Comment 69 for bug 1690085

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Tom Reynolds (tomreyn) wrote :

So, to sum up, we have several reports here that Ubuntu's default kernels freeze on low load situations with Ryzen CPUs (or a subset of Ryzen CPUs?) up to and including the latest mainline releases. This bug differs from #196481 (user space segfaults only on high CPU load).

For low load freezes we have a reliable workaround for Xenial (probably also for newer releases) across different hardware components (differences in exact Ryzen model, mainboards, memory models and configurations) and kernel versions:

* Update to latest UEFI
* Kernel >= 4.10.0 (4.14.5 mainline tested successfully) built with CONFIG_RCU_NOCB_CPU
* The result of `echo rcu_nocbs=0-$(($(nproc)-1))` appended to GRUB_CMDLINE_LINUX

In my experience, with the above changes applied, the following BIOS changes are NOT necessary:

* Disabling SMT
* Disabling ALSR

It is unclear whether it is strictly necessary to disable C6 in BIOS, comments welcome.

I would think CONFIG_RCU_NOCB_CPU is an acceptable compromise (for this CPU type) if it provides the stability AMD has, for exactly one year now, been unable to provide to Linux users on their current platform.