linux-kernel - Re: [PATCH 2/2] jump label: introduce static

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Date:	Wed, 05 Jan 2011 09:32:11 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Jason Baron <jbaron@...hat.com>, Ralf Baechle <ralf@...ux-mips.org>
CC:	peterz@...radead.org, mathieu.desnoyers@...ymtl.ca, hpa@...or.com,
	rostedt@...dmis.org, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, sam@...nborg.org,
	michael@...erman.id.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] jump label: introduce static_branch()

On 01/05/2011 07:43 AM, Jason Baron wrote:
> Introduce:
>
> static __always_inline bool static_branch(struct jump_label_key *key)
>
> to replace the old JUMP_LABEL(key, label) macro.
>
> The new static_branch(), simplifies the usage of jump labels. Since,
> static_branch() returns a boolean, it can be used as part of an if()
> construct. It also, allows us to drop the 'label' argument from the
> prototype. Its probably best understood with an example, here is the part
> of the patch that converts the tracepoints to use unlikely_switch():
>
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -146,9 +146,7 @@ static inline void tracepoint_update_probe_range(struct tracepoint *begin,
>   	extern struct tracepoint __tracepoint_##name;			\
>   	static inline void trace_##name(proto)				\
>   	{								\
> -		JUMP_LABEL(&__tracepoint_##name.key, do_trace);		\
> -		return;							\
> -do_trace:								\
> +		if (static_branch(&__tracepoint_##name.key))		\
>   			__DO_TRACE(&__tracepoint_##name,		\
>   				TP_PROTO(data_proto),			\
>   				TP_ARGS(data_args));			\
>
>
> I analyzed the code produced by static_branch(), and it seems to be
> at least as good as the code generated by the JUMP_LABEL(). As a reminder,
> we get a single nop in the fastpath for -02. But will often times get
> a 'double jmp' in the -Os case. That is, 'jmp 0', followed by a jmp around
> the disabled code. We believe that future gcc tweaks to allow block
> re-ordering in the -Os, will solve the -Os case in the future.
>
> I also saw a 1-2% tbench throughput improvement when compiling with
> jump labels.
>
> This patch also addresses a build issue that Tetsuo Handa reported where
> gcc v3.3 currently chokes on compiling 'dynamic debug':
>
> include/net/inet_connection_sock.h: In function `inet_csk_reset_xmit_timer':
> include/net/inet_connection_sock.h:236: error: duplicate label declaration `do_printk'
> include/net/inet_connection_sock.h:219: error: this is a previous declaration
> include/net/inet_connection_sock.h:236: error: duplicate label declaration `out'
> include/net/inet_connection_sock.h:219: error: this is a previous declaration
> include/net/inet_connection_sock.h:236: error: duplicate label `do_printk'
> include/net/inet_connection_sock.h:236: error: duplicate label `out'
>
>
> Thanks to H. Peter Anvin for suggesting this improved syntax.
>
> Suggested-by: H. Peter Anvin<hpa@...ux.intel.com>
> Signed-off-by: Jason Baron<jbaron@...hat.com>
> Tested-by: Tetsuo Handa<penguin-kernel@...ove.sakura.ne.jp>
> ---
>   arch/sparc/include/asm/jump_label.h |   25 ++++++++++++++-----------
>   arch/x86/include/asm/jump_label.h   |   22 +++++++++++++---------
>   arch/x86/kernel/jump_label.c        |    2 +-
>   include/linux/dynamic_debug.h       |   18 ++++--------------
>   include/linux/jump_label.h          |   26 +++++++++++---------------
>   include/linux/jump_label_ref.h      |   18 +++++++++++-------
>   include/linux/perf_event.h          |   26 +++++++++++++-------------
>   include/linux/tracepoint.h          |    4 +---
>   kernel/jump_label.c                 |    2 +-
>   9 files changed, 69 insertions(+), 74 deletions(-)
>
[...]

This patch will conflict with the MIPS jump label support that Ralf has 
queued up for 2.6.38.

David Daney
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