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

[Christophe Leroy]

Le 17/02/2022 à 15:50, Christophe Leroy a écrit :
> 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")

Of course not the above one (copy/paste error), but:
- ac7c3e4ff401 ("compiler: enable CONFIG_OPTIMIZE_INLINING forcibly")


> > 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