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Description of problem:
The /usr/lib/udev/rules.d/40-redhat.rules included in the normal systemd package includes a rule which brings any offline CPUs online.
SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ENV{CPU_LEAVE_OFFLINE}!="yes", ATTR{online}="1"
It looks like this is intended so that newly hotplugged CPUs will be automatically activated.
However, if any CPUs are present, but offline at boot - such as when the maxcpus kernel command line option is used - then those CPUs will be brought online by udevd, which is not what a user would probably expect.
In particular, this means that kdump setups using maxcpus=1 end up with SMP enabled, which is a complexity we don't want for something supposed to be as simple and reliable as possible.
Version-Release number of selected component (if applicable):
systemd-219-19.el7.ppc64le
How reproducible:
100%
Steps to Reproduce:
1. Boot up system, with maxcpus=1 on the command line
2. Once booted, check /proc/cpuinfo
Actual results:
All present CPUs are online.
Expected results:
Only one CPU is online.
Additional info:
* For kdump on x86, this is addressed by using "nr_cpus=1" instead of "maxcpus=1", which prevents more cpus being added later. Complexities with cpu threading modes means that nr_cpus=NN doesn't work properly on Power (bug 1222059 has some more information).
* An alternative approach specific to kdump would be to alter construction of the kdump initramfs to exclude this rule. However, I can't see a straightforward way of doing that without excluding 40-redhat.rules entirely, which might have other rules which are important during a kdump.
Description of problem: The /usr/lib/udev/rules.d/40-redhat.rules included in the normal systemd package includes a rule which brings any offline CPUs online. SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ENV{CPU_LEAVE_OFFLINE}!="yes", ATTR{online}="1" It looks like this is intended so that newly hotplugged CPUs will be automatically activated. However, if any CPUs are present, but offline at boot - such as when the maxcpus kernel command line option is used - then those CPUs will be brought online by udevd, which is not what a user would probably expect. In particular, this means that kdump setups using maxcpus=1 end up with SMP enabled, which is a complexity we don't want for something supposed to be as simple and reliable as possible. Version-Release number of selected component (if applicable): systemd-219-19.el7.ppc64le How reproducible: 100% Steps to Reproduce: 1. Boot up system, with maxcpus=1 on the command line 2. Once booted, check /proc/cpuinfo Actual results: All present CPUs are online. Expected results: Only one CPU is online. Additional info: * For kdump on x86, this is addressed by using "nr_cpus=1" instead of "maxcpus=1", which prevents more cpus being added later. Complexities with cpu threading modes means that nr_cpus=NN doesn't work properly on Power (bug 1222059 has some more information). * An alternative approach specific to kdump would be to alter construction of the kdump initramfs to exclude this rule. However, I can't see a straightforward way of doing that without excluding 40-redhat.rules entirely, which might have other rules which are important during a kdump.