Re: [PATCH] force inlining of spinlock ops

[Denys Vlasenko <dvlasenk@redhat.com>]

On 05/12/2015 12:19 AM, Andrew Morton wrote:
> On Mon, 11 May 2015 19:57:22 +0200 Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 
>> With both gcc 4.7.2 and 4.9.2, sometimes gcc mysteriously doesn't inline
>> very small functions we expect to be inlined. In particular,
>> with this config: http://busybox.net/~vda/kernel_config
>> there are more than a thousand copies of tiny spinlock-related functions:
>>
>> $ nm --size-sort vmlinux | grep -iF ' t ' | uniq -c | grep -v '^ *1 ' | sort -rn | grep ' spin'
>>     473 000000000000000b t spin_unlock_irqrestore
>>     292 000000000000000b t spin_unlock
>>     215 000000000000000b t spin_lock
>>     134 000000000000000b t spin_unlock_irq
>>     130 000000000000000b t spin_unlock_bh
>>     120 000000000000000b t spin_lock_irq
>>     106 000000000000000b t spin_lock_bh
>>
>> Disassembly:
>>
>> ffffffff81004720 <spin_lock>:
>> ffffffff81004720:       55                      push   %rbp
>> ffffffff81004721:       48 89 e5                mov    %rsp,%rbp
>> ffffffff81004724:       e8 f8 4e e2 02          callq  <_raw_spin_lock>
>> ffffffff81004729:       5d                      pop    %rbp
>> ffffffff8100472a:       c3                      retq
>>
>> This patch fixes this via s/inline/__always_inline/ in spinlock.h.
>> This decreases vmlinux by about 30k:
>>
>>     text     data      bss       dec     hex filename
>> 82375570 22255544 20627456 125258570 7774b4a vmlinux.before
>> 82335059 22255416 20627456 125217931 776ac8b vmlinux
> 
> See also https://lkml.org/lkml/2015/4/23/598 ("enforce function
> inlining for hot functions").
> 
> Presumably Hagen didn't see the issue with spinlock functions.  I
> wonder why not.
> 
> I suppose we should get both these consolidated into a coherent whole.
> 
> It's a bit irritating to have to do this: presumably gcc will get fixed
> and the huge sprinkling of __always_inline will become less and less
> relevant over time and people will have trouble distinguishing "real
> __always_inline which was put here for a purpose" from "dopey
> __always_inline to work around a short-term gcc glitch".

In my patches, I put __always_inline *only* on functions
where my measurements show a large size decrease from doing so.
*Not* on functions where "I think it may be a good idea".

So far, all such functions were so trivial that inlining decision there
is a no-brainer.

> and then use inline_for_broken_gcc everywhere.  That way, the reason
> for the marker is self-explanatory and we can later hunt all these
> things down and remvoe them.
> 
> Also, the inline_for_broken_gcc definition can be made dependent on
> particular gcc versions, which will allow us to easily keep an eye on
> the behaviour of later gcc versions.

I've seen it on gcc-4.7.2 and gcc-4.9.2, so this behavior is not
limited to a narrow range of gcc versions. I'd say by now about half
of running kernels can easily be affected.

-- 
vda