printk/index: Printk index feature documentation

Document the printk index feature. The primary motivation is to
explain that it is not creating KABI from particular printk() calls.
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Petr Mladek <pmladek@suse.com>

Documentation/core-api/index.rst

@@ -20,6 +20,7 @@ it.
    workqueue
    printk-basics
    printk-formats
+   printk-index
    symbol-namespaces
 
 Data structures and low-level utilities

Documentation/core-api/printk-index.rst

+.. 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
+logged.
+
+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 value: 0-7 for particular severity, -1 as default,
+	'c' as continuous line without an explicit log level
+   - :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
+prefixes.
+
+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.

MAINTAINERS

@@ -15585,6 +15585,7 @@ F:	kernel/printk/
 PRINTK INDEXING
 R:	Chris Down <chris@chrisdown.name>
 S:	Maintained
+F:	Documentation/core-api/printk-index.rst
 F:	kernel/printk/index.c
 K:	printk_index