Re: [PPC64] Remove silly debug path from get_vsid()

[Andreas Dilger <adilger@clusterfs.com>]

On May 28, 2004  13:16 +1000, David Gibson wrote:
> This patch removes the test, replacing it with a compile time switch.
> It seems to make a measurable, although small speedup to the SLB miss
> path (maybe 0.5%).

Looking at include/asm-ppc64/ppcdebug.h the ifppcdebug() macro is defined:

#ifdef CONFIG_PPCDBG
#define ifppcdebug(FLAGS) if (udbg_ifdebug(FLAGS))
#else
#define ifppcdebug(...) if (0)
#endif

so presumably if you don't select CONFIG_PPCDBG it is also a no-op unless
the compiler is broken and doesn't optimize away "if (0)" stanzas.  The
original construct gives you the option to enable this particular debugging
at runtime (without necessarily enabling all debugging at one time) if wanted.

> Index: working-2.6/include/asm-ppc64/mmu_context.h
> ===================================================================
> --- working-2.6.orig/include/asm-ppc64/mmu_context.h	2004-05-20 12:58:51.000000000 +1000
> +++ working-2.6/include/asm-ppc64/mmu_context.h	2004-05-27 14:22:01.088506616 +1000
> @@ -189,15 +189,15 @@
>  	ordinal = (((ea >> 28) & 0x1fff) * LAST_USER_CONTEXT) | (ea >> 60);
>  	vsid = (ordinal * VSID_RANDOMIZER) & VSID_MASK;
>  
> -	ifppcdebug(PPCDBG_HTABSTRESS) {
> -		/* For debug, this path creates a very poor vsid distribuition.
> -		 * A user program can access virtual addresses in the form
> -		 * 0x0yyyyxxxx000 where yyyy = xxxx to cause multiple mappings
> -		 * to hash to the same page table group.
> -		 */ 
> -		ordinal = ((ea >> 28) & 0x1fff) | (ea >> 44);
> -		vsid = ordinal & VSID_MASK;
> -	}
> +#ifdef HTABSTRESS
> +	/* For debug, this path creates a very poor vsid distribuition.
> +	 * A user program can access virtual addresses in the form
> +	 * 0x0yyyyxxxx000 where yyyy = xxxx to cause multiple mappings
> +	 * to hash to the same page table group.
> +	 */ 
> +	ordinal = ((ea >> 28) & 0x1fff) | (ea >> 44);
> +	vsid = ordinal & VSID_MASK;
> +#endif /* HTABSTRESS */
>  
>  	return vsid;
>  } 
> @@ -212,11 +212,11 @@
>  	ordinal = (((ea >> 28) & 0x1fff) * LAST_USER_CONTEXT) | context;
>  	vsid = (ordinal * VSID_RANDOMIZER) & VSID_MASK;
>  
> -	ifppcdebug(PPCDBG_HTABSTRESS) {
> -		/* See comment above. */
> -		ordinal = ((ea >> 28) & 0x1fff) | (context << 16);
> -		vsid = ordinal & VSID_MASK;
> -	}
> +#ifdef HTABSTRESS
> +	/* See comment above. */
> +	ordinal = ((ea >> 28) & 0x1fff) | (context << 16);
> +	vsid = ordinal & VSID_MASK;
> +#endif /* HTABSTRESS */
>  
>  	return vsid;
>  }
> 
> -- 
> David Gibson			| For every complex problem there is a
> david AT gibson.dropbear.id.au	| solution which is simple, neat and
> 				| wrong.
> http://www.ozlabs.org/people/dgibson
> -
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Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/