Re: [RFC][PATCH] net drivers and cache alignment

On Sat, Dec 07, 2002 at 03:29:16PM -0800, Andrew Morton wrote:

Jeff Garzik wrote:

quoted

David S. Miller wrote:

quoted

Can't the cacheline_aligned attribute be applied to individual
struct members?  I remember doing this for thread_struct on
sparc ages ago.

Looks like it from the 2.4 processor.h code.

Attached is cut #2.  Thanks for all the near-instant feedback so far :)
  Andrew, does the attached still need padding on SMP?

It needs padding _only_ on SMP.  ____cacheline_aligned_in_smp.

#define offsetof(t, m)  ((int)(&((t *)0)->m))

struct foo {
        int a;
        int b __attribute__((__aligned__(1024)));
        int c;
} foo;

main()
{
        printf("%d\n", sizeof(struct foo));
        printf("%d\n", offsetof(struct foo, a));
        printf("%d\n", offsetof(struct foo, b));
        printf("%d\n", offsetof(struct foo, c));
}

./a.out
2048
0
1024
1028

So your patch will do what you want it to do.  You should just tag the
first member of a group with ____cacheline_aligned_in_smp, and keep an
eye on things with offsetof().

Not sure why sizeof() returned 2048 though.

The structure contains an __aligned__(1024) item.  Think about an array
of 'struct foo' items.  They have to be 2048 bytes or you won't align
correctly.

C allows for empty space in structure padding, but not in arrays,
AFAIK.


-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer