linux-kernel - Re: [PATCH v3 7/7] dynamic_debug: add jump label support

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Date:	Wed, 06 Jul 2016 15:52:27 -0700
From:	Joe Perches <joe@...ches.com>
To:	Jason Baron <jbaron@...mai.com>, akpm@...ux-foundation.org
Cc:	peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/7] dynamic_debug: add jump label support

On Wed, 2016-07-06 at 17:42 -0400, Jason Baron wrote:
> Although dynamic debug is often only used for debug builds, sometimes its
> enabled for production builds as well. Minimize its impact by using jump
> labels. This reduces the text section by 7000+ bytes in the kernel image
> below. It does increase data, but this should only be referenced when
> changing the direction of the branches, and hence usually not in cache.
> 
>    text	   data	    bss	    dec	    hex	filename
> 8194852	4879776	 925696	14000324	 d5a0c4	vmlinux.pre
> 8187337	4960224	 925696	14073257	 d6bda9	vmlinux.post

Maybe to get some of that space back on the 32 bit builds
the 8 byte alignment can be relaxed.

> diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
[]
> @@ -33,6 +37,12 @@ struct _ddebug {
>  #define _DPRINTK_FLAGS_DEFAULT 0
>  #endif
>  	unsigned int flags:8;
> +#ifdef HAVE_JUMP_LABEL
> +	union {
> +		struct static_key_true dd_key_true;
> +		struct static_key_false dd_key_false;
> +	} key;
> +#endif
>  } __attribute__((aligned(8)));

Couldn't this be:

} __aligned(__alignof__(void *));
 
> @@ -60,7 +70,7 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
>  			  const struct net_device *dev,
>  			  const char *fmt, ...);
>  
> -#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)		\
> +#define DEFINE_DYNAMIC_DEBUG_METADATA_KEY(name, fmt, key, init)	\
>  	static struct _ddebug  __aligned(8)			\
>  	__attribute__((section("__verbose"))) name = {		\

And this shouldn't need the __aligned as it's already
in the structure definition.

Maybe:

	static struct _ddebug __section(__verbose) name = {

>  		.modname = KBUILD_MODNAME,			\