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

[Andrew Morton <akpm@digeo.com>]

Jeff Garzik wrote:
> 
> David S. Miller wrote:
> > 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.