Re: [PATCH v2 0/3] Deferrable timers support for timerfd API

[Alexey Perevalov <a.perevalov@samsung.com>]

On 02/19/2014 02:33 AM, Thomas Gleixner wrote:
> On Tue, 18 Feb 2014, Alexey Perevalov wrote:
>> On 02/16/2014 07:39 PM, Thomas Gleixner wrote:
>> I figured out with deviation, I described before.
> Which is wrong to begin with. Using the wrong method does not justify
> the results. Are you actually trying to understand what I'm saying?
>
>> It was due expires and especially softexpires is fixed (don't base on delay).
> That's how an interval timer is supposed to work by definition. End of
> discussion.
>
>> For example if we have a timer like this:
>> hrtimer_expire_entry: hrtimer=ffffffffa056f280 function=timerfd_tmrproc
>> [hrtimers_mod] now=191450988244 expires=191400000000 softexpires=191400000000
>> It was fired at 191450988244, but softexpire is  191400000000, 50ms delay, if
>> I'm not wrong.
>> Next trigger time is 191700000000, (hrtimer_start: hrtimer=ffffffffa056f280
>> function=timerfd_tmrproc [hrtimers_mod] expires=191700000000
>> softexpires=191700000000)
>> and if there is no cpu idle at next time, we'll get 250ms interval for such
>> timer.
> This is complete nonsense. You schedule your hrtimer on an absolute
> timeline:
>
> 191400000000
> 191700000000
> ...
>
> So it's supposed to fire every 300ms, but it is allowed to fire later
> when the system is idle. And that's what it does. If the system would
> be idle for 10.3 seconds from the point where the timer is started
> then it would expire the first timer at
>
> 191400000000 + 10 sec = 201400000000
>
> and then schedule the next one at
>
> 201400000000 + 300ms = 201700000000
>
> So if your system is not idle the timer can be expired. That's the
> same for deferrable timer list timers. We expire them when a non
> deferrable timer fires.
>
> And you do the same for your timer list timer according to your trace:
>
> expires=4298169903
> expires=4298169978
> expires=4298170053
> expires=4298170128
> expires=4298170204
> expires=4298170287
> expires=4298170362
> expires=4298170462
> expires=4298170558
> expires=4298170637
> expires=4298170712
> expires=4298170787
> expires=4298170862
>
> The delta is always 75 ticks. And the expiry times are
>
> now=4298169903
> now=4298169978
> now=4298170053
> now=4298170129
> now=4298170212
> now=4298170287
> now=4298170387
> now=4298170483
> now=4298170562
> now=4298170637
> now=4298170712
> now=4298170787
>
> Which results in the deferrements:
>
> Delta:   0.0ms
> Delta:   0.0ms
> Delta:   0.0ms
> Delta:   1.0ms
> Delta:   8.0ms
> Delta:   0.0ms
> Delta:  25.0ms
> Delta:  21.0ms
> Delta:   4.0ms
> Delta:   0.0ms
> Delta:   0.0ms
> Delta:   0.0ms
>
> Avg:  	 4.0ms
>
> And why? Because you scheduled your timer along an absolute
> timeline. And if you use an absolute timeline, then the deltas between
> the actual timer events are completely irrelevant. The only thing what
> matters is the delta between the expected and the real expiry time.
>
> Is it really that hard to understand?
>
>> But we want 300ms or more for DEFERRABLE timer.
> And I want a pony!
>
> If you want that then simply setup the timer in relative oneshot mode,
> i.e. interval = 0 and when it expires (deferred) rearm it relative to
> now from user space. Then you get exactly the behaviour you want. It's
> that simple, really.
>
>> Thomas what do you think about moving format expire/softexpire to _!now!_ in
>> run_hrtimer, right before we
>> invoke callback function? The prolongation of hrtimer usually comes from user
>> timer functions by
>> invoking hrtimer_forward, which moves expires/softexpires forward.
> You really don't want to know what I think about that.
>
>> +       trace_hrtimer_expire_entry(timer, now, 0);
>> +
>> +       if (deferrable)
>> +               hrtimer_set_expires(timer, *now);
>>          restart = fn(timer);
>>
>>
>> I got expected results (timer interval is 300ms):
> So you got your pony. But it's your private pony and it stays that
> way, because you made the timer interval relative. And you managed to
> do that in the most disgusting way.
>
> In course of that you broke the behaviour of the existing user space
> interfaces. You can do so in your own hackery, but it's not going to
> go near mainline.
>
> Read and understand:
>
>     man timer_create
>     man timerfd
>
> along with the relevant standards.
>
> And if you need further education feel free to get a lecture from
> Linus about user space interfaces or google one if you really want to
> know how that works out.
>
> We are not going to special case that deferrable stuff, simply because
> it breaks the user space interfaces and you can solve your issue with
> the existing user space interfaces already.
>
> There is a simple solution for the problem. You just need to
> understand what you try to solve and use the proper mechanisms. And
> don't tell me you can't do that, because you need to modify your user
> space code anyway as CLOCK*DEFERRABLE does not exist yet.
In that the whole point CLOCK*DEFERRABLE doesn't exist yet, and such
reset of timer's expire didn't affect another clockids. Ok no problem, 
to set new expire time based on _now_ from user space.

>
> Just because you can do it in the kernel does not mean that it is the
> correct approach.
>
> Thanks,
>
> 	tglx
>


-- 
Best regards,
Alexey Perevalov