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

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 26 May 2009, Stanislaw Gruszka wrote:
> On Mon, 25 May 2009 14:51:32 +0200
> Stanislaw Gruszka <sgruszka@redhat.com> wrote:
> 
> > On Mon, 25 May 2009 14:32:14 +0200 (CEST)
> > Thomas Gleixner <tglx@linutronix.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) 

  In time_init you add unconditionally:

> > > > @@ -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);

  I doubt that the compiler will happy about that as it expands to

      (1UL) = jiffies_to_cputime(1)
  or 
      jiffies_to_cputime(1) = jiffies_to_cputime(1)

  in the CONFIG_VIRT_CPU_ACCOUTING=n case.
 
> 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)

  My bad, I missed the ia64 hunk.
 
> Does we really need such optimization (because before usage of
> jiffies_to_cputime(1) was just fine) ?

Well, there is a subtle difference between working and nobody noticing
the overhead.

If we notice such heavy instructions in a code path we change then
ignoring the optimization with the argument that it worked before is
just plain wrong.

Thanks,

	tglx