linux-kernel - Re: [PATCH 0/3] Deferrable timers support for timerfd API

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Message-id: <52C9B39C.2000901@samsung.com>
Date:	Sun, 05 Jan 2014 23:33:48 +0400
From:	Alexey Perevalov <a.perevalov@...sung.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel@...r.kernel.org, anton@...msg.org,
	kyungmin.park@...sung.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 0/3] Deferrable timers support for timerfd API

On 01/04/2014 04:18 AM, John Stultz wrote:
> On 01/03/2014 09:45 AM, Alexey Perevalov wrote:
>> On 01/03/2014 03:17 AM, John Stultz wrote:
>>> On 01/02/2014 10:30 AM, Alexey Perevalov wrote:
>>>> This version introduces new clockid (CLOCK_DEFERRABLE) , for
>>>> timerfd_create, instead of
>>>> new flag (TFD_TIMER_DEFERRABLE) for timerfd_settime introduced in
>>>> previous version.
>>> So why did you make this change?
>>>
>>> thanks
>>> -john
>>>
>>>
>> I looked at alarm timers and found approach of making timer behavior
>> persistent per file descriptor is better than
>> changeable by timerfd_settime. I think "end user wake up from suspend"
>> and "don't wake up in idle" is the same thing on the same abstraction
>> level.
>>
>> Yes Anton's previous  patches worked with CLOCK_MONOTONIC only and I
>> didn't intend to use it with CLOCK_REALTIME, cause it's hard to me to
>> find such use case.
>> Another way - it's stay as was Anton's patch, I mean as flag for the
>> timerfd_settime, but in original patch set both hrtimer and deferrable
>> timers initialized in timerfd_create, I think it's not needed. Also
>> ability to change timer behavior looks not good if you couldn't change
>> alarm timer behavior, not unified API.
> So while the alarm timers are a reasonable precedent, I think they were
> introduced prior to the timerfd interface, so it seemed at the time
> having new clockids for the functionality was required to work with the
> existing syscalls that use the clockid (Though in retrospect, I question
> if it would have been better to use timer flags to introduce the alarm
> functionality rather then introducing it via a clockid, as it would
> simplify the clockid definitions).
As I understood alarm and deferrability it's type of repetition (timer 
trigger condition),
but REALTIME, BOOTTIME, MONOTONIC it's a type of time representation.
Mixed it in one clockid, maybe it's a controversially. Which flags do 
you want to use, flags of timerfd_settime?

>
> Now that we have the timerfd interface, and if this functionality is
> really limited to the timerfds, then we may want to consider what might
> be, at least to me, the cleaner approach of using the flag.
>
> Either way, I'd like to make sure we have a sound rational. My worry is
> that deferrable timers would be desired on more then just
> CLOCK_MONOTONIC, so we could quite likely end up with quite a few new
> clockids (ie: CLOCK_BOOTTIME_DEFERRED, CLOCK_TAI_DEFERRED,
> CLOCK_REALTIME_DEFERRED).
>
Here I'm totally agree CLOCK_DEFERRABLE is not maintainable named constant.


>> If I'm right, only high resolution timer could be REALTIME, and there
>> is no deferrable behavior for hrtimer only for timer_list.
>>
>>
> I'm not sure I understood this part. Could you explain further?
I meant, we couldn't use hrtimer for deferrability, right now.

>
> thanks
> -john
>
>


-- 
Best regards,
Alexey Perevalov
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