Merge tag 'printk-for-5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Offload writing printk() messages on consoles to per-console
   kthreads.

   It prevents soft-lockups when an extensive amount of messages is
   printed. It was observed, for example, during boot of large systems
   with a lot of peripherals like disks or network interfaces.

   It prevents live-lockups that were observed, for example, when
   messages about allocation failures were reported and a CPU handled
   consoles instead of reclaiming the memory. It was hard to solve even
   with rate limiting because it would need to take into account the
   amount of messages and the speed of all consoles.

   It is a must to have for real time. Otherwise, any printk() might
   break latency guarantees.

   The per-console kthreads allow to handle each console on its own
   speed. Slow consoles do not longer slow down faster ones. And
   printk() does not longer unpredictably slows down various code paths.

   There are situations when the kthreads are either not available or
   not reliable, for example, early boot, suspend, or panic. In these
   situations, printk() uses the legacy mode and tries to handle
   consoles immediately.

 - Add documentation for the printk index.

* tag 'printk-for-5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  printk, tracing: fix console tracepoint
  printk: remove @console_locked
  printk: extend console_lock for per-console locking
  printk: add kthread console printers
  printk: add functions to prefer direct printing
  printk: add pr_flush()
  printk: move buffer definitions into console_emit_next_record() caller
  printk: refactor and rework printing logic
  printk: add con_printk() macro for console details
  printk: call boot_delay_msec() in printk_delay()
  printk: get caller_id/timestamp after migration disable
  printk: wake waiters for safe and NMI contexts
  printk: wake up all waiters
  printk: add missing memory barrier to wake_up_klogd()
  printk: cpu sync always disable interrupts
  printk: rename cpulock functions
  printk/index: Printk index feature documentation
  MAINTAINERS: Add printk indexing maintainers on mention of printk_index

2 files changed, 138 insertions, 0 deletions

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.
    workqueue
    printk-basics
    printk-formats
+   printk-index
    symbol-namespaces
 
 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..3062f37d119b
--- /dev/null
+++ b/Documentation/core-api/printk-index.rst
@@ -0,0 +1,137 @@
+.. 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.