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== 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
devices:

KDUMP_CMDLINE_APPEND="irqpoll maxcpus=1
cio_ignore=all,!condev,!0.0.e934"

For more information on the "cio_ignore=" kernel parameter see:
https://github.com/torvalds/linux/blob/master/Documentation/s390/CommonIO

Even with "cio_ignore=" we still get out-of-memory with
"crashkernel=128M".

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.

** Affects: makedumpfile (Ubuntu)
     Importance: Undecided
     Assignee: Skipper Bug Screeners (skipper-screen-team)
         Status: New


** Tags: architecture-s39064 bugnameltc-139866 severity-critical targetmilestone-inin---
-- 
128M is not enough for kdump on s390 LPARs
https://bugs.launchpad.net/bugs/1564475
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