle state, we need to set the sched
> > +        * bit to trigger reselect the new suitable C-state, it
> > +        * will be helpful for power.
> > +       */
> > +       if (is_idle_task(current))
> > +               set_tsk_need_resched(current);
> >
> 
> Mmh, shouldn't we inspect the polling flag instead ? Peter (Cc'ed) did some
> changes around this and I think we should ask its opinion. I am not sure
> this code won't make all cpu to return to the scheduler and go back to the
> idle task.

Yes, this is wrong.. Also cpuidle should not know about this, so this is
very much the wrong place to go fix this. Lemme have a look.

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