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

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Message-ID: <53ECC4EB.20407@linaro.org>
Date:	Thu, 14 Aug 2014 16:17:15 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	"Liu, Chuansheng" <chuansheng.liu@...el.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"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

On 08/14/2014 04:10 PM, Liu, Chuansheng wrote:
>
>
>> -----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())

Or alternatively, something like:

	while (!need_resched() || kickme) {
		...
	}
			

smp_callback()
{
	kickme = 1;
}

kickme is a percpu variable and set to zero when exiting the 'enter' 
callback.

So we don't mess with the polling flag, which is already a bit tricky.

This patch is very straightforward to illustrate the idea.

>                          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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