Do Printk - Revision history

Chris at 10:46, 14 July 2007

2007-07-14T10:46:59Z

RBot at 03:05, 8 March 2007

2007-03-08T03:05:49Z

RBot: Bot (Edward's framework)

2007-03-06T03:32:26Z

Bot (Edward's framework)

New page

Table Of Contents:

== Description ==
This page describes the "DO_PRINTK" feature. This is a small tweak on the already-supported
printk configuration option in the kernel, which allows a developer to configure whether the
kernel will be compiled with or without printk messages.

This feature allows you to turn off printk messages, while preserving the printk function
itself. Then, you can turn on the printk messages from an individual file, or set of files.
This allows you greater granularity of control over the printk messages that are compiled
into the kernel.

=== Rationale ===
Control over the printk messages in the kernel is important because the kernel messages
comprise a substantial portion of the overall kernel size (ranging from 5 to 10% of overall
kernel image size.)

The kernel (as of this writing, version 2.6.17) already supports an option to turn off all
printk messages. However, sometimes this is too much. Sometimes, an embedded developer
may want to see a few printk messages from specific files or routines, while still not
enabling every single printk message in the kernel. The DO_PRINTK feature provides
this fine-grained control.

== Downloads ==
=== Patch ===
- Patch for 2.6.17.4 is here: [[Media:do-printk.patch]] (3KB)

== How To Use ==
To use DO_PRINTK, perform the following steps:

* Apply the patch
* Configure your kernel with CONFIG_PRINTK_FUNC=y and CONFIG_PRINTK=n
** These options appear under "General Setup", "Configure standard kernel features (for small systems)" in 'make menuconfig' {{{
General Setup --->
[*] Configure standard kernel features (for small systems) --->
[*] Enable printk function in the kernel
[ ] Enable support for printk messages
}}}
** With CONFIG_PRINTK=n, the compiler will by default omit (compile away) the calls to printk in all files
* Select the individual files you would like to still emit printk messages from
** Add "#define DO_PRINTK" to the files where you wish printk messages to be preserved
*** Make sure this definition appears '''before''' any <code>#include</code> lines in the file.
** Alternatively, add -DDO_PRINTK to the compiler flags for files you wish to preserve printks in.
*** (see below for details)
* Compile the kernel
* (optionally) Examine the kernel size, and rejoice at your size savings
* Install the kernel

=== Ways to force printks to be preserved ===
There are actually several ways to get DO_PRINTK defined for a particular file:
1) add to the top of the file a line consisting of:

<pre>#define DO_PRINTK
</pre>

** this must appear before any <code>#include</code> lines for the file
** this definition will override the CONFIG_PRINTK=n directive, and allow printks in the file to work normally
<hr />

2) add an option to CFLAGS to the Makefile which controls the building of the file(s) you want to affect
** To affect an individual file, add -DDO_PRINTK for the CFLAGS for that file:
** For example, in the Makefile that compiles foo.c , add the line:

<pre>CFLAGS_foo.o += -DDO_PRINTK
</pre>

** To affect all the files in a directory, set EXTRA_CFLAGS, like so:

<pre>EXTRA_CFLAGS += -DDO_PRINTK
</pre>

<hr />

== Sample Results ==
Examples of use with measurement of the effects.
=== Case Study 1 ===
On an x86 machine, using kernel.org 2.6.17.4 kernel, I got the results below, using the DO_PRINTK feature in combination with the PRINTK configuration option.

Here is the key for the following tables:
{|
|-
| kernel
| CONFIG_PRINTK_FUNC
| CONFIG_PRINTK
| DO_PRINTK used with files...
| Notes
|-
| func.yes-printk.yes-do.na/vmlinux
| yes
| yes
| not applicable
| This is a pretty regularly-configured kernel (used for baseline in first set of comparisons)
|-
| func.yes-printk.no-do.none/vmlinux
| yes
| no
| none
| Turn off all messages, but leave printk function in place
|-
| func.no-printk.na-do.na/vmlinux
| no
| not applicable
| not applicable
| Turn off all messages and remove printk function
|-
| func.yes-printk.no-do.init.star/vmlinux
| yes
| no
| init/*.c
| Turn off most messages, preserve messages from files in 'init' source directory
|-
| func.yes-printk.no-do.init.main.c/vmlinux
| yes
| no
| init/main.c
| Turn off most messages, preserve messages from init/main.c
|}

==== Summary of image size results ====
Kernel sizes (sorted by decreasing size):
{|
|-
| text
| data
| bss
| dec
| hex
| filename
|-
| 2621433
| 564958
| 137072
| 3323463
| 32b647
| func.yes-printk.yes-do.na/vmlinux (a.k.a. baseline)
|-
| 2376317
| 564958
| 137072
| 3078347
| 2ef8cb
| func.yes-printk.no-do.init.star/vmlinux
|-
| 2375508
| 564958
| 137072
| 3077538
| 2ef5a2
| func.yes-printk.no-do.init.main.c/vmlinux
|-
| 2374733
| 564958
| 137072
| 3076763
| 2ef29b
| func.yes-printk.no-do.none/vmlinux (a.k.a. printk-off baseline)
|-
| 2371463
| 564514
| 120688
| 3056665
| 2ea419
| func.no-printk.na-do.na/vmlinux
|}

==== Image size results compared to baseline ====
The following tables show the image sizes of various kernels compared with a baseline
kernel. The baseline kernel has a fairly normally i386 configuration, with
CONFIG_PRINTK_FUNC=y and CONFIG_PRINTK=y.

Size delta results:
{|
|-
| section
| baseline/vmlinux
| func.no-printk.na-do.na/vmlinux
| change
| percent
|-
| text
| 2621433
| 2371463
| -249970
| -9%
|-
| data
| 564958
| 564514
| -444
| 0%
|-
| bss
| 137072
| 120688
| -16384
| -11%
|-
| total
| 3323463
| 3056665
| -266798
| bgcolor="#B0FFB0" | -8% *
|-
| bgcolor="#B0FFB0" | * Full size reduction using printk elimination is 266K or 8% of vmlinux
|}

<hr />
Size delta results:
{|
|-
| section
| baseline/vmlinux
| func.yes-printk.no-do.init.main.c/vmlinux
| change
| percent
|-
| text
| 2621433
| 2375508
| -245925
| -9%
|-
| data
| 564958
| 564958
| 0
| 0%
|-
| bss
| 137072
| 137072
| 0
| 0%
|-
| total
| 3323463
| 3077538
| -245925
| -7%
|}

<hr />
Size delta results:
{|
|-
| section
| baseline/vmlinux
| func.yes-printk.no-do.init.star/vmlinux
| change
| percent
|-
| text
| 2621433
| 2376317
| -245116
| -9%
|-
| data
| 564958
| 564958
| 0
| 0%
|-
| bss
| 137072
| 137072
| 0
| 0%
|-
| total
| 3323463
| 3078347
| -245116
| -7%
|}

<hr />
Size delta results:
{|
|-
| section
| baseline/vmlinux
| func.yes-printk.no-do.none/vmlinux
| change
| percent
|-
| text
| 2621433
| 2374733
| -246700
| -9%
|-
| data
| 564958
| 564958
| 0
| 0%
|-
| bss
| 137072
| 137072
| 0
| 0%
|-
| total
| 3323463
| 3076763
| -246700
| -7%
|}

<hr />

==== Image size results compared to 'printk-off' baseline ====
The following tables show the image sizes of various kernels compared with a baseline
kernel. The baseline kernel has 'printk-off' configuration, with
CONFIG_PRINTK_FUNC=y and CONFIG_PRINTK=n.

Size delta results:
{|
|-
| section
| func.yes-printk.no-do.none/vmlinux
| func.no-printk.na-do.na/vmlinux
| change
| percent
|-
| text
| 2374733
| 2371463
| bgcolor="#FFB0B0" | -3270 *
| 0%
|-
| data
| 564958
| 564514
| bgcolor="#FFB0B0" | -444 *
| 0%
|-
| bss
| 137072
| 120688
| bgcolor="#FFB0B0" | -16384 *
| -11%
|-
| total
| 3076763
| 3056665
| bgcolor="#FFB0B0" | -20098 *
| 0%
|-
| bgcolor="#FFB0B0" | * Size of printk function alone is about 3.6k in image and 20k in static memory
|}

<hr />
Size delta results:
{|
|-
| section
| func.yes-printk.no-do.none/vmlinux
| func.yes-printk.no-do.init.main.c/vmlinux
| change
| percent
|-
| text
| 2374733
| 2375508
| bgcolor="#FFFFB0" | 775 *
| 0%
|-
| data
| 564958
| 564958
| 0
| 0%
|-
| bss
| 137072
| 137072
| 0
| 0%
|-
| total
| 3076763
| 3077538
| 775
| 0%
|-
| bgcolor="#FFFFB0" | * Size of printk messages from a single C file (init/main.c in this case) was 775 bytes. Obviously, this is heavily dependent on the number of printk calls and the size of the messages in that file.
|}

<hr />
Size delta results:
{|
|-
| section
| func.yes-printk.no-do.none/vmlinux
| func.yes-printk.no-do.init.star/vmlinux
| change
| percent
|-
| text
| 2374733
| 2376317
| 1584
| 0%
|-
| data
| 564958
| 564958
| 0
| 0%
|-
| bss
| 137072
| 137072
| 0
| 0%
|-
| total
| 3076763
| 3078347
| 1584
| 0%
|}

<hr />
Size delta results:
{|
|-
| section
| func.yes-printk.no-do.none/vmlinux
| func.yes-printk.yes-do.na/vmlinux
| change
| percent
|-
| text
| 2374733
| 2621433
| 246700
| 10%
|-
| data
| 564958
| 564958
| 0
| 0%
|-
| bss
| 137072
| 137072
| 0
| 0%
|-
| total
| 3076763
| 3323463
| 246700
| 8%
|}

<hr />

=== Case Study 2 ===
[need to post PPC and MIPS results here also]

== Status ==
* Status: tested on multiple platforms
* Architecture Support:
(for each arch, one of: unknown, patches apply, compiles, runs, works, accepted)
** i386: works
** ARM: compiles, not run-tested yet
** PPC: works
** MIPS: works
** SH: unknown

== Future Work/Action Items ==
Here is a list of things that could be worked on for this feature:
- nothing right now

← Older revision		Revision as of 10:46, 14 July 2007
Line 479:		Line 479:
	Here is a list of things that could be worked on for this feature:		Here is a list of things that could be worked on for this feature:
	- nothing right now		- nothing right now
		+
		+	[[Category:Printk]]

@@ Line 1: / Line 1: @@
 == Description ==
 This page describes the "DO_PRINTK" feature.  This is a small tweak on the already-supported
@@ Line 25: / Line 22: @@
 == Downloads ==
 === Patch ===
- - Patch for 2.6.17.4 is here: [[Media:do-printk.patch]] (3KB)
+Patch for 2.6.17.4 is here: [[Media:do-printk.patch]] (3KB)
 == How To Use ==
@@ Line 32: / Line 29: @@
 * Apply the patch
 * Configure your kernel with CONFIG_PRINTK_FUNC=y and CONFIG_PRINTK=n
-** These options appear under "General Setup", "Configure standard kernel features (for small systems)" in 'make menuconfig' {{{
+** These options appear under "General Setup", "Configure standard kernel features (for small systems)" in 'make menuconfig'
      General Setup --->
      [*] Configure standard kernel features (for small systems)  --->
      [*] Enable printk function in the kernel
        [ ]  Enable support for printk messages
-   }}}
+</nowiki></pre>
 ** With CONFIG_PRINTK=n, the compiler will by default omit (compile away) the calls to printk in all files
 * Select the individual files you would like to still emit printk messages from
@@ Line 50: / Line 48: @@
 === Ways to force printks to be preserved ===
 There are actually several ways to get DO_PRINTK defined for a particular file:
-<pre>#define DO_PRINTK
+<pre>EXTRA_CFLAGS += -DDO_PRINTK</pre>
-   </pre>
 <hr />
-) add an option to CFLAGS to the Makefile which controls the building of the file(s) you want to affect
+== Sample Results ==
-** To affect an individual file, add -DDO_PRINTK for the CFLAGS for that file:
-** For example, in the Makefile that compiles foo.c , add the line:
+Examples of use with measurement of the effects.
-<pre>CFLAGS_foo.o += -DDO_PRINTK
-   </pre>
 On an x86 machine, using kernel.org 2.6.17.4 kernel, I got the results below, using the DO_PRINTK feature in combination with the PRINTK configuration option.
 Here is the key for the following tables:
 {|
 |-
@@ Line 125: / Line 116: @@
 ==== Summary of image size results ====
 Kernel sizes (sorted by decreasing size):
 {|
 |-
@@ Line 171: / Line 163: @@
 ==== Image size results compared to baseline ====
 The following tables show the image sizes of various kernels compared with a baseline
 kernel.  The baseline kernel has a fairly normally i386 configuration, with
@@ Line 176: / Line 169: @@
 Size delta results:
 {|
 |-