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

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Date:	Wed, 5 Jan 2011 16:16:45 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>, 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 Wed, Jan 05, 2011 at 09:32:11AM -0800, David Daney wrote:
> 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

indeed. If you look at the x86 or sparc bits the fixup should be quite
small. The bulk of the changes are in the common code.

thanks,

-Jason
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