linux-kernel - RE: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM

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Date:	Thu, 14 Aug 2014 14:10:27 +0000
From:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Liu, Changcheng" <changcheng.liu@...el.com>,
	"Wang, Xiaoming" <xiaoming.wang@...el.com>,
	"Chakravarty, Souvik K" <souvik.k.chakravarty@...el.com>
Subject: RE: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS
 back to DEFAULT



> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@...radead.org]
> Sent: Thursday, August 14, 2014 9:13 PM
> To: Liu, Chuansheng
> Cc: Daniel Lezcano; Rafael J. Wysocki; linux-pm@...r.kernel.org; LKML; Liu,
> Changcheng; Wang, Xiaoming; Chakravarty, Souvik K
> Subject: Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS
> back to DEFAULT
> 
> On Thu, Aug 14, 2014 at 11:24:06AM +0000, Liu, Chuansheng wrote:
> > If inspecting the polling flag, we can not fix the race between poll_idle and
> smp_callback,
> > since in poll_idle(), before set polling flag, if the smp_callback come in, then
> no resched bit set,
> > after that, poll_idle() will do the polling action, without reselection
> immediately, it will bring power
> > regression here.
> 
> -ENOPARSE. Is there a question there?

Lezcano suggest to inspect the polling flag, then code is like below:
smp_callback() {
if (polling_flag)
  set_resched_bit;
}

And the poll_idle code is like below:
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;
}

The race is:
Idle task:
poll_idle
  local_irq_enable()
<== IPI interrupt coming, check the polling flag is not set yet, do nothing;
Come back to poll_idle, it will stay in the poll loop for a while, instead break
it immediately to let governor reselect the right C-state.






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