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SubjectRe: [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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