linux-kernel - Re: [PATCH] force inlining of spinlock ops

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Message-Id: <20150511151913.86c37cde9294700b4b0e26c4@linux-foundation.org>
Date:	Mon, 11 May 2015 15:19:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Graf <tgraf@...g.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Bart Van Assche <bvanassche@....org>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Hagen Paul Pfeifer <hagen@...u.net>
Subject: Re: [PATCH] force inlining of spinlock ops

On Mon, 11 May 2015 19:57:22 +0200 Denys Vlasenko <dvlasenk@...hat.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".

__always_inline is one of those things where a usage site should always
be commented, because it's near impossible to work out why someone
chose to use it.  Quick, tell me what's happening in include/linux/slab.h.

Perhaps we should do

/*
 * Comment goes here.  It is very specific about gcc versions.
 */
#define inline_for_broken_gcc __always_inline

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.

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