linux-kernel - Re: [PATCH 0/2] itimers: periodic timers fixes

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Date:	Wed, 15 Apr 2009 14:22:32 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] itimers: periodic timers fixes

Hi Ingo.

On Thu, 2 Apr 2009 18:57:53 +0200
Ingo Molnar <mingo@...e.hu> wrote:
> > We found the periodic timers ITIMER_PROF and ITIMER_VIRT are 
> > unreliable, they have systematic timing error. For example period 
> > of 10000 us will not be represented by the kernel as 10 ticks, but 
> > 11 (for HZ=1000). The reason is that the frequency of the hardware 
> > timer can only be chosen in discrete steps and the actual 
> > frequency is about 1000.152 Hz. So 10 ticks would take only about 
> > 9.9985 ms, the kernel decides it must never return earlier than 
> > requested, so it rounds the period up to 11 ticks. This results in 
> > a systematic multiplicative timing error of -10 %. The situation 
> > is even worse where application try to request with 1 thick 
> > period. It will get the signal once per two kernel ticks, not on 
> > every tick. The systematic multiplicative timing error is -50 %. 
> > He have program [1] that shows itimers systematic error, results 
> > are below [2].
> > 
> > To fix situation we wrote two patches. First one just simplify 
> > code related with itimers. Second is fix, it change intervals 
> > measurement resolutions and correct times when signal is 
> > generated. However this add some drawback, that I'm not sure if 
> > are acceptable:
> > 
> > - the time between two consecutive tics can be smaller than 
> >   requested interval
> > 
> > - intervals values which are returned to user by getitimer() are 
> >   not rounded up
> > 
> > Second drawback mean that applications which first call 
> > setitimer() then call getitimer() to see if interval was round up 
> > and to correct timings, will potentially stop works. However this 
> > can be only problem with requested interval smaller than 1/HZ, as 
> > for intervals > 1/Hz we can generate signals with proper 
> > resolution.
> 
> Converting those to GTOD sampling instead of jiffies sampling is a 
> worthwile change IMO and a good concept.
> 
> The unificaton of ITIMER_PROF and ITIMER_VIRT is a nice observation 
> and a good patch.
> 
> The second one, changing all the sampling from cputime to ktime_t is 
> nicely done too:
> 
> We could do more though, there's still a bit of cputime legacies 
> around:
> 
> +	cputime_t cval, nval;
> 
> Couldnt all of that go over into the ktime_t space as well, phasing 
> out cputime logic from the itimer code?
> 
> The user ABI is struct timeval based, so there's no need to have 
> cputime anywhere. The scheduler does nanoseconds accurate stats so 
> it can be connected up there too.

Could the patches be merged and possible other work done in later time?
Or perhaps I should rework on them?

Regards
Stanislaw
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