Re: [PATCH net v3] net: Force inlining of checksum functions in net/checksum.h

[Christophe Leroy]

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
>> 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