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Ubuntu 20.04, 5.13.0-39-generic AMD Ryzen 9 3900x (12c/24t), B550 Aorus pro (rev 1.0). with recent updates 12 cores out of 24 are not detected. htop only shows 12 , where as it used to show 24 core two weeks back.

  1. Tried updating grub with below options but no positive result.
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pcie_aspm=off pci=assign-busses apicmaintimer idle=poll reboot=cold,hard acpi=on"
  1. Updated BIOS with latest version.
  2. With Dual boot to windows 10 , all 24 cores are shown !
  3. Tried disabling CCD control from BIOS, This lead to show only 6 cores instead of 12
  4. Tried disabling Global C-State control from BIOS. Nothing improved.

Note: I was using this setup from more than 6 months with 24 cores shown. I wonder what could be wrong.

lscpu shows : Off-line CPU(s) list: 12-23. Any suggestion to make these Off-line CPU to active (to be part of Online) ?

lscpu
...
CPU(s):                          24
On-line CPU(s) list:             0-11
Off-line CPU(s) list:            12-23
Thread(s) per core:              1
Core(s) per socket:              12
Socket(s):                       1
NUMA node(s):                    1
gprasad
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  • According to your own question, your CPU only has 12 cores. Why all of the GRUB kernel options? Show me `sudo dmidecode -s bios-version`. – heynnema Apr 12 '22 at 14:50
  • `sudo dmidecode -s bios-version` -> `F15a` . Ryzen 9 3900x has 12 cores / 24 threads. `Why all of the GRUB kernel options?` : These were suggestion from [amd-ryzen-5-3600-ubuntu-20-04](https://askubuntu.com/questions/1234299/amd-ryzen-5-3600-ubuntu-20-04-problems?rq=1) . – gprasad Apr 12 '22 at 15:48
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    Your BIOS is current. Copying a two year old thread suggestions is not always the best idea. Try editing /etc/default/grub so that your default looks like **GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"**, then `sudo update-grub`, `reboot`, and check your CPU's again. Report back. Start comments to me with @heynnema or I'll miss them. – heynnema Apr 12 '22 at 16:22
  • Also, at the GRUB menu, try booting to an older 5.11 kernel, and see if your CPUs come back. – heynnema Apr 12 '22 at 16:25
  • @heynnema Thank you for quick response. 1. clean up GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" 2. Booted with 5.11.0-41 (tried updating to kernel 5.17.xx also. for both there were no change , I could still see `Off-line CPU(s) list: 12-23` – gprasad Apr 12 '22 at 16:48
  • You did `sudo update-grub`, yes? Check your BIOS for any settings that might have changed. Make sure that everything is enabled in the hardware. – heynnema Apr 13 '22 at 01:21
  • @heynnema, Yes i did ran `update-grub` , looked various bios setting by enabling / disabling one by one, Tried setting everything to default and also. Please note: with same BIOS (Dual boot windows 10 and Ubuntu) setting when i boot to windows all 16/24t are shown and functional. – gprasad Apr 13 '22 at 06:00
  • Boot to a Ubuntu Live USB and see how many CPUs you get. Report back. – heynnema Apr 13 '22 at 13:44
  • @heynnema, `lscpu -> CPU(s) : 24 on-line CPU(s) list: 0-23` booting from Ubuntu Live USB shows all the CPU(s). Thanks once again. Any suggestion on looking a boot log and know whey half of the CPU(s) are in offline. – gprasad Apr 13 '22 at 18:43
  • You must have made some kind of change to your running system, but I can't imagine what. We tried an older kernel. Your BIOS is current. You've removed all of the GRUB mods. You may just need to reinstall Ubuntu to fix this one. You **could** try... `grep -i cpu /var/log/syslog*` and see if you get any hits. – heynnema Apr 13 '22 at 19:49
  • You haven't overclocked your CPU or RAM, correct? – heynnema Apr 13 '22 at 19:51
  • Show me `grep GRUB_CMDLINE_LINUX /etc/default/grub`. – heynnema Apr 13 '22 at 19:52

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