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SubjectRe: [PATCH] force inlining of spinlock ops
On Mon, May 11, 2015 at 07:57:22PM +0200, Denys Vlasenko 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

Frame pointers make this even more awful, since without them this could
just become a single jmp. (Assuming _raw_spin_lock shouldn't be
inlined too.)

> 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

Nice improvement. Given that this actually makes the kernel *smaller*,
presumably in addition to faster, this forced inlining seems completely
reasonable.

> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Thomas Graf <tgraf@suug.ch>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: linux-kernel@vger.kernel.org

Reviewed-by: Josh Triplett <josh@joshtriplett.org>


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