Re: [PATCH 24/27] Convert gfp_zone() to use a table of precalculatedvalues

[Christoph Lameter]

On Wed, 18 Mar 2009, Mel Gorman wrote:

> On Mon, Mar 16, 2009 at 03:12:50PM -0400, Christoph Lameter wrote:
> > On Mon, 16 Mar 2009, Mel Gorman wrote:
> >
> > > +int gfp_zone_table[GFP_ZONEMASK] __read_mostly;
> >
> > The gfp_zone_table is compile time determinable. There is no need to
> > calculate it.
> >
>
> The cost of calculating it is negligible and the code is then freed later
> in boot. Does having a const table make any difference?

Should it not enable the compiler to determine the value at
compile time and therefore make things like gfp_zone(constant) a
constant?

> > const int gfp_zone_table[GFP_ZONEMASK] = {
> > 	ZONE_NORMAL,		/* 00 No flags set */
> > 	ZONE_DMA,		/* 01 Only GFP_DMA set */
> > 	ZONE_HIGHMEM,		/* 02 Only GFP_HIGHMEM set */
> > 	ZONE_DMA,		/* 03 GFP_HIGHMEM and GFP_DMA set */
> > 	ZONE_DMA32,		/* 04 Only GFP_DMA32 set */
> > 	ZONE_DMA,		/* 05 GFP_DMA and GFP_DMA32 set */
> > 	ZONE_DMA32,		/* 06 GFP_DMA32 and GFP_HIGHMEM set */
> > 	ZONE_DMA,		/* 07 GFP_DMA, GFP_DMA32 and GFP_DMA32 set */
> > 	ZONE_MOVABLE,		/* 08 Only ZONE_MOVABLE set */
> > 	ZONE_DMA,		/* 09 MOVABLE + DMA */
> > 	ZONE_MOVABLE,		/* 0A MOVABLE + HIGHMEM */
> > 	ZONE_DMA,		/* 0B MOVABLE + DMA + HIGHMEM */
> > 	ZONE_DMA32,		/* 0C MOVABLE + DMA32 */
> > 	ZONE_DMA,		/* 0D MOVABLE + DMA + DMA32 */
> > 	ZONE_DMA32,		/* 0E MOVABLE + DMA32 + HIGHMEM */
> > 	ZONE_DMA		/* 0F MOVABLE + DMA32 + HIGHMEM + DMA
> > };
> >
> > Hmmmm... Guess one would need to add some #ifdeffery here to setup
> > ZONE_NORMAL in cases there is no DMA, DMA32 and HIGHMEM.
> >
>
> Indeed, as I said, this is somewhat error prone which is why the patch
> calculates the table at run-time instead of compile-time trickery.

One would need to define some macros to make it simpler I guess

Write something like

#ifdef CONFIG_ZONE_DMA
#define TZONE_DMA ZONE_DMA
#else
#define TZONE_DMA ZONE_NORMAL
#endif

for each configurable item. Then just add the T to the above table.
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