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

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Date:	Fri, 3 Apr 2009 14:59:03 +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

On Thu, 2 Apr 2009 18:57:53 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> 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.

Removing cputime stuff from itimers has probably only sense when
utime, stime and related fields in task_struct would be represented
as ktime or u64 variable in nanoseconds accurate. This mean a lot of
work. I'm not sure if is worth to do in the meaning that as result we
get better (faster and perhaps smaller) code. 

I was thinking about removing cputime as whole, make utime and stime
64 bit variables and account them in nanoseconds resolution. Remove
sum_exec_runtime from struct task_cputime and related CPUCLOCK_SCHED
code as duplicate of nanosecond accounted stime and CPUCLOCK_PROF code.
But that were too intrusive changes for me with unknown performance
impact.

My primary goal is to improve periodic itimers accuracy (see 
https://bugzilla.redhat.com/show_bug.cgi?id=441134), these patches
are just enough to achieve the goal.

Cheers
Stanislaw Gruszka
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