Re: RFC: remove __read_mostly

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Adrian Bunk a écrit :
I tried the following patch with a full x86 .config [1]:

--- a/include/asm-x86/cache.h
+++ b/include/asm-x86/cache.h
-#define __read_mostly __attribute__((__section__(".data.read_mostly")))
+/* #define __read_mostly __attribute__((__section__(".data.read_mostly"))) */

The result [2,3] was:

-rwxrwxr-x 1 bunk bunk 46607243 2007-12-13 19:50 vmlinux.old
-rwxrwxr-x 1 bunk bunk 46598691 2007-12-13 21:55 vmlinux

It's not a surprise that the kernel can become bigger when __read_mostly gets used, especially in cases where __read_mostly prevents gcc optimizations.

My question is:
Is there anywhere in the kernel a case where __read_mostly brings a measurable improvement or can it be removed?

Yes, there are many cases where read_mostly brings huge improvements.

I did test your idea on a 4 CPUS server, and system time was roughly doubled, from 11% to 20%

Of course, you noticed that puting a __read_mostly attribute force the linker to reserve space for the variable. So a null variable previously in bss section (no space in vmlinux file) is now in .data.read_mostly. Not a big deal.

If you want, you could play some .lds games to create sort of a "bss.read_mostly" section to save 10000 bytes in vmlinux.

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