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SubjectRe: [PATCH] [3/5] Mark complex bitops.h inlines as __always_inline
On Wed, 7 Jan 2009, Ingo Molnar wrote:
>
> > Hugh Dickins noticed that released gcc versions building the kernel with
> > CONFIG_OPTIMIZE_INLINING=y don't inline some of the bitops - sometimes
> > generating very inefficient pageflag tests, and many instances of
> > constant_test_bit().
>
> Could you quantify that please?
>
> We really dont want to reintroduce __always_inline just for performance /
> code size reasons. If GCC messes up and makes a larger / more inefficient
> kernel, GCC will be fixed. CONFIG_OPTIMIZE_INLINING is default-off, so
> enable it only if it improves your kernel.

I'm happy with the config option defaulting off, but the Kconfig help
text seems too optimistic, and I doubt defconfigs should say Y yet.

http://lkml.org/lkml/2008/11/14/203 CONFIG_OPTIMIZE_INLINING fun
was my original mail to you on this: somewhat rambling, but worth
looking at just for the laughable 4.2.1 Page disassemblies.

Quantification: you may have a better idea of what numbers to look
at, but here are some.

I've tried "make defconfig" on 2.6.28 x86_32 kernel trees, building
first with the openSUSE 10.3 version of gcc 4.2.1, then the openSUSE
11.1 version of gcc 4.3.2 (Fedora 10 and Ubuntu 8.10 versions gave
similar results, though I didn't go very far with those), then the
latest gcc 4.4 snapshot. I was building x86_64 a few weeks ago,
and that gave similar results.

I've built five configs: vmlinux.nn is the defconfig, except for
CONFIG_CC_OPTIMIZE_FOR_SIZE and CONFIG_OPTIMIZE_INLINING both off;
vmlinux.yn has SIZE=y, vmlinux.ny has INLINING=y, vmlinux.yy is the
actual defconfig with both y, vmlinux.yya is that with Andi's bitops
patch applied.

For background I'm showing "size" output, and the number of lines in
System.map (supposing that to give some idea of how much inlining).

Then how many "Page" lines appear in System.map: ideally none, a
few are okay, but lots mean it's being stupid about PageLocked etc -
4.2.1 SIZE=y INLINING=y is afflicted by that, the rest are okay.

Then how many "constant_test_bit"s (each sized 27 bytes): clearly
Andi's patch eliminates those. Contrary to expectations, 4.4 does
little better than 4.3.2 with them; and though 4.2.1's yy case is
worst, its ny case is actually much better than with 4.3 and 4.4
(or maybe there's a different interpretation than numbers suggest).

I think these numbers show that that even gcc 4.4 is not yet ready
for CONFIG_OPTIMIZE_INLINING=y, but that Andi's patch helps in one
important area. grep for something else and maybe a different story.

gcc version 4.2.1 (SUSE Linux):

text data bss dec hex filename
6847905 1151180 622592 8621677 838e6d vmlinux.nn
45069 wc -l <System.map.nn
0 grep -c Page System.map.nn
0 grep -c constant_test_bit System.map.nn

text data bss dec hex filename
6848053 1151180 622592 8621825 838f01 vmlinux.ny
45130 wc -l <System.map.ny
0 grep -c Page System.map.ny
5 grep -c constant_test_bit System.map.ny

text data bss dec hex filename
5921243 1124336 618496 7664075 74f1cb vmlinux.yn
44949 wc -l <System.map.yn
0 grep -c Page System.map.yn
0 grep -c constant_test_bit System.map.yn

text data bss dec hex filename
5906470 1124336 618496 7649302 74b816 vmlinux.yy
49223 wc -l <System.map.yy
98 grep -c Page System.map.yy
228 grep -c constant_test_bit System.map.yy

text data bss dec hex filename
5877907 1124336 618496 7620739 744883 vmlinux.yya
48786 wc -l <System.map.yya
98 grep -c Page System.map.yya
0 grep -c constant_test_bit System.map.yya

gcc version 4.3.2 [gcc-4_3-branch revision 141291] (SUSE Linux):

text data bss dec hex filename
7017366 1151180 622592 8791138 862462 vmlinux.nn
43755 wc -l <System.map.nn
0 grep -c Page System.map.nn
0 grep -c constant_test_bit System.map.nn

text data bss dec hex filename
7014542 1151180 622592 8788314 86195a vmlinux.ny
44165 wc -l <System.map.ny
2 grep -c Page System.map.ny
79 grep -c constant_test_bit System.map.ny

text data bss dec hex filename
5965795 1124336 618496 7708627 759fd3 vmlinux.yn
43790 wc -l <System.map.yn
0 grep -c Page System.map.yn
0 grep -c constant_test_bit System.map.yn

text data bss dec hex filename
5917632 1124336 618496 7660464 74e3b0 vmlinux.yy
44816 wc -l <System.map.yy
4 grep -c Page System.map.yy
151 grep -c constant_test_bit System.map.yy

text data bss dec hex filename
5898313 1124336 618496 7641145 749839 vmlinux.yya
44602 wc -l <System.map.yya
0 grep -c Page System.map.yya
0 grep -c constant_test_bit System.map.yya

gcc version 4.4.0 20090102 (experimental) (GCC)

text data bss dec hex filename
6960329 1151180 622592 8734101 854595 vmlinux.nn
44121 wc -l <System.map.nn
0 grep -c Page System.map.nn
0 grep -c constant_test_bit System.map.nn

text data bss dec hex filename
6952421 1151180 622592 8726193 8526b1 vmlinux.ny
44564 wc -l <System.map.ny
1 grep -c Page System.map.ny
60 grep -c constant_test_bit System.map.ny

text data bss dec hex filename
5897459 1124336 618496 7640291 7494e3 vmlinux.yn
44149 wc -l <System.map.yn
0 grep -c Page System.map.yn
0 grep -c constant_test_bit System.map.yn

text data bss dec hex filename
5843498 1124336 618496 7586330 73c21a vmlinux.yy
45655 wc -l <System.map.yy
4 grep -c Page System.map.yy
148 grep -c constant_test_bit System.map.yy

text data bss dec hex filename
5816020 1124336 618496 7558852 7356c4 vmlinux.yya
45452 wc -l <System.map.yya
0 grep -c Page System.map.yya
0 grep -c constant_test_bit System.map.yya

Hugh


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