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    SubjectRe: [PATCH] printk: support structured and multi-facility log messages

    * Kay Sievers <> wrote:

    > From: Kay Sievers <>
    > Subject: printk: support structured and multi-facility log messages
    > Kernel log messages are the primary source of information about the overall
    > state of the system and connected devices. Traditional kernel messages are
    > mostly human language, targeted at a human reading them. This part of the
    > picture works very well since a very long time.
    > However, most machines run unattended almost all of their time, and
    > software, and not humans, need to process the kernel messages. Having
    > a machine making sense out of human language messages is inefficient,
    > unreliable, and sometimes plain impossible to get right. With human
    > language messages all useful information about their context,
    > available at the time of creation of the messages, is just thrown
    > away. Later, software consumers of the messages will need to apply
    > magic to reconstruct what the context might have been, to be able to
    > interpret the messages.
    > This patch extends printk() to be able to attach arbitrary key/value
    > pairs to logged messages, to carry machine-readable data which
    > describes the context of the log message at time of its
    > creation. Users of the log can retrieve, along with the human-readable
    > message, a key/value dictionary to reliably identify specific devices,
    > drivers, subsystems, classes and types of messages.
    > Various features of this patch:
    > - Record-based stream instead of the traditional byte stream
    > buffer. All records carry a 64 bit timestamp, the syslog facility
    > and priority in the record header.
    > - Records consume almost the same amount, sometimes less memory than
    > the traditional byte stream buffer (if printk_time is enabled). The record
    > header is 16 bytes long, plus some padding bytes at the end if needed.
    > The byte-stream buffer needed 3 chars for the syslog prefix, 15 char for
    > the timestamp and a newline.
    > - Buffer management is based on message sequence numbers. When records
    > need to be discarded, the reading heads move on to the next full
    > record. Unlike the byte-stream buffer, no old logged lines get
    > truncated or partly overwritten by new ones. Sequence numbers also
    > allow consumers of the log stream to get notified if any message in
    > the stream they are about to read gets discarded during the time
    > of reading.
    > - Better buffered IO support for KERN_CONT continuation lines, when printk()
    > is called multiple times for a single line. The use of KERN_CONT is now
    > mandatory to use continuation; a few places in the kernel need trivial fixes
    > here. The buffering could possibly be extended to per-cpu variables to allow
    > better thread-safety for multiple printk() invocations for a single line.
    > - Full-featured syslog facility value support. Different facilities
    > can tag their messages. All userspace-injected messages enforce a
    > facility value > 0 now, to be able to reliably distinguish them from
    > the kernel-generated messages. Independent subsystems like a
    > baseband processor running its own firmware, or a kernel-related
    > userspace process can use their own unique facility values. Multiple
    > independent log streams can co-exist that way in the same
    > buffer. All share the same global sequence number counter to ensure
    > proper ordering (and interleaving) and to allow the consumers of the
    > log to reliably correlate the events from different facilities.
    > - Output of dev_printk() is reliably machine-readable now. In addition
    > to the printed plain text message, it creates a log dictionary with the
    > following properties:
    > SUBSYSTEM= - the driver-core subsytem name
    > DEVICE=
    > b12:8 - block dev_t
    > c127:3 - char dev_t
    > n8 - netdev ifindex
    > +sound:card0 - subsystem:devname
    > - Support for multiple concurrent readers of /dev/kmsg, with read(),
    > seek(), poll() support. Output of message sequence numbers, to allow
    > userspace log consumers to reliably reconnect and reconstruct their
    > state at any given time. After open("/dev/kmsg"), read() always
    > returns *all* buffered records. If only future messages should be
    > read, SEEK_END can be used. In case records get overwritten while
    > /dev/kmsg is held open, or records get faster overwritten than they
    > are read, the next read() will return -EPIPE and the current reading
    > position gets updated to the next available record. The passed
    > sequence numbers allow the log consumer to calculate the amount of
    > lost messages.
    > $ cat /dev/kmsg
    > PRIORITY=5
    > SEQNUM=0
    > MESSAGE=Linux version 3.4.0-rc1+ (kay@mop) (gcc version 4.7.0 20120315 ...
    > ...
    > PRIORITY=7
    > SEQNUM=268
    > TIMESTAMP=399682
    > MESSAGE=pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored)
    > SUBSYSTEM=acpi
    > DEVICE=+acpi:PNP0A03:00
    > ...
    > PRIORITY=6
    > SEQNUM=863
    > TIMESTAMP=2479024
    > MESSAGE=udevd[71]: starting version 182
    > ...
    > PRIORITY=6
    > SEQNUM=1012
    > TIMESTAMP=4069447
    > MESSAGE=usb 2-1.4: MAC-Address: 02:80:37:e6:12:00
    > SUBSYSTEM=usb
    > DEVICE=c189:130

    Hm, this seems to be a conceptual duplication of the console
    tracepoint that include/trace/events/printk.h already offers.

    Have you looked at using that and extending it with a new
    tracepoint and new fields (with log priority level) where

    If you use it via a perf fd then you'll have structured logging,
    a record formatted ring-buffer, multiple readers support,
    filtering support, etc. - and of course you'll have access to a
    lot more tracepoints as well, should the system logging facility
    decide to log MCE events, etc.

    I.e. it will provide (vastly) more capabilities, it would be
    much more extensible - and the patch from you would shrink as
    well, drastically.

    IMHO the last thing we need is Yet Another Logging Facility.



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