Re: [PATCH] printk/index: Printk index feature documentation

From: Chris Down
Date: Tue Apr 05 2022 - 20:40:20 EST

Petr Mladek writes:
Document the printk index feature. The primary motivation is to
explain that it is not creating KABI from particular printk() calls.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Acked-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>

This looks great, thanks a lot for spending the time to write this up. Just some very minor comments.

Reviewed-by: Chris Down <chris@xxxxxxxxxxxxxx>

Changes against RFC[*]:

+ Updated text by feedback from Darrick.
+ Updated index.rst and MAINTAINERS.
+ Tested build of the html documentation.

Did not add Sergey's ack because there were some changes.


Documentation/core-api/index.rst | 1 +
Documentation/core-api/printk-index.rst | 136 ++++++++++++++++++++++++
3 files changed, 138 insertions(+)
create mode 100644 Documentation/core-api/printk-index.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 972d46a5ddf6..8a3fce1fe582 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -20,6 +20,7 @@ it.
+ printk-index

Data structures and low-level utilities
diff --git a/Documentation/core-api/printk-index.rst b/Documentation/core-api/printk-index.rst
new file mode 100644
index 000000000000..0de380b4a67c
--- /dev/null
+++ b/Documentation/core-api/printk-index.rst
@@ -0,0 +1,136 @@
+.. SPDX-License-Identifier: GPL-2.0
+Printk Index
+There are many ways how to monitor the state of the system. One important
+source of information is the system log. It provides a lot of information,
+including more or less important warnings and error messages.
+There are monitoring tools that filter and take action based on messages
+The kernel messages are evolving together with the code. As a result,
+particular kernel messages are not KABI and never will be!
+It is a huge challenge for maintaining the system log monitors. It requires
+knowing what messages were updated in a particular kernel version and why.
+Finding these changes in the sources would require non-trivial parsers.
+Also it would require matching the sources with the binary kernel which
+is not always trivial. Various changes might be backported. Various kernel
+versions might be used on different monitored systems.
+This is where the printk index feature might become useful. It provides
+a dump of printk formats used all over the source code used for the kernel
+and modules on the running system. It is accessible at runtime via debugfs.
+The printk index helps to find changes in the message formats. Also it helps
+to track the strings back to the kernel sources and the related commit.
+User Interface
+The index of printk formats are split in into separate files. The files are
+named according to the binaries where the printk formats are built-in. There
+is always "vmlinux" and optionally also modules, for example::
+ /sys/kernel/debug/printk/index/vmlinux
+ /sys/kernel/debug/printk/index/ext4
+ /sys/kernel/debug/printk/index/scsi_mod
+Note that only loaded modules are shown. Also printk formats from a module
+might appear in "vmlinux" when the module is built-in.
+The content is inspired by the dynamic debug interface and looks like::


+ $> head -1 /sys/kernel/debug/printk/index/vmlinux; shuf -n 5 vmlinux
+ # <level[,flags]> filename:line function "format"
+ <5> block/blk-settings.c:661 disk_stack_limits "%s: Warning: Device %s is misaligned\n"
+ <4> kernel/trace/trace.c:8296 trace_create_file "Could not create tracefs '%s' entry\n"
+ <6> arch/x86/kernel/hpet.c:144 _hpet_print_config "hpet: %s(%d):\n"
+ <6> init/do_mounts.c:605 prepare_namespace "Waiting for root device %s...\n"
+ <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"
+, where the meaning is::
+ - level: log level

Maybe worth noting that level may not be there if we have a KERN_CONT.

+ - flags: optional flags: currently only 'c' for KERN_CONT
+ - filename:line: source filename and line number of the related
+ printk() call. Note that there are many wrappers, for example,
+ pr_warn(), pr_warn_once(), dev_warn().
+ - function: function name where the printk() call is used.
+ - format: format string
+The extra information makes it a bit harder to find differences
+between various kernels. Especially the line number might change
+very often. On the other hand, it helps a lot to confirm that
+it is the same string or find the commit that is responsible
+for eventual changes.
+printk() Is Not a Stable KABI
+Several developers are afraid that exporting all these implementation
+details into the user space will transform particular printk() calls
+into KABI.
+But it is exactly the opposite. printk() calls must _not_ be KABI.
+And the printk index helps user space tools to deal with this.
+Subsystem specific printk wrappers
+The printk index is generated using extra metadata that are stored in
+a dedicated .elf section ".printk_index". It is achieved using macro
+wrappers doing __printk_index_emit() together with the real printk()
+call. The same technique is used also for the metadata used by
+the dynamic debug feature.
+The metadata are stored for a particular message only when it is printed
+using these special wrappers. It is implemented for the commonly
+used printk() calls, including, for example, pr_warn(), or pr_once().
+Additional changes are necessary for various subsystem specific wrappers
+that call the original printk() via a common helper function. These needs
+their own wrappers adding __printk_index_emit().
+Only few subsystem specific wrappers have been updated so far,
+for example, dev_printk(). As a result, the printk formats from
+some subsystes can be missing in the printk index.
+Subsystem specific prefix
+The macro pr_fmt() macro allows to define a prefix that is printed
+before the string generated by the related printk() calls.
+Subsystem specific wrappers usually add even more complicated
+These prefixes can be stored into the printk index metadata
+by an optional parameter of __printk_index_emit(). The debugfs
+interface might then show the printk formats including these prefixes.
+For example, drivers/acpi/osl.c contains::
+ #define pr_fmt(fmt) "ACPI: OSL: " fmt
+ static int __init acpi_no_auto_serialize_setup(char *str)
+ {
+ acpi_gbl_auto_serialize_methods = FALSE;
+ pr_info("Auto-serialization disabled\n");
+ return 1;
+ }
+This results in the following printk index entry::
+ <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"
+It helps matching messages from the real log with printk index.
+Then the source file name, line number, and function name can
+be used to match the string with the source code.
index fd768d43e048..ad8625130ea9 100644
@@ -15855,6 +15855,7 @@ F: kernel/printk/
R: Chris Down <chris@xxxxxxxxxxxxxx>
S: Maintained
+F: Documentation/core-api/printk-index.rst
F: kernel/printk/index.c