1.. SPDX-License-Identifier: GPL-2.0
  2
  3============
  4Printk Index
  5============
  6
  7There are many ways to monitor the state of the system. One important
  8source of information is the system log. It provides a lot of information,
  9including more or less important warnings and error messages.
 10
 11There are monitoring tools that filter and take action based on messages
 12logged.
 13
 14The kernel messages are evolving together with the code. As a result,
 15particular kernel messages are not KABI and never will be!
 16
 17It is a huge challenge for maintaining the system log monitors. It requires
 18knowing what messages were updated in a particular kernel version and why.
 19Finding these changes in the sources would require non-trivial parsers.
 20Also it would require matching the sources with the binary kernel which
 21is not always trivial. Various changes might be backported. Various kernel
 22versions might be used on different monitored systems.
 23
 24This is where the printk index feature might become useful. It provides
 25a dump of printk formats used all over the source code used for the kernel
 26and modules on the running system. It is accessible at runtime via debugfs.
 27
 28The printk index helps to find changes in the message formats. Also it helps
 29to track the strings back to the kernel sources and the related commit.
 30
 31
 32User Interface
 33==============
 34
 35The index of printk formats are split in into separate files. The files are
 36named according to the binaries where the printk formats are built-in. There
 37is always "vmlinux" and optionally also modules, for example::
 38
 39   /sys/kernel/debug/printk/index/vmlinux
 40   /sys/kernel/debug/printk/index/ext4
 41   /sys/kernel/debug/printk/index/scsi_mod
 42
 43Note that only loaded modules are shown. Also printk formats from a module
 44might appear in "vmlinux" when the module is built-in.
 45
 46The content is inspired by the dynamic debug interface and looks like::
 47
 48   $> head -1 /sys/kernel/debug/printk/index/vmlinux; shuf -n 5 vmlinux
 49   # <level[,flags]> filename:line function "format"
 50   <5> block/blk-settings.c:661 disk_stack_limits "%s: Warning: Device %s is misaligned\n"
 51   <4> kernel/trace/trace.c:8296 trace_create_file "Could not create tracefs '%s' entry\n"
 52   <6> arch/x86/kernel/hpet.c:144 _hpet_print_config "hpet: %s(%d):\n"
 53   <6> init/do_mounts.c:605 prepare_namespace "Waiting for root device %s...\n"
 54   <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"
 55
 56, where the meaning is:
 57
 58   - :level: log level value: 0-7 for particular severity, -1 as default,
 59	'c' as continuous line without an explicit log level
 60   - :flags: optional flags: currently only 'c' for KERN_CONT
 61   - :filename\:line: source filename and line number of the related
 62	printk() call. Note that there are many wrappers, for example,
 63	pr_warn(), pr_warn_once(), dev_warn().
 64   - :function: function name where the printk() call is used.
 65   - :format: format string
 66
 67The extra information makes it a bit harder to find differences
 68between various kernels. Especially the line number might change
 69very often. On the other hand, it helps a lot to confirm that
 70it is the same string or find the commit that is responsible
 71for eventual changes.
 72
 73
 74printk() Is Not a Stable KABI
 75=============================
 76
 77Several developers are afraid that exporting all these implementation
 78details into the user space will transform particular printk() calls
 79into KABI.
 80
 81But it is exactly the opposite. printk() calls must _not_ be KABI.
 82And the printk index helps user space tools to deal with this.
 83
 84
 85Subsystem specific printk wrappers
 86==================================
 87
 88The printk index is generated using extra metadata that are stored in
 89a dedicated .elf section ".printk_index". It is achieved using macro
 90wrappers doing __printk_index_emit() together with the real printk()
 91call. The same technique is used also for the metadata used by
 92the dynamic debug feature.
 93
 94The metadata are stored for a particular message only when it is printed
 95using these special wrappers. It is implemented for the commonly
 96used printk() calls, including, for example, pr_warn(), or pr_once().
 97
 98Additional changes are necessary for various subsystem specific wrappers
 99that call the original printk() via a common helper function. These needs
100their own wrappers adding __printk_index_emit().
101
102Only few subsystem specific wrappers have been updated so far,
103for example, dev_printk(). As a result, the printk formats from
104some subsystems can be missing in the printk index.
105
106
107Subsystem specific prefix
108=========================
109
110The macro pr_fmt() macro allows to define a prefix that is printed
111before the string generated by the related printk() calls.
112
113Subsystem specific wrappers usually add even more complicated
114prefixes.
115
116These prefixes can be stored into the printk index metadata
117by an optional parameter of __printk_index_emit(). The debugfs
118interface might then show the printk formats including these prefixes.
119For example, drivers/acpi/osl.c contains::
120
121  #define pr_fmt(fmt) "ACPI: OSL: " fmt
122
123  static int __init acpi_no_auto_serialize_setup(char *str)
124  {
125	acpi_gbl_auto_serialize_methods = FALSE;
126	pr_info("Auto-serialization disabled\n");
127
128	return 1;
129  }
130
131This results in the following printk index entry::
132
133  <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"
134
135It helps matching messages from the real log with printk index.
136Then the source file name, line number, and function name can
137be used to match the string with the source code.