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[Bug 1564475] Re: 128M is not enough for kdump on s390 LPARs



If I understand correctly cio_ignore command needs to be on the running
instance that is about to crash, rather than on the re-exec kernel. Thus
it needs to be computed and added to e.g. /etc/zipl.conf. At the moment
we do not generate/update /etc/zipl.conf in an automated way, but the
more I think about it the better it sounds to e.g. able to specify all
crashdump parameters, computed cio_ignore, correct root= argument, and
generate menu items for every installed ubuntu kernel (rather than just
the last one), and recovery stanzas too. I'll open a wishlist bug about

BTW, does it make sense to compute and use `cio_ignore -k -u` generated
command line by default? on one hand kernel will use less memory, on the
other hand `lszdev` will be quite empty and one will have a harder time
to discover additional devices one can bring online.

I have, for now, added things to https://wiki.ubuntu.com/S390X -> feel
free to edit and/or improve that. And I will open a new bug report to
get those updates into the Ubuntu Server Guide.

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  128M is not enough for kdump on s390 LPARs

Status in Ubuntu on IBM z Systems:
  Fix Released
Status in makedumpfile package in Ubuntu:
Status in s390-tools package in Ubuntu:
  Fix Released
Status in zipl-installer package in Ubuntu:
  Fix Released
Status in makedumpfile source package in Xenial:
Status in s390-tools source package in Xenial:
  Fix Released
Status in zipl-installer source package in Xenial:
  Fix Released

Bug description:
  == Comment: #0 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 10:59:26 ==
  With the current Ubuntu default setting "crashkernel=128M" kdump on LPARs crashes with out-of-memory (see attachment "dmesg_lpar_out_of_mem_128M.txt").

  On z/VM guests 128M seems to be sufficient.

  One reason on our test LPAR is that a lot of devices are attached (see
  attachment "lscss_lpar.txt") which are not required for kdump but
  consume a lot of memory because the s390 CIO layer allocates data
  structures in the kernel for those devices.

  We can disable the devices by using the "cio_ignore=" kernel parameter
  in "/etc/default/kdump-tools". For example, on our LPAR that uses DASD
  0.0.e934 for /var/crash, we added the following line to disable the

  KDUMP_CMDLINE_APPEND="irqpoll maxcpus=1

  For more information on the "cio_ignore=" kernel parameter see:

  Even with "cio_ignore=" we still get out-of-memory with

  With "crashkernel=196M" and "cio_ignore=" we are able to create a dump
  on our LPAR. We currently do not know why kdump with "cio_ignore=" on
  LPAR consumes more memory than on z/VM guests.

  == Comment: #1 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 11:03:15 ==
  Kernel messages of kdump out-of-memory crash on LPAR with many devices without cio_ignore parameter and 128M crashkernel memory.

  == Comment: #2 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 11:04:10 ==
  Output of lscss showing all attached (not online) devices on the LPAR.

  == Comment: #3 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 11:07:35 ==
  To solve this issue our recommendation is:

  1) Increase "crashkernel=" default to 196M on Ubuntu for s390.

  2) Document that KDUMP_CMDLINE_APPEND with "cio_ignore=" can be used
  to decrease memory consumption for kdump on systems with many devices
  that are not required for kdump.

  The most user friendly solution would be to automatically determine
  the required kdump devices and set the correct "cio_ignore=" kernel
  parameter. But this is not trivial, because it can be difficult to
  find out the required devices for stacked setups like LVM or for
  network dump.

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