linux-kernel - Re: [RFC] Sectionized printk data

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Message-ID: <20080210002718.GT4352@ghostprotocols.net>
Date:	Sat, 9 Feb 2008 22:27:18 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	Sam Ravnborg <sam@...nborg.org>,
	Johannes Weiner <hannes@...urebad.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Sectionized printk data

Em Sun, Feb 10, 2008 at 01:18:18AM +0100, Jan Engelhardt escreveu:
> 
> On Feb 9 2008 21:54, Arnaldo Carvalho de Melo wrote:
> >> To drop strings that are only shown once anyway, such as:
> >> 
> >> static int __init ebtables_init(void)
> >> {
> >>         int ret;
> >> 
> >>         mutex_lock(&ebt_mutex);
> >>         list_add(&ebt_standard_target.list, &ebt_targets);
> >>         mutex_unlock(&ebt_mutex);
> >>         if ((ret = nf_register_sockopt(&ebt_sockopts)) < 0)
> >>                 return ret;
> >> 
> >> ->      printk(KERN_INFO "Ebtables v2.0 registered\n");
> >>         return 0;
> >> }
> >> 
> >> >If you say "saving memory" then please let us know with specific examples
> >> >in what area these savings will really pay off.
> >
> >[...]
> >With a tool like this the advantage is that no source code has to be
> >changed, strings in __init functions are automagically moved to
> >.init.data, the disadvantage is that not all strings can be moved to
> >.init.data as there were (are?) subsystems that keep pointers to the
> >string passed and another tool would be involved in the build process.
> 
> There is one corner case to consider:
> 
> 
> 	static char abc[] = "foo";
> 
> 	int __init init_module(void)
> 	{
> 		printk(abc);
> 	}
> 
> I am not sure if gcc/ld is smart enough to figure out that abc is
> only ever used from within an __init function and that it could hence
> be moved to __initdata.

The initstr tool mentioned doesn't touches this case, as it doesn't
searches specific functions such as printk, it looks for strings inside
__init marked functions. In the above example abc won't be marked as
__initdata.

So if there are two places where the same string is used, with one being
in a __init function one copy goes to .init.data and another to .data.

- Arnaldo
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