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Date:	Thu, 06 Feb 2014 21:38:45 +0400
From:	Alexey Perevalov <>
To:	Thomas Gleixner <>,
	John Stultz <>
Subject: Re: [PATCH v2 0/3] Deferrable timers support for timerfd API

On 02/06/2014 02:16 AM, Thomas Gleixner wrote:
> On Wed, 5 Feb 2014, John Stultz wrote:
>> On 02/05/2014 01:41 PM, Thomas Gleixner wrote:
>>> On Wed, 5 Feb 2014, Alexey Perevalov wrote:
>>>> On 02/04/2014 08:10 PM, Thomas Gleixner wrote:
>>>>> On Mon, 27 Jan 2014, Alexey Perevalov wrote:
>>>>>> On 01/21/2014 11:12 PM, John Stultz wrote:
>>>>>>> Thomas: Any thought here? Should we be trying to unify the timerfd flags
>>>>>>> and the posix timer flags (specifically things like TIMER_CANCEL_ON_SET,
>>>>>>> which is currently timerfd-only)?  Should a deferrable flag be added to
>>>>>>> the hrtimer core or left to the timer wheel?
>>>>> The timer cancel on set was added only to timerfd because timerfd is a
>>>>> non posix interface and we are halfways free to add stuff to
>>>>> it. Adding extra flags to the real posix timer interfaces is a
>>>>> different story.
>>>> And what about "deferrable" possibility for hrtimers, do you consider it
>>>> reasonable?
>>> In principle, I have no objections, but we need a proper technical
>>> solution. Just adding a flag and keeping the timers in the same rbtree
>>> like we do for the timer wheel timers is not going to happen.
>>> The only feasible solution is to have separate clock ids,
>>> e.g. CLOCK_*_DEFERRABLE, which would enable the deferrable
>>> functionality for all user space interfaces. No need for magic flags
>>> and complex search for non deferrable timers.
>> So of course, I was actually arguing against having a new clockid (which
>> was Alexey's first approach).
> Mooo.
>> My reasoning was that the deferrablity isn't a clock domain, and is more
>> of a modifier. Thus to keep the interfaces somewhat sane (and avoiding
>> having to add N new clockids for each new modifier), we should utilize
>> the flag arguments to timers. So instead of just having TIMER_ABSTIME,
>> we could add TIMER_DEFER, etc, which we could utilize instead.
> I can see the point. I have no objections against that approach as
> long as we map that against separate internal bases.
>> Internally we can still keep separate bases, much as your patch does, to
>> keep the next-event searching overhead more limited.
> It's not only more limited, it's bound.
>> I mainly wanted to get your thoughts on extending the flags, and doing
>> so in a consistent manner between the timerfd and other timer interfaces.
> So the only interface which does not support that is sys_nanosleep()
> but that's not really an issue. sys_nanosleep() should die anyway :)
>> Of course, all this is after I added the _ALARM clockids... so you can
>> decide if its hypocrisy or experience.
>> (The "old wisdom comes from experience and experience comes from bad
>> decisions" bit ;).
> Well, you have a valid point about the clock ids. I did not realize in
> the first place that we can avoid that business if we use the flags to
> select the internal representation.
> Either way is preferred over reintroducing the timer wheel mess....
> Thanks,
> 	tglx

As I truly understand, you decided - flags is better than new clockids, 
and internals of timerfd could be a mix of timer_list and hrtimer.
If so, it's in v2 patch set.

Best regards,
Alexey Perevalov
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