linux-kernel - Re: [RESEND PATCH v2 2/2] power/idle: enhance the precision of sleep

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Date:	Thu, 23 Jun 2016 15:26:40 +0800
From:	Zhaoyang Huang <zhaoyang.huang@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geng Ren <geng.ren@...eadtrum.com>,
	Alex Wang <alex.wang@...eadtrum.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	mingo@...hat.com, peterz@...radead.org,
	Zhaoyang Huang (黄朝阳) 
	<zhaoyang.huang@...eadtrum.com>
Subject: Re: [RESEND PATCH v2 2/2] power/idle: enhance the precision of sleep_length

On 23 June 2016 at 15:01, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 22 Jun 2016, Zhaoyang Huang wrote:
>> On 20 June 2016 at 09:14, Zhaoyang Huang <zhaoyang.huang@...aro.org> wrote:
>> > On 17 June 2016 at 19:50, Thomas Gleixner <tglx@...utronix.de> wrote:
>> >> On Fri, 17 Jun 2016, Zhaoyang Huang wrote:
>> >>> On 17 June 2016 at 17:27, Thomas Gleixner <tglx@...utronix.de> wrote:
>> >>> > On Fri, 17 Jun 2016, Zhaoyang Huang wrote:
>> >>> >> There should be a gap between tick_nohz_idle_enter and
>> >>> >> tick_nohz_get_sleep_length when idle, which will cause the
>> >>> >> sleep_length is not very precised. Change it in this patch.
>> >>> >
>> >>> > What kind of imprecision are we talking about? Seconds, nanoseconds or
>> >>> > lightyears?
>> >>> >
>> >>> > Your changelog lacks any form of useful information.
>> >>> >
>> >>> sorry for the confusion. The imprecision can be caused by, for
>> >>> example, the callback function registered for CPU_PM_ENTER, which may
>> >>> consume a period of time within the 'idle' time. Besides, I also
>> >>> wonder why not calc the 'sleep_length' in the
>> >>> tick_nohz_get_sleep_length?  This value is calculated at very
>> >>> beginning of the idle in current approach.
>> >>
>> >> You still are not explaining the amount of imprecision. What are you talking
>> >> about and is it really relevant in any way or are you just trying to solve an
>> >> acedemic issue?
>> >>
>> >> Thanks,
>> >>
>> >>         tglx
>> >>
>> > Indeed, it is depends on how deep the idle state is. For example, the
>> > lightest level for my current platform is 1100us, which sums up the
>> > entry,exit and min time. However, there is a callback which do memory
>> > management(merge the same page) in CPU_PM_ENTER will consume at least
>> > 500us. In current approach, it cause 50% imprecision for this level of
>> > idle state.
>> Hi Thomas,
>> Any further comments on that?
>
> Yes. Why on earth do we have a 500us callback in the idle entry path? That's
> just insane and needs to be fixed.
>
> Thanks,
>
>         tglx
Thomas, I agree with you, I have discussed the modification with the
call back owner. However, I wonder if we can make the idle's framework
to be more precised without the assumption of short CPU_PM_ENTER
callbacks. Thank you!