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[Dimitri John Ledkov <launchpad@xxxxxxxxxxxx>]

$ cat /boot/config-5.0.0-16-generic | grep LOG_BUF
CONFIG_LOG_BUF_SHIFT=18
CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT=13

$ uname -a
Linux doerfel 5.0.0-16-generic #17-Ubuntu SMP Wed May 15 10:54:19 UTC 2019 aarch64 aarch64 aarch64 GNU/Linux

And journal shows complete kernel boot messages, thus this all now good
on disco.

** Tags removed: verification-needed-disco
** Tags added: verification-done-disco

** No longer affects: systemd (Ubuntu Xenial)

** No longer affects: systemd (Ubuntu Bionic)

** No longer affects: systemd (Ubuntu Cosmic)

** No longer affects: systemd (Ubuntu Disco)

** No longer affects: systemd (Ubuntu Eoan)

** No longer affects: systemd (Ubuntu)

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https://bugs.launchpad.net/bugs/1824864

Title:
  CONFIG_LOG_BUF_SHIFT set to 14 is too low on arm64

Status in linux package in Ubuntu:
  Confirmed
Status in linux source package in Xenial:
  Confirmed
Status in linux source package in Bionic:
  Confirmed
Status in linux source package in Cosmic:
  Confirmed
Status in linux source package in Disco:
  Fix Committed
Status in linux source package in Eoan:
  Confirmed

Bug description:
  [Impact]

   * Too small dmsg kernel buf ring size leads to loosing/missing early
  boot kernel messages which happen before journald starts slurping them
  up and storing them on disc. This results in messages similar to this
  one on boot "missed NN kernel messages on boot". This is especially
  pronounced on arm64 as the default setting there is way lower than any
  other 32bit or 64bit architecture we ship. Also amd64 appears to have
  the highest setting of 18 among all architectures we ship. The best
  course of action to bump all 64bit arches to 18, and keep all 32bit
  arches at the current & upstream default of 17.

  [Test Case]

   * $ cat /boot/config-`uname -r` | grep CONFIG_LOG_BUF_SHIFT

  on 64bit arches result should be: CONFIG_LOG_BUF_SHIFT=18
  on 32bit arches result should be: CONFIG_LOG_BUF_SHIFT=17

   * run systemd adt test, the boot-and-services test case should not
  fail journald tests with "missed kernel messages" visible in the error
  logs.

  [Regression Potential]

   * Increasing the size of the log_buf, will increase kernel memory
  usage which cannot be reclaimed. It will now become 256kb on arm64,
  ppc64el, s390x instead of 8kB/128kb/128kb respectively. 32bit arches
  remain unchanged at 128kb.

  [Other Info]
   
   * Original bug report

  CONFIG_LOG_BUF_SHIFT
  policy<{
  'amd64'  : '18',
  'arm64'  : '14',
  'armhf'  : '17',
  'i386'   : '17',
  'ppc64el': '17',
  's390x'  : '17'}>

  Please set CONFIG_LOG_BUF_SHIFT to at least 17 on arm64.

  Potentially bump all 64-bit arches to 18 (or higher!) as was done on
  amd64, meaning set 18 on arm64 s390x ppc64el.

  I have a systemd autopkgtest test that asserts that we see Linux
  kernel command line in the dmesg (journalctl -k -b). And it is
  consistently failing on arm64 scalingstack KVM EFI machines with
  messages of "missing 81 kernel messages".

  config LOG_BUF_SHIFT
          int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
          range 12 25
          default 17
          depends on PRINTK
          help
            Select the minimal kernel log buffer size as a power of 2.
            The final size is affected by LOG_CPU_MAX_BUF_SHIFT config
            parameter, see below. Any higher size also might be forced
            by "log_buf_len" boot parameter.

            Examples:
                       17 => 128 KB
                       16 => 64 KB
                       15 => 32 KB
                       14 => 16 KB
                       13 =>  8 KB
                       12 =>  4 KB

  14 sounds like redictiously low for arm64. given that 17 is default
  across 32-bit arches, and 18 is default on amd64.

  On a related note, we have CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT                policy<{'amd64': '13', 'arm64': '13', 'armhf': '13', 'i386': '13', 'ppc64el': '13', 's390x': '13'}>
  I'm not sure if we want to bump these up to LOG_BUF_SHIFT size or not.

  Please backport this to xenial and up.

  === systemd ===

  systemd, boot-and-services test case can bump the ring buffer before
  running the tests.

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