linux-kernel - Re: [PATCH resend4 2/3] itimers: fix periodic tics precision

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Date:	Tue, 26 May 2009 08:44:55 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linuxppc-dev@...abs.org
Subject: Re: [PATCH resend4 2/3] itimers: fix periodic tics precision

On Mon, 25 May 2009 14:51:32 +0200
Stanislaw Gruszka <sgruszka@...hat.com> wrote:

> On Mon, 25 May 2009 14:32:14 +0200 (CEST)
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Mon, 25 May 2009, Stanislaw Gruszka wrote:
> > > @@ -904,6 +905,7 @@ void __init time_init(void)
> > >  	tb_ticks_per_usec = ppc_tb_freq / 1000000;
> > >  	tb_to_us = mulhwu_scale_factor(ppc_tb_freq, 1000000);
> > >  	calc_cputime_factors();
> > > +	cputime_one = jiffies_to_cputime(1);
> > 
> >   1) The variable name is misleading.
> 
> What about cputime_one_jiffy ?
> 
> >   2) The patch breaks all powerpc platforms which have
> >   CONFIG_VIRT_CPU_ACCOUNTING=n and ia64 with
> >   CONFIG_VIRT_CPU_ACCOUNTING=y
> 
> Stupid me, in asm-generic/cputime.h should be 
> #define cputime_one jiffies_to_cputime(1)

Hmmm, I'm confused. Perhaps I missed something, but I think patch was ok.
For powerpc and ia64 and CONFIG_VIRT_CPU_ACCOUNTING=n definitions from
asm-generic/cputime.h where used. In this file was:

#define cputime_one (1UL) 

and that correct as jiffies_to_cputime(x) is just (x)

For CONFIG_VIRT_CPU_ACCOUTING=y:
- For powerpc additional variable was  declared and computed in
  initialization  time. Declaration of was in __KERENEL__ scope.
- For ia64: cputime_one was defined as jiffies_to_cputime(1)

Anyway I didn't try to even compile the patch on other architectures than x86.
Of cource I will test my patch, but first I would like to know what You think?
Does we really need such optimization (because before usage of
jiffies_to_cputime(1) was just fine) ?

Cheers
Stanislaw
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