1 .. SPDX-License-Identifier: GPL-2.0
2
3 ============
4 Printk Index
5 ============
6
7 There are many ways to monitor the state of the system. One important
8 source of information is the system log. It provides a lot of information,
9 including more or less important warnings and error messages.
10
11 There are monitoring tools that filter and take action based on messages
12 logged.
13
14 The kernel messages are evolving together with the code. As a result,
15 particular kernel messages are not KABI and never will be!
16
17 It is a huge challenge for maintaining the system log monitors. It requires
18 knowing what messages were updated in a particular kernel version and why.
19 Finding these changes in the sources would require non-trivial parsers.
20 Also it would require matching the sources with the binary kernel which
21 is not always trivial. Various changes might be backported. Various kernel
22 versions might be used on different monitored systems.
23
24 This is where the printk index feature might become useful. It provides
25 a dump of printk formats used all over the source code used for the kernel
26 and modules on the running system. It is accessible at runtime via debugfs.
27
28 The printk index helps to find changes in the message formats. Also it helps
29 to track the strings back to the kernel sources and the related commit.
30
31
32 User Interface
33 ==============
34
35 The index of printk formats are split in into separate files. The files are
36 named according to the binaries where the printk formats are built-in. There
37 is 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
43 Note that only loaded modules are shown. Also printk formats from a module
44 might appear in "vmlinux" when the module is built-in.
45
46 The 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
67 The extra information makes it a bit harder to find differences
68 between various kernels. Especially the line number might change
69 very often. On the other hand, it helps a lot to confirm that
70 it is the same string or find the commit that is responsible
71 for eventual changes.
72
73
74 printk() Is Not a Stable KABI
75 =============================
76
77 Several developers are afraid that exporting all these implementation
78 details into the user space will transform particular printk() calls
79 into KABI.
80
81 But it is exactly the opposite. printk() calls must _not_ be KABI.
82 And the printk index helps user space tools to deal with this.
83
84
85 Subsystem specific printk wrappers
86 ==================================
87
88 The printk index is generated using extra metadata that are stored in
89 a dedicated .elf section ".printk_index". It is achieved using macro
90 wrappers doing __printk_index_emit() together with the real printk()
91 call. The same technique is used also for the metadata used by
92 the dynamic debug feature.
93
94 The metadata are stored for a particular message only when it is printed
95 using these special wrappers. It is implemented for the commonly
96 used printk() calls, including, for example, pr_warn(), or pr_once().
97
98 Additional changes are necessary for various subsystem specific wrappers
99 that call the original printk() via a common helper function. These needs
100 their own wrappers adding __printk_index_emit().
101
102 Only few subsystem specific wrappers have been updated so far,
103 for example, dev_printk(). As a result, the printk formats from
104 some subsystems can be missing in the printk index.
105
106
107 Subsystem specific prefix
108 =========================
109
110 The macro pr_fmt() macro allows to define a prefix that is printed
111 before the string generated by the related printk() calls.
112
113 Subsystem specific wrappers usually add even more complicated
114 prefixes.
115
116 These prefixes can be stored into the printk index metadata
117 by an optional parameter of __printk_index_emit(). The debugfs
118 interface might then show the printk formats including these prefixes.
119 For 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
131 This 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
135 It helps matching messages from the real log with printk index.
136 Then the source file name, line number, and function name can
137 be used to match the string with the source code.
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