Adding Ingo, Andrew and Nick as they were involved in the subjet,

Le 17/02/2022 à 14:36, David Laight a écrit :

From: Christophe Leroy

quoted

Sent: 17 February 2022 12:19

All functions defined as static inline in net/checksum.h are
meant to be inlined for performance reason.

But since commit ac7c3e4ff401 ("compiler: enable
CONFIG_OPTIMIZE_INLINING forcibly") the compiler is allowed to
uninline functions when it wants.

Fair enough in the general case, but for tiny performance critical
checksum helpers that's counter-productive.

There isn't a real justification for allowing the compiler
to 'not inline' functions in that commit.

Do you mean that the two following commits should be reverted:

- 889b3c1245de ("compiler: remove CONFIG_OPTIMIZE_INLINING entirely")
- 4c4e276f6491 ("net: Force inlining of checksum functions in 
net/checksum.h")

It rather seems backwards.
The kernel sources don't really have anything marked 'inline'
that shouldn't always be inlined.
If there are any such functions they are few and far between.

I've had enough trouble (elsewhere) getting gcc to inline
static functions that are only called once.
I ended up using 'always_inline'.
(That is 4k of embedded object code that will be too slow
if it ever spills a register to stack.)

I agree with you that that change is a nightmare with many small 
functions that we really want inlined, and when we force inlining we 
most of the time get a smaller binary.

And it becomes even more problematic when we start adding 
instrumentation like stack protector.

According to the original commits however this was supposed to provide 
real benefit:

- 60a3cdd06394 ("x86: add optimized inlining")
- 9012d011660e ("compiler: allow all arches to enable 
CONFIG_OPTIMIZE_INLINING")

But when I build ppc64le_defconfig + CONFIG_CC_OPTIMISE_FOR_SIZE I get:
     112 times  queued_spin_unlock()
     122 times  mmiowb_spin_unlock()
     151 times  cpu_online()
     225 times  __raw_spin_unlock()


So I was wondering, would we have a way to force inlining of functions 
marked inline in header files while leaving GCC handling the ones in C 
files the way it wants ?

Christophe