linux-kernel - Re: [PATCH 2/8] dynamic debug

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Date:	Fri, 20 Jun 2008 11:38:22 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Greg KH <greg@...ah.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	joe@...ches.com, nick@...k-andrew.net, randy.dunlap@...cle.com
Subject: Re: [PATCH 2/8] dynamic debug - core infrastructure

On Wed, Jun 18, 2008 at 09:08:12AM -0700, Greg KH wrote:
> On Fri, Jun 13, 2008 at 03:00:39PM -0400, Jason Baron wrote:
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -595,6 +595,22 @@ extern const char *dev_driver_string(struct device *dev);
> >  #ifdef DEBUG
> >  #define dev_dbg(dev, format, arg...)		\
> >  	dev_printk(KERN_DEBUG , dev , format , ## arg)
> > +#elif defined(CONFIG_DYNAMIC_PRINTK_DEBUG)
> > +#define dev_dbg(dev, format, ...) do {				    	     \
> > +	static char mod_name[]						     \
> > +	__attribute__((section("__verbose_strings")))			     \
> > +	 = KBUILD_MODNAME;						     \
> > +	static struct mod_debug foobar					     \
> > +	__used								     \
> > +	__attribute__((section("__verbose"), aligned(8))) =		     \
> > +	{ mod_name, mod_name, "0", "0", NULL };				     \
> 
> Does the compiler merge all of these variables together into one within
> the same file?
> 

no.

> If not, should we have a new macro, one per file, for this information?
> 

With this feature enabled, the above 'meta-data' added ~32K to the .init.data
section. This memory is freed when the system boots because it is placed in the
.init.data section. The same is true for modules, the 'meta-data' is freed when
the module loads.

We could reduce the vmlinux by having 'register()' type calls for this
infrastructure, or a new marco per file as you suggest, and i had something
like that in an earlier iteration, but I think the memory savings is very
small when compared to the increased complexity. The simplicity to just put a
'pr_debug()" anywhere and just have it do the right thing is nice.

Also, it should be noted that the total increase in the size of the 'vmlinux' 
was 3%, or 2M, this is due to the above 'meta-data', the new code, and probably 
most significantly, the strings, for the additinal printks. Thus, compressing
the above meta-data, or eliminating it by some other means would only decrease
the size of this infrastructure by 2% or so.

thanks,

-Jason

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