Re: get_cycles() on i386

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

Any concerns? 

On 4 Nov 2003, john stultz wrote:

> On Tue, 2003-11-04 at 15:22, Joel Becker wrote:
> > Folks,
> > 	Certain distributions are building all of their SMP kernels
> > NUMA-aware.  This is great, as the kernels support boxes like the x440
> > with no trouble.  However, this implicitly disables CONFIG_X86_TSC.
> > While that is good for NUMA systems, and fine from a kernel timing
> > standpoint, it also eliminates any generic access to the TSC via
> > get_cycles().  With CONFIG_X86_TSC not defined, get_cycles() always
> > returns 0.
> > 	Given that >95% of machines will not be x440s, this means that a
> > user of that kernel cannot access a high resolution timer via
> > get_cycles().  I don't want to have to litter my code with rdtscll()
> > when I managed to remove it!
> > 	The proposed patch is trivial.  If the system has a TSC, it is
> > available get_cycles().  This makes no change to the other parts of the
> > kernel protected by CONFIG_X86_TSC.
> 
> CONFIG_X86_TSC be the devil. Personally, I'd much prefer dropping the
> compile time option and using dynamic detection. Something like (not
> recently tested and i believe against 2.5.something, but you get the
> idea):
> 
> 
> diff -Nru a/include/asm-i386/timex.h b/include/asm-i386/timex.h
> --- a/include/asm-i386/timex.h	Mon Feb 24 21:09:32 2003
> +++ b/include/asm-i386/timex.h	Mon Feb 24 21:09:32 2003
> @@ -40,14 +40,10 @@
>  
>  static inline cycles_t get_cycles (void)
>  {
> -#ifndef CONFIG_X86_TSC
> -	return 0;
> -#else
> -	unsigned long long ret;
> -
> -	rdtscll(ret);
> +	unsigned long long ret = 0;
> +	if(cpu_has_tsc)
> +		rdtscll(ret);
>  	return ret;
> -#endif
>  }
>  
>  extern unsigned long cpu_khz;

John, Joel, 

I believe this is reliable. I'll apply it.

Any concerns?