linux-kernel - Re: [PATCH] enforce function inlining for hot functions

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Date:	Fri, 24 Apr 2015 12:49:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, x86@...nel.org
Subject: Re: [PATCH] enforce function inlining for hot functions

On Thu, 23 Apr 2015 23:40:01 +0200 Hagen Paul Pfeifer <hagen@...u.net> wrote:

> GCC inlining heuristics are sometimes quizzical. Especially with inline
> assembler constructs GCC seems to have issues. A allyesconfig show a rather
> long list of functions where GCC inlining decisions are questionable (not
> inlined).

I can't reproduce this with either gcc-4.8.2 or gcc-4.4.4.  The patch
makes zero difference to `size vmlinux' and a bit of poking around with
nm doesn't show any out-of-lined versions of the functions you
identify.

So.  More details, please.  How to demonstrate this, gcc versions, etc.

> Furthermore, because the functions are declared with static
> linkage each function is copied n times - and n can be rather high:
> 
>               atomic_inc: 544 duplicates
>          rcu_read_unlock: 453 duplicates
>            rcu_read_lock: 383 duplicates
>              get_dma_ops: 271 duplicates
>   arch_local_irq_restore: 258 duplicates
>               atomic_dec: 215 duplicates
>                  kzalloc: 185 duplicates
>            cpumask_check: 157 duplicates
>         test_and_set_bit: 156 duplicates
>             cpumask_next: 146 duplicates
>                 list_del: 131 duplicates
>                 kref_get: 126 duplicates

That's pretty pathetic.
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