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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 . – 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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