Many Ryzen CPUs for a long time have been affected by random freezes and reboots, which some managed to narrow down to C6 power states. Even RMA often didn't help with these.

Recently I found an actual kernel bug report about it: https://bugzilla.kernel.org/show_bug.cgi?id=196683

Apparently, AMD said that they are going to release some microcode update (or MB manufacturers are going to release some firmware updates?), which handle this. But until that will happen, you can work around it without disabling C6 states, if you build the kernel with

CONFIG_RCU_NOCB_CPU=y

And then use rcu_nocbs=0-... kernel boot parameter to enable it.

I was bugged by this issue for a long time, and recently decided to build a kernel like above. It happens to be quite easy with Debian. It indeed works around the problem.

Here is an example HOWTO on doing it:
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If this is useful to anyone, here is what I did on Debian testing:

Install current Linux source package and some tools and libraries:

sudo apt-get install linux-source build-essential kernel-package libncurses5-dev libelf-dev libssl-dev

Note: in my case I pulled linux-source from Debian sid, since the default kernel in testing still didn't get bumped to Linux 4.15.x, even though 4.15 is available in general.

Unpack the source for example to $HOME/build.

I like using explicit tar parameters even though they are long, it just makes it hugely more readable and easy to understand:

linux_ver="4.15"
config_ver="4.15.0-1-amd64"

linux_src="/usr/src/linux-source-${linux_ver}.tar.xz"
build_dir="${HOME}/build"
source_dir="${build_dir}/linux-source-${linux_ver}"

mkdir -p $build_dir
tar --extract --verbose --use-compress-program "xz --decompress" --file ${linux_src} --directory ${build_dir}
cd $source_dir

Now you are in the unpacked source directory. You'd need to create proper .config file. My goal was to make only minimal tweaks from the stock Debian kernel, so I copied default config from /boot first:

cp -v /boot/config-${config_ver} ${source_dir}/.config

Now, you need to enable the actual workaround. There is a useful tool for configuring kernel parameters - menuconfig (that's why libncurses5-dev was needed above).

make menuconfig

It will build the tool in place and will run it. Find RCU options under General Setup > RCU Subsystem





And there enable "Make expert-level adjustments to RCU configuration" and "Offload RCU callback processing from boot-selected CPUs"



Then select Save (into .config), and a few times Exit to exit the tool.

One more thing is needed - comment out CONFIG_SYSTEM_TRUSTED_KEYS in resulting ${source_dir}/.config, otherwise [the build will fail](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=823107).

perl -p -i -e 's/^CONFIG_SYSTEM_TRUSTED_KEYS=/#CONFIG_SYSTEM_TRUSTED_KEYS=/' .config

Now you are ready to build it (I chose suffixes -rcu, and -1 for versions):

make -j$(nproc) deb-pkg LOCALVERSION=-rcu KDEB_PKGVERSION=$(make kernelversion)-1

Press Enter a few times to complete the configuration caused by the modifications, and it should proceed with the build. After it completes, you should have the ready package in $build_dir:

cd ..
ls -1 linux-image*
linux-image-4.15.4-rcu_4.15.4-1_amd64.deb
linux-image-4.15.4-rcu-dbg_4.15.4-1_amd64.deb

Install the result:

sudo dpkg -i linux-image-4.15.4-rcu_4.15.4-1_amd64.deb

Now open /etc/default/grub with your editor (under sudo), and add this to your kernel boot parameters (assuming you have 8 core / 16 thread processor).

GRUB_CMDLINE_LINUX_DEFAULT="... rcu_nocbs=0-15"

... here means whatever was there before the change, don't remove what was there, just add the new parameter after a space!

Save and update system grub:

sudo update-grub

That's it. Reboot the system and you are ready to use the workaround kernel.