kernel log buffer to small...

[Daniel J Blueman]

Binary package hint: linux-image

When debugging a kernel problem, after eg a 'echo t >/proc/sysrq-trigger', the kernel log buffer simply overflows; 2^17 (128KB) is too small.

Fix is to change kernel configuration from:
CONFIG_LOG_BUF_SHIFT=17

to:
CONFIG_LOG_BUF_SHIFT=18

This would help in particular cases where we're reliant on this mechanism, for reporting/fixing kernel bugs.

[Daniel J Blueman (danielblueman)]

When debugging the state of kernel tasks, eg by sending sysctrl-T, there is too much information with a typical login session for the default log buffer size, thus if the process is started earlier, the information is lost (either from dmesg, or /var/log/messages) and the first part of the output truncated:

$ cat /var/log/messages
Sep 19 11:42:32 exige kernel: [<ffffffff812732aa>] ? __up_read+0x9a/0xc0
Sep 19 11:42:32 exige kernel: [ 1803.421542] [<ffffffff811293f9>] do_vfs_ioctl+0x79/0x370
Sep 19 11:42:32 exige kernel: [ 1803.421542] [<ffffffff81077f79>] ? up_read+0x9/0x10
Sep 19 11:42:32 exige kernel: [ 1803.421542] [<ffffffff81525fe4>] ? do_page_fault+0x194/0x370
Sep 19 11:42:32 exige kernel: [ 1803.421542] [<ffffffff81129771>] sys_ioctl+0x81/0xa0
<snip>

I think it's important to be able to debug kernel problems fully, so this is a quick and safe fix.

[Tim Gardner (timg-tpi)]

How about raising it to 256K (2^18) for all flavours _except_ i386 generic?

[Daniel J Blueman (danielblueman)]

This also benefits crashdump kernels, where the only kernel log messages may be available via the static kernel message buffer.

Keeping it at 128KB/2^17 for i386 seems entirely reasonable, in-line with keeping the memory requirements minimal, setting to 256KB/2^18 elsewhere.

Thanks!

[Tim Gardner (timg-tpi)]

http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-karmic.git;a=commit;h=68625adedbf3e2a804254d636b3408f29877a708

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux - 2.6.31-11.37

---------------
linux (2.6.31-11.37) karmic; urgency=low

  [ Tim Gardner ]

  * [Config] Increase kernel log buffer to 256K for amd64 flavours
    - LP: #424810
  * [Config] Set HZ=100 for amd64 flavours
    - LP: #438234
  * [Upstream] e1000e: Emit notice instead of an error when
    pci_enable_pcie_error_reporting() fails
    - LP: #436370

  [ Upstream Kernel Changes ]

  * n_tty: honor opost flag for echoes
  * n_tty: move echoctl check and clean up logic
    - LP: #438310

  * Revert "[Upstream] drm/i915: Check that the relocation points to within
    the target" - Use upstream cherry-pick.
  * drm/i915: Check that the relocation points to within the target
    - LP: #429241

  * drm/i915: fix tiling on IGDNG
  * drm/i915: add B43 chipset support
  * agp/intel: Add B43 chipset support
    Intel request from kernel team mailing list.

  * HID: completely remove apple mightymouse from blacklist
    - LP: #428111

 -- Tim Gardner <email address hidden> Mon, 28 Sep 2009 11:47:29 -0600