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

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Message-ID: <CAN2waFs2tQFYs-ZGvY2i1e+PSBAjNwn1+C6N_J3tm8BkvXXVcQ@mail.gmail.com>
Date:	Wed, 22 Jun 2016 11:01:06 +0800
From:	Zhaoyang Huang <zhaoyang.huang@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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 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?