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I'm testing linux cpu hotpluging on normal desktop 8-core AMD machine. Problem seems that after onlining a CPU again taskset and kernel refuses to move and/or schedule any tasks on the core. E.g. doing:

echo 0 | sudo tee /sys/devices/system/cpu/cpu7/online
echo 1 | sudo tee /sys/devices/system/cpu/cpu7/online
taskset -c 7 bash

Fails... I don't know why taskset refuses to migrate any threads to the hotpluged cores. I have intend to use this to improve the kernel NO_HZ tickless operation by forcing Local timer interupts (LOC) off from cores 2-7. The kernel NO_HZ documentation suggests this possibility.

I'm running mainline 4.4.6 PREEMPT kernel on Linux Mint 17.3. The kernel is self-compiled though as stock mint kernels are missing several features/drivers that I require.

JATothrim
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  • The same happens on me. Once `offline`d, the cpus don't come online. Kernel 4.1.1. Did you solve this? – davide May 14 '16 at 15:45
  • No, I did not. It is still an issue. I would really need this to work so I don't have to force cpu affinity with cpusets. I wrote an upstart service that sweeps periodically selected tasks to core 0 using cpusets. But that still won't stop the LOC interrupts triggering on cpus 1-7. At least the NO_HZ mode is now utilized some what. – JATothrim May 14 '16 at 19:17
  • Might be interested: https://bugzilla.kernel.org/show_bug.cgi?id=118221 – davide May 14 '16 at 19:50
  • I found a functional solution to this, which allows CPUs hotplug without LXC disruption. See my duplicate thread here: http://superuser.com/questions/1076920/cpu-cores-remain-offline-after-hotplug/1077898#1077898 – davide May 17 '16 at 16:20

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